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��Common Channel Signalling by S. Welch (Edited/OCRed by Omega)��
��

This TadXF File is split into two pieces. TADXF #004 which is just the ASCII
text and TADXF #005 which are the digitized figures and tables in PCX format.

Release Date: 26th of November 1994. The text was taken from a book from
Samuel Welch called 'Signalling in Telecommunications Networks' The ISBN
number is : 0 906048 46 X. The pages OCRed are page 259-321.

=��=
Table of contents

Common channel signalling
=��=

11.1 Introduction
11.2 Basic common channel signalling
11.3 Association between c.c.s. and speech (or equivalent networks)
11.4 Network centralised service signalling
11.5 CCITT No. 6 signalling system
11.5.1 Basic concepts
11.5.2 Signal codings
11.5.3 Error control
11.5.4 Analysis of the system 6 error control method
11.5.5 Synchronisation
11.5.6 System 6 analogue and digital versions
11.6 Common channel signalling loading
11.7 Signalling link security and load sharing
11.7.1 Security
11.7.2 Load sharing
11.8 Changeover, retrieval and changeback
11.9 Continuity check of the speech path
11.10 Signal Priority
11.11 CCITT No. 7 optimised digital common channel signalling system
11.11.1 Basic concepts
11.11.2 Signalling bit rate
11.11.3 Error control
11.11.4 Message Structure
References

=��=
11.1 Introduction
=��=

Conventional signalling on speech path systems (d.c., v.f., outband, m.f.)
have a number of limitations:

(a) Relatively slow signalling. This is usually tolerable for telephony pur-
poses in most networks in view of the acceptable postdialling delays, but
the application of computer-like techniques (stored program control) to
the control of switching introduces new factors.

(b) Limited information capacity.

(c) Limited capability to convey signalling information which is not call re-
lated and the inability of some systems to signal during the speech
period.

(d) The signalling systems tend to be designed for specific application con-
ditions, which results in a relatively large number of different systems
in the one network with consequential economic and administrative
problems.

(e) Expensive due to the per-speech circuit provision of most of the systems.

Conventional line-signalling systems incorporating decadic address signal-
ling are slow and inflexible, but meet the needs of most directacting non-
common control switching systems. Wired-logic common control switching systems
demand more flexibility, more facilities and the application of interregister
signalling systems as discussed, which systems are common provision, line-
signalling systems being necessary to deal with the supervisory signals. Time-
sharing of conventional analogue line-signalling systems is not usually
practicable and they are invariably per-speech circuit provided, and expensive
in total network signalling cost.

Stored program (processor) control (s.p.c.) of switching and networks has
prompted a reappraisal of the signalling technique. Processor control makes
possible the concentration of signalling logic for a large number of infor-
mation circuits, e.g. speech, with consequent cost-reduction. Unless care is
taken, however, an exchange processor may be given an excessive load in merely
scanning speech circuits to detect the signalling condition, with a consequent
loss of capacity for other functions, such as switching control, to be per-
formed by the processor. It would also be necessary for the processor, or
other suitable interface, to translate the various analogue conditions of
speechpath signalling to digital, and vice versa. Thus with s.p.c., it is in-
efficient for the processor which works in the digital mode, to deal with sig-
nalling on the speech path. A much more efficient way of transferring infor-
mation between s.p.c. exchanges is to provide a bidirectional high-speed data
link between the two processors over which they transfer signals in digital
form by means of coded-bit fields. A group of circuits (many hundreds) thus
shares a common channel signalling (c.c.s.) link in the time-shared mode. In
preferred c.c.s., all the signals between two exchanges are passed over a sig-
nalling link which is separate from speech, c.c.s. thus replacing speech-path
signalling systems such as d.c., v.f., outband and m.f. While c.c.s. could be
applied to non-s.p.c. exchanges, the need to provide means of directing the
information contained in the data link signals to or from the registers, and,
in the case of tandem connections, to transfer such information across the
exchange, would tend to make c.c.s. uneconomic; c.c.s. is thus mainly suited
for s.p.c. exchanges.

Many administrations are at present programming c.c.s. application, but
owing to the well known inertia of networks to change, it will require a
number of years for c.c.s. to make significant application impact in a net-
work, and conventional signalling systems will exist, and continue to be
applied, for a long time to come.

=��=
11.2 Basic common channel signalling
=��=

General

With c.c.s., independent signalling is performed in the respective sig-
nalling directions on the respective signalling channels, the two signal-
ling channels comprising the c.c.s. link (Fig. 11.1). Modems are used
to transmit and receive serial binary data over analogue transmission
channels, the multiplex performing this function on digital transmission
channels. C.C.S. has the following merits:

(i) signalling is completely separate from switching and speech
transmission and thus may evolve without the constraints normally
associated with such factors

------------------------[See fig11-1.pcx in tadxf005]------------------------
Fig. 11.1 Basic schematic common channel signalling
-----------------------------------------------------------------------------

(ii) significantly faster signalling

(iii) potential for a large number of signals

(iv) freedom to handle signals during the speech period

(v) flexibility to change or add signals

(vi) potential for network centralised-service signalling (e.g. network
management, network maintenance, centralised call accounting, etc.)

(vii) is economic for large speech circuit groups

(viii) can be economic for the smaller speech circuit groups owing to
the quasiassociated and dissociated signalling capabilities

(ix) unlike v.f. signalling, signalling line splits are not necessary;
this eliminates the problem arising on v.f. signalling systems where
a fast nonrepeated verbal answer response may be clipped, or lost,
owing to the speech path being split on electrical-answer signal
transmission.
It was the recognition by the CCITT of the signficance of this
problem in the international service which produced the initial
thoughts for a c.c.s. system of the type under discussion.

(x) allows possibility for signalling rationalisation in networks.

C.C.S., however, gives rise to requirements which do not arise with
signalling on speech-path systems such as:

(a) high order error rate performance
(b) signalling link security backup
(c) assurance of speech-path continuity as, unlike speech-path sig-
nalling, c.c.s. does not establish speech path integrity.

A c.c.s. system serving many speech circuits must have a much
greater signal dependability than signalling on speech-path systems, as
random errors on the signalling link would disturb an appreciable
number of signals, and therefore speech circuits. For this reason,
provision must be made in c.c.s. to control errors, and as a generalisa-
tion, an undetected error rate of the order of 1 in 10^8 to 1 in 10^10 is
desired. Automatic diverting of signalling traffic to an alternative
signalling facility occurs on excessive error rate, or on complete failure
of a c.c.s. link.

Time-shared signalling requires each signal to have identification
of the speech circuit to which it belongs, the identification being on
a time basis when a speech circuit has exclusive use of a signalling
facility, as in built-in p.c.m. signalling (Section 6). This time-assigned
signalling cannot apply when, as in c.c.s., a speech circuit does not
have exclusive use of a signalling facility in the time-shared signalling,
and a signal is labelled to give the speech circuit identity. With circuit
labelling, the coding of a bit-field in the signal itself gives the address
of the speech circuit, the number of bits depending on the number
of speech circuits to be identified. This depends on the maximum
number of speech circuits per c.c s. link-loading adopted, which
is influenced by the capacity of the c.c.s. link to transfer signals.
This, in tum, depends on the signalling bit rate and the size of signal
unit.

With c.c.s., the speech-path connection may be set up in parallel
with the signalling connection set-up, or retrospectively. The
former is adopted for simplicity and assurance of speech-path avail-
ability. Speech-path availability could be assured with the retrospec-
tive set-up by marking, but not switching, relevant speech circuits
in the connection build up, but there appears to be little point in this.

General features of c.c.s.
--------------------------

The following general features apply:

(a) Information may be transferred as signal messages of varying bit
length, or by defined signal units. Signal units will be assumed for
the present.

(b) Each signalling channel operates in the synchronous mode with
its continuous bit stream divided into contiguous signal units which
all contain the same number of bits for the same application or service.
The two signalling channels of a link need not necessarily be syn-
chronised to each other, and, if not, drift may arise between the signal
units in the respective directions, which, in certain implementations
of c.c s. (e.g. CCITT 6 system) may require compensation arrangements.

(c) Synchronisation (idle) units are transmitted when message signals
are not being transmitted, to maintain signal unit synchronism on the
signalling channel.

(d) The signal unit is divided into a number of constituent bit fields
each having its own function in the system, typically, heading, signal
information, circuit label, parity check, etc.

(e) As the c.c.s. link is time-shared, each message signal requires
identification of the speech circuit (and thus the call) to which it be-
longs. This is by means of a circuit label bit-field of size depending on
the number of speech circuits to be identified.

(f) Unlike time-assigned, channel-associated, time-shared p.c.m.
signalling (Sections 6.2 and 6.3), a speech circuit does not have the
exclusive use of a signalling facility in time-shared circuit-label ad-
dressed c.c.s. and queueing delays arise; message signals being placcd
in a queueing store and offered in turn for transfer over the signalling
channel. Thus the heavier the signal loading the greater the queue,
and the queueing delay, and the slower the speed of signal transfer.

(g) As c.c s. signals do not prove the continuity of the speech path
selected, other arrangements (e.g. per-call continuity check, routine
testing of idle paths) must be made.

(h) Errors are liable to occur in the signal-transfer process and some
form of error control is required as the uncorrected error-rate of
transmission plant is usually unacceptable for c.c s. Error detection is
based on redundant coding, the parity check bits being, typically,
part of each signal unit. Error correction can be, and usually is in
telephony c.c.s., by retransmission. Despite the incorporation of error
control, undetected errors may arise. Even with a high degree of error
correction a signalling link could be unusable for varying periods,
which requires signalling security backup.

(i) For signalling security, signals may be directed, by automatic
procedures, from a regular signalling link to an alternative signalling
facility when an excessive error rate, or complete failure, of the
regular link is detected.

(j) On multilink connections, signalling information is transmitted
on a link-by-link basis, the signals being transferred from one link to
the next only after processing. This is an inherent feature of c.c.s.

(k) since the circuit label may require a substantial proportion of the
bits available in a signal unit, comparatively few bits may remain for
coding the information a message has to convey. It follows that when
the information is even moderately extensive, and particularly when it
has a data content which can vary from one message to another, it
cannot be transmitted efficiently by a succession of single signal unit
messages. Instead, the initial circuit label carrying signal unit may be
augmented by one or more subsequent signal units in which all the
available bits can be used for carrying the information, thus producing
an efficient multiunit message. Thus lone signal units (LSU) and multi-
unit messages (MUM) may apply with c.c.s., and in the latter case the
initial signal unit (ISU) carries the circuit label bit-field for the
whole MUM. Each SU in a MUM carries its own check bit-field.

=��=
11.3 Association between c.c.s. and speech (or equivalent) networks
=��=

The signal messages relating to a given group of speech circuits between
two switching centres using a c.c.s. system can be transferred in the
following ways:

(a) Associated mode of operation: The signal messages are trans-
ferred between the two switching centres over a c.c.s. Iink which
terminates at the same switching centres as the group of speech circuits
to which the signalling link is assigned (Fig. 11.2a). The term
'associated signalling' applied to c.c.s. should not be confused with
speech path, or channel, associated signalling used to describe sig-
nalling systems in which the signals are passed over the speech paths
comprising the connections to which they relate, or over paths which are
permanently and individually related to the speech paths.

(b) Nonassociated mode of operation: The signal messages are
transferred between the two switching centres over two or more c.c.s.
links in tandem, and thus on a different routing from the relevant
speech-circuit group, the signal messages being processed and forwarded
through one or more intermediate signal-transfer points. It follows from
this definition that there may be a range of nonassociated modes of
operation which vary in the degree of rigidity imposed on the choice of
the path used by the signal messages. The extremes of this range can be
described as the fully dissociated mode and the quasiassociated mode.

------------------------[See fig11-2.pcx in tadxf005]------------------------
Fig. 11.2 Examples of associated and quasiassociated signalling
Associated signalling between A and B
b Quasiassociated signalling between A and B
c Quasiassociated signalling between A and B
-----------------------------------------------------------------------------

(c) Fully dissociated mode of operation: The signal messages are
transferred between the two switching centres via any available path
in the switching network according to the routing principles and
rules of that network. The great flexibility in the routing of signal
messages demands a more comprehensive message-addressing scheme
than is needed for associated signalling. This is the ultimate of the
principle of separate paths for signalling, and a completely separate
c.c.s. network has been suggested.

(d) Quasiassociated mode of operation: The signal messages are
transferred between the two switching centres over two or more c.c.s.
links in tandem, but only over predetermined paths and through
predetermined signal transfer points. The predetermined routing
permits the same method of addressing (circuit labelling) the signal
messages as in the associated mode of operation.

The constituent signalling routes of a quasiassociated signalling
relationship may be associated-signalling in their own right (Fig. 11.2b),
in which case the constituent routes carry the quasiassociated signalling
traffic in addition to the associated-signalling traffic. Alternatively,
the constituent signalling routes need not be associated-signalling
in their own right (Fig. 11.2c). When the constituent signalling routes
(AC and CB) in the quasiassociated signalling relationship are associated-
signalling in their own right (Fig. 11.2b), the circuit labels for the
speech-circuit group AB must be different from those of speech-
circuit groups AC and CB. Thus for the necessary signalling discrimi-
nation, the circuit labelling of signalling routes AC and CB must be
increased to accommodate for this. When the constituent signalling
routes AC and CB in the quasiassociated signalling relationship are not
associated-signalling in their own right (Fig. 11.2c), the circuit labelling
concerns the speech circuit group AB only in the typical case shown.

Quasiassociated signalling may be adopted when a speech-circuit
group is too small to justify economic application of associated-signalling
The mode may also be used for signalling security backup for associated-
signalling and for backup for another quasiassociated signalling relation-
ship, but the circuit labelling tends to be complex in the latter case.

With quasiassociated signalling, the number of signal transfer points
in the signalling path for a group of speech circuits between two switch-
ing centres should be as few as practicable to minimise the signalling
time of those circuits, and to minimise the total signal processing
load of the network. Normally one or two signal transfer points should
suffice in a quasiassociated signalling relationship.

Signal transfer point
---------------------

In a nonassociated mode of operation, a signal transfer point is a
signalling centre which forwards signal messages received on one c.c.s.
link for onward transmission over another c.c.s. Iink. It follows from
this that there is no need for a signal transfer point to have any con-
nection with a switching centre (Fig. 11.2c). Alternatively, it may
have (Fig. 11.2b).

The main functions of a signal transfer point are:

(i) to analyse the circuit label and the priority indication of every
received signal message to determine the routing and, hence, to offer
the message to the proper signalling link for onward transmission of
the message taking account of the appropriate priority

(ii) in doing so, it may be necessary to change (translate) the circuit
label of the message according to some preset rules

(iii) if for some reason, a signal transfer point is unable to transfer
signal messages, a procedure is desired to notify the preceding
signalling centre(s) so that the signal messages may be sent via
an alternative route, if available.

While c.c.s. has the potential for the various signalling modes
discussed, it is thought that a combination of associated and quasi-
associated signalling would meet the needs of most networks. The
predetermined signal routings possible with these modes would be
attractive in facilitating an orderly and efficient traffic-dimensioned
c.c.s. network.

=��=
11.4 Network centralised service signalling
=��=

Compared with wired-logic, processor (s.p.c.) control of switching
has the potential for a far more sophisticated and greater degree of
network-centralised services, and it is thought that this will be the
undoubted trend. The combination of s.p.c. and c.c.s. will enable
networks to be controlled and exploited to a greater extent than
previously attainable. Many administrations are at present actively
studying the extent and desired features of centralised services, typical
services being:

Speech-network management. Typically to initiate temporary
changes of traffic routing patterns for reasons of congestion,
catastrophic failure, etc.
Signalling network management
Network maintenance
Centralised call accounting etc.

Fig. 11.3 shows a possible basic arrangement in simplified form.

Various implementations are possible, e.g. the various services may
or may not be combined in the one centre, and the centres could be
hierarchical (local, regional, national). The processors (CC) at relevant
exchanges could be connected to relevant centralised service centres
by data-signalling links, the signalling not being concerned with call
handling. Should a hierarchical service-centre structure apply, further
data links would interconnect the hierarchical centres as appropriate.

------------------------[See fig11-3.pcx in tadxf005]------------------------
Fig. 11.3 General arrangements network centralised services
-----------------------------------------------------------------------------

The present interest is in the relevant signalling arrangements in
the c.c.s. environment and it is thought that the switched-network
c.c s. links should be capable of carrying centralised service in addition
to the normal call-handling signalling to allow for the following pos-
sibilities:

(a) switched-network c.c.s. links used as security backup against
failure of the centralised-service data links, e.g. on failure of the data
link from exchange 1, the centralised service signalling passed on the
c.c.s. link between exchanges 1 and 2 and then on the data link from
exchange 2 (Fig. 11.3)

(b) switched-network c.c.s. links utilised as data links to carry the
centralised-service signalling en route to the service centres

(c) when an exchange does not have direct data-link access to the service
centres, access is obtained by means of a switched-network c.c s. link
via an exchange having direct data-link access.

It is clearly desirable that the switched-network c.c s. links should
have adequate spare capacity to carry the centralised service in addition
to the call-handling signalling, the centralised-service signalling con-
forming to the switched-network c.c.s. signalling features (error control,
signal unit size, etc.) in this situation. It would not be essential for the
centralised service data links to be of same type as the switched network
call handling c.c.s. links, but in certain circumstances there could be merit
in uniformity if they were.

=��=
11.5 CCITT No. 6 signalling system
=��=

=��=
11.5.1 Basic concepts.
=��=

The CCITT 6 signalling system was the first c.c.s. system to be specified,
designed and tested. While specified primarily for international use, it is
equally suitable for national application and some administrations are
presently programming its use in their national networks. Initially, system 6
was specified for 2.4 kbit/s analogue application. Subsequently, digital
versions 4 kbit/s and 56 kbit/s were included in the specification but without
change to the initial basic signalling arrangements formulated for analogue.
Other signalling bit rates may be adopted as desired (e.g. 4.8 kbit/s
analogue).

It is convenient to present the pioneer system 6 solutions to the many
problems arising in c.c.s. as a general basis before discussing improved
solutions which may be adopted in some areas, improvements that arise from the
greater knowledge now available in the c.c.s. art and from the natural evo-
lution of the technique.

Each signalling channel is operated synchronously but the two channels are
not synchronised with each other, and a continuous stream of data flows in
both directions. The data stream is divided into signal units (SUs) of 28 bits
each, of which the last 8 are check bits, all SUs being of the same length for
ease of synchronisation in particular. The SUs in turn are grouped into blocks
of 12, the 12th and last SU of each block being an acknowledgment SU coded to
indicate the number of the block being transmitted and carrying the ack-
nowledgment i.e., the number of the block in the other direction being acknow-
ledged, and whether or not each of the 11 SUs of the block being acknowledged
was received without detected errors. Blocks are sequenced numbered but indi-
vidual message SUs in the block are not, being identified by their position in
the block. The first 11 SUs of a block consist of message or synchronisation
SUs; the latter, transmitted only in the absence of other signalling traffic,
facilitate achieving or maintaining SU and block sychronisation, and are coded
to indicate the number of the position they occupy within the block to facili-
tate locating the acknowledgment SU.

A signalling bit-rate of 2.4kbit/s adopted for the analogue version is the
maximum rate for transmission with acceptable error rate over a phase-
equalised speech-band channel. At this bit-rate, one c.c.s. Iink can carry all
the signals required by some 1500-2000 speech circuits in the telephony ser-
vice without excessive delay to individual signals. Compared with the
2.4kbit/s analogue version, the 4kbit/s and 56 kbit/s signalling bit-rate
digital versions allow, in theory, more speech circuits to be served per
c.c.s. link due to the reduced emission time of the signals. In practice, the
same size circuit label bit-field applies for both the analogue and digital
versions in the international specification. National network variants of the
specification may, of course, extend or reduce the size of the circuit label
bit field as required. Optional 4.8 kbit/s analogue is presently being con-
sidered.

The signals, which may be LSUs or MUMs, are formatted in the processor func-
tion and delivered to an output buffer, which delivers the highest-priority
signal awaiting transmission to a coder in serial form in the next available
time slot. Each SU is encoded by the coder by the addition of check bits in
accordance with the check-bit polynomial. The signal is then modulated and
transmitted over the link in serial form. At the receive end, serial data is
delivered to a decoder where each SU is checked for error on the basis of the
associated check bits, SUs received with detected errors being discarded.
Message SUs which are error-free are transferred to an input buffer after de-
letion of the check bits, the input buffer delivering the SUs to the processor
function for appropriate action.

A transmission path error rate of 1 in 10^6 to 1 in 10^7 was assumed and as
the requirement of system 6 was to achieve an undetected error rate of 1 in
10^10 for significant signals and 1 in 10^8 for others, error control was
adopted with error detection by redundant coding and error correction by
retransmission. Message SUs are retransmitted on error detected, synchronisa-
tion SUs are not. A data channel failure detector complements the decoder for
the longer error bursts. On excessive error rate, or on complete failure,
automatic procedures direct the signal traffic from a 'failed' link to an
alternative signalling facility.

=��=
11.5.2 Signal Codings
=��=

Basic philosophy

The binary codings of bit fields in c.c.s. allows potential for a consider-
able signal repertoire depending upon the bit field, and SU, size. Information
is transferred in system 6 by means of one (LSU) or more (MUM) SUs, a SU being
the smallest defined group of bits on the signal channel, and, in system 6,
contains 28 bits (20 information, 8 check). A LSU may transmit either a single
telephone signal, a system control signal (e.g. ACU) or a centralised service
signal. Basically, a MUM, which conveys a number of related signals (e.g.
address digit signals), may consist of any desired number of SUs, but in
system 6 consists of 2, 3, 4, 5 or 6 SUs, the constituent SUs of a MUM being
identified as ISU (the first) and SSU (the second and any following). In the
c.c.s. art (but not 6) the FSU is the final SU of a MUM, the FSU being a SSU
and is so called in system 6. The ISU, SSU (and FSU) facilitate MUM length
indication in the signalling system. If the MUM is speech-circuit related, the
circuit label is included in the ISU only.

Basically, a defined c.c.s. system can be regarded as being merely a means
of transferring bit-coded messages, individual administrations being free to
adopt their own codings as appropriate. It is logical, however, to adopt some
uniformity in the codings and constituent bit-field sizes, for ease of inter-
working between national, regional and international applications of system 6,
the approach that has been adopted. The CCITT specification details the signal
codings for the international application, with adequate spare capacity allo-
cated to cater for particular national network requirements not included in
the international specification. As there is a high degree of commonality in
international and national signalling requirements, considerable uniformity
will apply which is the objective, but it must be accepted that national
network variants may well arise.

(a) Telephone signals

LSU (lone signal unit): A number of bits within each SU must be allocated
to distinguish the specific message being transmitted and in the telephony
LSU nine bits perform this function, divided into two fields; heading field
bits 1-5 and signal information bits 6-9 (Fig. 11.4a). The LSU Fig. 11.4a is
also the basic format of the ISU. The two bit-fields permit ease of admini-
stration of the signals, a heading being allocated to a class of signals, e.g.
address digit signals, individual signals within each class being distin-
guished by the relevant coding of the signal information field. These two bit-
fields more than cover existing requirements (Table 11.1) and leave capacity
for new signals and classes of signals not yet defined.

Where appropriate, the coded bits 1-5 also give indication of the signalling
direction (forward, backward) of the signal identified by the signal infor-
mation bits 6-9 (e.g. heading code 11011 indicates that all the signals under
this heading are backward signals--Table 11.1). While the heading generally
consists of bits 1-5, there are two exceptions:

------------------------[See fig11-4.pcx in tadxf005]------------------------
Fig. 11.4 CCITT 6 system: typical message signal formats
a Basic format of LSU and ISU of MUM
b Format of SSU of MUM
c Example of 3-unit MUM
-----------------------------------------------------------------------------

(i) all SSUs of a MUM are identified by the same 2-bit heading code 00 (bits
1-2), Fig. 11.4b.

(ii) the acknowledgment signal unit ACU is identified by a 3-bit heading code
011 (bits 1-3), Fig. 11.5a.

The heading codes (bits 1-5) are allocated:

00 SSU

01000 -|
01001 | Spare
01010 |
01011 -|

011 ACU
10000 ISU of IAM or of MUM

10001 -|
10010 |
10011 | SAM (one-unit message or multiunit message)
10100 |
10101 |
10110 |
10111 -|

11000 -|
11001 | International telephone signals
11010 |
11011 -|

11100 Spare
11101 Signalling system control signals (Except ACU)
and management signals

11110 -| Spare
11111 -|

The signal information codes (bits 6-9) are allocated as shown in Table 11.1.

MUM(multiunit message): A MUM, which may be address messages, centralised
service messages, or messages conveying other types of information, consists
of an ISU and a number of SSUs. The basic format of the ISU is the same as
that of the LSU (Fig. 11.4a) the heading code identifying the ISU. SSUs are
identified by the heading code bits 1-2 and have a 2-bit field (bits 3-4)
coded to indicate the number of SSUs in the MUM (Fig. 11.4b), thus serving as
MUM length indication. In system 6, an IAM MUM may be 3-6 SUs and all other
MUMs 2-5 SUs, and each SSU of a MUM carries the same relevant length indicator
coded as follows:

Length indication in SSU
(bits 3-4)
Number of SSUs - - - - - - - - - - - - - - - - - - - - - -
in MUM IAM MUM Other MUMs
--------------------------------------------------------------------------
1 - 00
2 01 01
3 10 10
4 11 11
5 00 -

This leaves 16 bits (bits 5-20) in each SSU to carry information (Fig. 11.4b).

The term MUM should not be confused with the term block in the block trans-
mission of system 6. The block is simply a group of 12 consecutive SUs, which
may be LSUs, a mixture of LSUs and SYUs, a mixture of MUM (or part MUM) and
LSUs, two MUMs, etc.

IAM (initial address message): The IAM is the first message of a call
connection set-up. Unlike conventional speech path signalling systems, a dis-
crete seizure signal is not required in c.c.s., the IAM performing this
function. As the IAM conveys information (e.g. routing) in addition to address
digits, more than one SU is always necessary and the IAM on any call is always
a MUM, consisting of an ISU (Fig. 11.4a) and SSUs (Fig. 11.4b). As in SSUs of
MUMs in general, bits 1-4 of SSUs of IAMs give SSU identification and MUM
length indication. The information bit-field (bits 5-20) of SSUs 2-5 of IAMs
in the international system 6 is subdivided into four 4-bit parts so that four
address-digit signals can be conveyed in each SSU. The information bit-field
(bits 5-20) of SSU 1 of IAMs is used for routing information required in the
setting up of connections in the international system 6. This field could be
used for other purposes in regional and/or national application of system 6 as
required. Also as in SSUs of MUMs in general, SSUs of IAMs do not require the
5-bit heading or the circuit label bit-fields as these items of information
are already contained in the ISU.

In the detail, the codes used in the IAM are (Table 11.1):

(i) ISU: The 5-bitheading code 10000 (bits 1-5) in combination with the
signal information code 0000 (bits 6-9) identify that the ISU is the
ISU of an IAM.

(ii) SSU 1: The heading code 00 (bits 1-2) identifies the SSU and bits 3-4
are coded to give the appropriate length indication. In the interna-
tional application, the special routing information required in the
setting up of connections is as follows:

Bit 5 Country code included or not in the IAM. This feature can be
utilised to inform an incoming international exchange to function as
terminal or as transit.

Bit 6 Nature of circuit indicator. In particular, this bit is used in
satellite routing restriction and records if a satellite link has been
used.

Bit 7 Echo suppressor control. Whether or not an echo suppressor is
required.

Bit 8 Spare (reserved for international)

Bits 9-12 Spare (reserved for regional and/or national)

Bits 13-16 Calling party's category, such as operator, ordinary sub-
scriber, data service, etc.

Bits 17-20 Spare (reserved for regional and/or national)

(iii) SSUs 2-5 - telephone call: As in SSU 1, the heading code 00 (bits 1-2)
identifies the SSU and bits 3-4 give the appropriate length indication.
The four 4-bit parts of the signal information field bits 5-20 contain
address-digit signals in sequence, bits 5-8, 9-12, 13-16, and 17-20,
respectively, being coded as follows (Table 11.1):

0000 filler (no information)
0001 digit I
0010 digit 2
0011 digit 3
0100 digit 4
0101 digit 5
0110 digit 6
0111 digit 7
1000 digit 8
1001 digit 9
1010 digit 0
1011 code 11 operator
1100 code 12 operator
1101 spare
1110 spare
1111 ST (end of pulsing)

The filler code 0000 is used where needed to complete the signalling infor-
mation of the last SSU of the IAM if this SSU is partially used for carrying
address digits.

------------------------[See tabl11-1.pcx in tadxf005]-----------------------
Table 11.1 CCITT System 6 allocation of heading and signal information codes
-----------------------------------------------------------------------------

Signal abbreviations, System 6:

ACU Acknowledgment signal unit-error control
ADC Address complete signal, charge
ADI Address incomplete signal
ADN Address complete signal, no charge
ADX Address complete signal, coin box
AFC Address complete signal, subscriber free, charge
AFN Address complete signal, subscriber free, no charge
AFX Address complete signal, subscriber free, coin box
ANC Answer signal, charge
ANN Answer signal, no charge
BLA Blocking Acknowledgment Signal
BLO Blocking Signal
CB1-3 Clear Back signal No. 1 - No. 3
CLF Call Failure Signal
COF Confusion signal
COT Continuity signal
CSSN Circuit state sequence number
FOT Forward Transfer Signal
IAM Initial Address Message
ISU Initial Signal Unit
LOS Line Out-of-service Signal
LSU Lone Signal Unit
MBS Multiblock Synchronisation Signal
MRF Message-refusal Signal
MUM Multiunit Message
NMM Network Management and maintenance signal
NNC National Network Congestion Signal
RA1-3 Reanswer Signal No. 1 - No. 3
RLG Release guard signal
SAM1-7 Subsequent Adress Message No. 1 - No. 7
SCU System Control Signal Unit
SEC Switching Equipment Congestion Signal
SNM Signalling Network Management Signal
SSB Subscriber Busy Signal (Electrical)
SST Subscriber Transferred Signal
SSU Subsequent Signal Unit
SU Signal Unit
SYU Synchronisation Signal Unit
UBA Unblocking Acknowledgment Signal
UBL Unblocking Signal
VNN Vacant National Number Signal

Fig. 11.4c shows a typical three-unit IAM.

In bothway operation of c.c.s., a double seizure is detected by an exchange
when it receives an IAM for a speech circuit for which it has sent an IAM.
At the analogue signalling bit-rate 2.4 kbit/s, each SU has an emission time
of approximately 11.7ms and with a three-unit IAM, four address digits can be
transmitted in some 35ms. Typically, a ten-digit national number, if transmit-
ted en bloc, would require a five-unit IAM and transmitted in some 58.5 ms at
a rate of 170 digits per second. This is much faster than any speech-path
telephony-signalling system currently in use. Digital c.c.s. transfers address
information at still faster speeds.

SAM (subsequent address message): A SAM, which may be a LSU or a MUM, is used
to transmit additional address information not available for transmission when
the IAM is formed, or (but not in international system 6) when the trans-
mission of address information after the IAM is controlled by backward request
signals. The format of an LSU SAM and of the ISU of a MUM SAM, is the same as
that of the normal LSU (Fig. 11.4a). The LSU SAM carries one address digit in
the signal-information field-bits 6-9. The ISU of a SAM does not carry an
address digit, bits 6-9 being coded 0000 filler (no information). The format
of an SSU of a MUM SAM is the same as that of the SSU of any MUM (Fig. 11.4b)
except that, unlike SSU 1 of an IAM (which does not carry address information
in international system 6), SSU 1 of a SAM may carry address information in
the information field bits 5-20. Thus all SSUs of SAMs may carry four address
digits each, bits 5-20 being subdivided into four 4-bit parts. The 4-bit
address digit fields are coded as for SSUs 2-5 of an IAM to transmit address
information. The Code 11 and Code 12 operator codes are not used in SAMs, as
this information will have been given by the IAM. The filler code 0000 is
used, where needed, to complete the signal information field of the last SSU
of a SAM.
Heading codes (bits 1-5) in the range 10001-10111 are used in LSU, or the
ISU of, SAMs, depending upon the sequence number of the SAM concerned
(Table 11.1), i.e.
10001 first SAM
10010 second SAM
10011 third SAM, etc.

This sequence numbering of SAMs is necessary in system 6 as messages can get
out of sequence at the processor function in the working of the c.c.s. system,
and the sequence numbering facilitates the restoration of SAMs to correct the
sequence at the processor. It is preferred to limit the number of SAMs on a
call connection set-up, but if more than seven are sent, the sequence is re-
cycled so that the eighth SAM uses code 10001

(b) Management signals

In international system 6, management signalling may include network manage-
ment, network maintenance and signalling network management, the signals being
transferred as LSUs or MUMs. The management signalling detail is not yet
resolved for international system 6 and specific formats are not yet
specified. The basic format of the normal LSU (Fig. 11.4a) will apply for
management LSUs and ISUs of management MUMs, and the heading code 11101 (bits
1-5) is assigned. Appropriate coding of the signal information field (bits
6-9) distinguish the various categories of management signalling, i.e. network
management maintenance, etc. (Table 11.1).
Most management signals will require a band number (bits 10-16 Fig. 11.4a)
to route the signal information on the switched network via appropriate signal
transfer points. As management signals will not require a circuit number in
the band, bits 17-20 (Fig. 11.4a) are used for management information.

(c) Signalling system control signals

Control signals in system 6, always LSUs, are necessary for the proper func-
tioning of c.c.s. systems. They are not related to telephone signalling
information and are thus not speech-circuit related. In system 6, the control
signalling comprises:

(i) acknowledgment ACU signals in the error control
(ii) synchronisation (idle) SYU signal units
(iii) multiblock monitoring and acknowledgment signals MBS for multiblock
synchronisation
(iv) system control signal units SCU, a group of signals concerned with
signalling link failure and the various consequential procedures.

The heading field (bits 1-5 Fig. 11.4a) is coded 11101 for signalling system
control signals, except the ACU which is coded 011 in heading bits 1-3. Signal
information bits 6-9 are coded to distinguish the category:

1100 signalling link failure procedure signals (SCU)
1101 SYU
1011 MBS

Bits 10-20 give further signalling information in each category, bits that
would otherwise be used for the circuit label in speech-circuit-related LSUs.

SYU (synchronisation signal units): This consists of a 16-bit fixed pattern
for bit (analogue c.c.s.) and SU synchronisation, and a 4-bit field for block
synchronisation, the latter identifying the location of the SYU within the
block (i.e. 1st, 2nd . . . or 11th unit). The SYU (Fig. 11.5b) completes with
the eight check bits. The particular 16-bit fixed pattern of the SYU presents
an easily recognisable pattern with a variation at the end to aid in its
detection. It also provides six-bit transitions to aid the attainment of bit-
synchronisation by the modems in analogue c.c.s. (Section 11.5.5(a)), bit-
synchronisation being maintained by the transition between dibits.

MBS (Multiblock synchronisation signal unit): The original system 6 specifi-
cation, limited to 2.4kbit/s analogue c.c.s., allowed for a maximum of eight
blocks in the error-control loop. This was adequate for a maximum delay of 740
ms (single-hop satellite) and accounted for the 3-bit (8 possibilities) block-
completed and block-acknowledged fields in the ACU. The revision of the
specification to include 4kbit/s and 56 kbit/s digital versions was coincident
with a requirement to meet a maximum error-control loop delay of 1200ms
(double hop satellite), requiring more than eight blocks in the errorcontrol
loop at all the signalling bit rates specified i.e. 2.4, 4 and 56kbit/s. The
revised specification treats eight consecutive blocks as a multiblock and
allows for up to 32 multiblocks. Thus the maximum number of blocks in the
error-control loop is 256 (3072 SUs), which is adequate for a 1200ms loop-
delay signalling link at 56 kbit/s in the telephony service. The full 256-
block capability need not be handled in all applications. Block memory may be
limited to that required for the expected range of loop delays and signalling
bit-rates at which the system is applied in particular situations.
The concept of the multiblock gives rise to the requirement for two signals
of a control nature (Fig. 11.5c) i.e.

(a) multiblock monitoring signal, required on links where the number of blocks
in the error-control loop exceeds eight, and sent to check multiblock synchronism
(b) multiblock acknowledgment signal, sent in response to the multiblock moni-
toring signal and used by the terminal receiving it to verify multiblock
synchronism.

Of the MBS as shown in Fig. 11.5c:

Bits 10-12 are coded 000 for the monitoring signal and 100 for the acknow-
ledgment.
Bits 13-17 are coded to indicate the sequence number of the multiblock in
which the multiblock monitoring signal is sent.
Bits 10-20 are coded to indicate the sequence number of the block in which the
multiblock monitoring signal is sent (or placed into the output
buffer).

------------------------[See fig11-5.pcx in tadxf005]------------------------
Fig. 11.5 CCITT 6 system: typical control signal formats
a Format of ACU
b Format of SYU
c Format of MBS
-----------------------------------------------------------------------------

The above covers the main signals of CCITT system 6, the international
specification details the format and codings of other signals included in the
specification. Fig. 11.6 shows a typical signalling sequence with system 6.

=��=
11.5.3 Errorcontrol
=��=

In system 6, provision is made in every SU to detect errors due to noise, line
faults or transmission faults causing a mutilation of bits.

------------------------[See fig11-6.pcx in tadxf005]------------------------
Fig. 11.6; Typical signal sequence CCITT 6 system
-----------------------------------------------------------------------------

Error detection is by redundant coding, a number of check bits being generated
in the form of a cyclic code, each SU having an 8-bit check field. The gene-
rator polynomial selected for system 6 to generate the check-bit pattern
provides maximum protection for short noise bursts affecting up to four
consecutive bits. To detect for longer noise bursts, a data channel-failure
detector supplements the error detection by the check bits. This detects, and
takes effect after a delay of about 5 ms, the following:

(a) failure of the data carrier or an excessive noise level relative to the
carrier level in analogue c.c.s.

(b) loss of frame alignment in digital c.c.s.

Not all errors will be detected by the error control, undetected errors will
arise.

Error detection: Error detection is performed by check bits (bits 21-28) in
each SU, with coders and decoders equipped at the transmit and receive
terminals, respectively, for the purpose. The principle of operation is that
the digital sequence constituting the message is treated as a polynomial and
is divided at the transmitter and at the receiver by a preagreed number. The
remainder is transmitted from the transmitter to the receiver as check bits.
The process is implemented using a modulo-2-division by a shift register with
a number of stages equal to the number of check bits. For a given number of
check bits C, there are 2^(C-1) different generating polynomials which can be
chosen for the divisor, these polynomials having different characteristics
which are indicated by their modulo-2 factors.

Error burst patterns can also be considered as polynomials. Any error burst
whose polynomial is the same as the generating polynomial is undetectable as
the division process will change the quotient by one bit, but not alter the
remainder. If the length of the error polynomial is less than the generating
polynomial, the remainder will be changed and the error detected. When the
error-burst polynomial has more terms than the generating polynomial, the
errors will only be undetected if the generating polynomial forms a factor in
the error-burst polynomial. In system 6, the coder generates eight check bits
based on the polynomial:

P(X) = (X+1)(X^7 + X^6 + X^5 + X^4 + X^3 + X^2 + 1) = X^8 + X^2 + X + 1

A characteristic of cyclic codes is their inferior protection in the case of a
slip in frame synchronisation in p.c.m. Security is maintained by arranging
for the check bits C to be inverted before transmission and reinverted before
checking at the receiver. If there has been a slip, the reinversion is applied
on an incorrect sequence of bits. When the decoder at the receive terminal has
received all 28 bits of the SU after the check bits have been reinverted, it
will indicate whether or not the signal has been checked as correct. This
information is stored for inclusion in the acknowledgment indicator field of
an ACU to be emitted in the return direction.

The cyclic code for error detection is supplemented by a datachannel failure
detector (Section 11.8). Indication of data-channel failure owing to unsatis-
factory transmission conditions causes rejection of SUs in the process of
reception, and during the alarm condition all SUs are rejected (i.e. acknow-
ledged as corrupted) regardless of check.

The system 6 specification requires that an undetected error rate of not
more than one erroneous SU out of each 10^8 transmitted should result in a
false operation and not more than 1 in 10^10 should cause malfunction such as
false metering or false release. The specification is worded in this way as it
was anticipated that most signals with undetected errors would result from the
disturbance of idle units which predominate even during the busy hour. It was
also evident that many false signals would cause no trouble as they would be
addressed to nonassigned circuit labels, or would occur in conditions in which
they were meaningless.

Error correction: Error correction is by retransmission in system 6, which
necessitates a retransmission store at the transmit end, correctly received
SUs being cleared from the store. On error-detected, the SU must be discarded
at the receive end and the information retransmitted. This requires a method
of acknowledging signals and a means of identifying SUs. The latter is
achieved in system 6 by transmitting SUs in blocks and allocating identifying
numbers to the blocks, individual SUs can then be identified by position
within a block. Blocks are of a 12-unit fixed length, comprising 11 SUs (which
may be message or synchronising) and an acknowledgment unit (ACU) in the 12th
position (Fig. 11.7). Positive/negative block acknowledgment is employed. A
positive acknowledgment indicates that the relevant SU has been received
error-free, a negative acknowledgment indicates error detected and is thus a
request for a retransmission.

Each ACU (Fig. 11.5a) contains:

(a) a 3-bit fixed pattern (011) to identify the unit as an ACU
(b) a 3-bit code to identify the block of 12 units completed by this
ACU (providing for a count of 8)
(c) a 3-bit code to identify the block being acknowledged by this ACU
(providing for a count of 8)
(d) the signal acknowledgment mechanism: eleven bits are allocated to identify
the SUs of blocks received in the other direction on a one-to-one basis,
zero (0) or (1) being written in the ACU depending on whether the corres-
ponding SU was received error-free or in error
(e) 8 check bits.

As a MUM may spread over two blocks, break-in to a MUM may take place due to
an ACU, whose position in the block is fixed.

------------------------[See fig11-7.pcx in tadxf005]------------------------
Fig. 11.7 CCITT 6 system: block signalling
-----------------------------------------------------------------------------

With the block structure used, the block sequence numbers must repeat every
1120 ms at 2.4 kbit/s. In the original specification this was sufficiently
long at 2.4 kbit/s to ensure that under normal conditions, there could be no
ambiguity as to what is being acknowledged even when a single-hop satellite is
included in the signalling link in the telephony service (but see Section
11.5.3 'Influence of digital versions system 6 on the error control').

Retransmission procedures: The following procedures apply in error conditions:

(i) When a SU is corrupted, the appropriate acknowledgment indicator in an
ACU is set negative. This procedure is also used if for any reason the
receive terminal cannot handle a SU.
(ii) When the corrupted SU is a LSU, that LSU is retransmitted (in another
block).
(iii) When a SU (or SUs) of a MUM is corrupted, the whole MUM is retrans-
mitted. This avoids associating SSUs with a corrupted ISU.
(iv) Corrupted SYUs (idle units) are not retransmitted.
(v) It will be noted that a received ACU is not itself acknowledged.

If an ACU, always known as such by its 12th position in the block, is
corrupted, it is assumed that all its acknowledgment indicators are negative
and all message SUs in the block waiting to be acknowledged are retransmitted.
In the limit, a block of 11 message SUs could be retransmitted. This means
that on occasions the same SU may be received error-free more than once.

Processor reasonableness checks: The following irregularities on the same
call may arise with system 6:

(a) Unreasonable messages:

(i) in incorrect sequence, as may arise on retransmission due to error
detected
(ii) an incorrect signal direction.

(b) Duplicated (superfluous) messages, as may arise owing to corruption of an
ACU, or to drift compensation.

To resolve such irregularities, special processor procedures are defined and
are included in reasonableness check tables which cover all possible stages in
the signalling sequences. The reasonableness check tables for system 6 are
based on the logical signal processes and sequences which should apply on a
call. Typically, if two IAMs are received owing to corruption of an ACU or to
drift compensation, the two are compared. If they are identical, one is
discarded, and if they are different, a confusion signal can be sent.

The action taken by the processor on unreasonableness detected depends on
the nature of the irregularity, the various actions being:

(i) Rejecting: Messages or SUs recognised to be unreasonable or superfluous are discarded.

(ii) Waiting: Unreasonable messages or SUs which may become meaningful at a
later stage of the signal sequence are provisionally held. The waiting
time should be longer than the retransmission delay of the delayed
message. The provisionally-held SUs are processed if the arrival of
retransmitted signals within the waiting period makes them meaningful.
Otherwise, if they are still meaningless at the end of the waiting
period, they are rejected with the exception of the cases where the
held signal is a clear forward. In this case, the release-guard signal
must be sent.

(iii) Clearing If owing to an abnormal signal sequence an ambiguous situation
arises which would result in a circuit being held unduly for a prolonged
period, the circuit is cleared by the release sequence.

(iv) Confusion signal: If none of the above actions is suitable for resolving
the situation created by the irregularity, the confusion signal is
returned. On receipt of this, the distant exchange sends the clear-
forward signal to release the connection and on certain irregularities
an automatic repeat attempt is made to set up the call. The confusion
signal will not be sent subsequent to sending the address complete
signal or other signal causing the clearing from the store of address
and routing information at the preceding exchange.

The irregularities arise in the message-transfer process of the signalling
system. Some reasonableness checking is always necessary at the processor
regardless of the type of error control involved in the message-transfer
control. It is a question of degree. Error-control methods which do not allow
incorrect sequence and duplicated messages to be passed from the signalling
terminal to the processor would permit relatively simple reasonableness-
checking covering undetected errors in the main. The system 6 error-control
method allows out-of-sequence and duplicated messages to be passed to the
processor, resulting in significant irregularity in addition to that due to
other causes, such as undetected errors. As a result, the reasonableness
checks associated with system 6 are complex and load the processor.

Drift compensation: As the two signalling channels of a signalling link are
not synchronised with each other, the usual condition for telephony c.c.s.,
the two signalling terminals may get out of step. The following applies in
system 6:

(a) The faster terminal may be ready to send an ACU before it has a block to
acknowledge. In this case the ACU of the previous block is repeated.

(b) The slower terminal may find it has two blocks to acknowledge when it is
ready to send an ACU. In this case it will omit to acknowledge one block
altogether, the 12th unit being sent to complete the block but will not
contain acknowledgment information. The far end then retransmits all
message SUs of the unacknowledged block.

Influence of digital versions system 6 on the error control

The revision of the orignal analogue 2.4 kbit/s signalling specification to
include 4 kbit/s and 56 kbit/s digital versions was coincident with a require-
ment to meet a maximum error-control loop delay of some 1200 ms (double-hop
satellite). This requires more than eight blocks in the error-control loop at
all the signalling bit-rates specified (2.4, 4 and 56 kbit/s). With the
retention of the 3-bit block-sequence number field in the ACU, an implicit
approach was adopted, the block-completed and block-acknowledged indications
being based on a so called 'dynamic indexing' plan.

The plan takes advantage of the fact that after a signalling link is in
synchronism, much of the information transmitted in the block-sequence number
fields of the ACU is redundant. Once block counts have been established in
the initial synchronisation, and as long as synchronism is maintained, local
block counts kept at each end are entirely adequate. Block acknowledgment
only, without block-sequence numbering, would suffice. As, however, drift may
arise due to the two signalling channels not being synchronised with each
other, some redundancy in block-number information is necessary in each block
to permit compensation for drift and for detection of loss of block synchro-
nisation. The 3-bit block-sequence number fields in the ACU performs this
function.

Each signalling terminal has two block counters, each of up to 8-bit
capacity (256 possibilities):

(i) Block completed counter: This indicates the sequence number of the last
block transmitted by the terminal. The last 3 bits of this number are
also sent in the ACU of the block in the 3-bit block completed field of
the ACU.

(ii) Block acknowledgment counter: This is updated by the 3-bit block-acknow-
ledgment sequence number in the incoming ACU and thus indicates the
sequence number of the block being acknowledged by the last received ACU.
To keep it updated even when the ACUs are corrupted, the block acknow-
ledgement counter is also incremented whenever the 12th unit of a block
is received corrupted.

Thus the signalling terminal and/or processor keeps track of the block
numbers and identifies blocks in the error-control loop. The subsequent return
in the block-acknowledged field of a received ACU of the last 3 bits of the
maximum 8-bit block-completed number of the block-completed counter in the
ACU in the opposite direction gives the block-acknowledgment indication to a
transmit terminal. Should the 3-bit block acknowledged number in the received
ACU be different from the last 3 bits of the block transmitted, drift or loss
of block sychronism will have occurred.

=��=
11.5.4 Analysis of the system 6 error-control method
=��=

The following main areas are analysed, many points of less significance being
omitted:

(a) Correct sequence of messages: Messages can get out of correct sequence on
retransmission and would be passed in incorrect sequence from the signal-
ling terminal to the processor.

(b) Duplicated messages: Messages may be received correctly more than once for
a number of reasons e.g.

(i) corruption of ACU, resulting in all message SUs in the block being
retransmitted, could result in the retransmission of messages al-
ready received correctly
(ii) drift compensation resulting in a block not being acknowledged
(iii) loss of an ACU during failure conditions.
Such duplicated messages would be passed from the signalling terminal to
the processor.

(c) Reasonableness checks: The incorrect sequence and duplicated messages
passed to the processor require complex reasonableness checks at the
processor to deal with the situation. This complicates the processor and
increases the processor load.

(d) Unnecessary retransmissions: These can occur owing to

(i) complete MUMs being retransmitted on corruption of constituent
message SU(s) of MUMs.
(ii) Corruption of message SU(s) in unrequested retransmissions.
SYUs are known to be such at the transmit end by virtue of their
positions in the block and thus by their positions in the block store.
Corrupted SYUs, while returning negative acknowledgments in the relevant
indicator bits in the ACU, do not cause retransmission of SYUs, or any
other signals. Thus unnecessary retransmissions do not occur on corrupted
SYUs.

(e) Unrequested retransmissions: These are said to arise when acknowledgments
are corrupted. All message SUs in a block are retransmitted on corrupted
ACU. Approximately 50% of all retransmissions will be unrequested due to
this cause.

(f) Drift compensation: While drift is always liable to occur when the two
signalling channels of a link are not synchronised with each other,
complex compensation procedures are necessary when the ACU is in a fixed
position in a block.

(g) Unique ACU signal unit. The use of a complete signal unit (ACU) for ack-
nowledgment reduces the traffic handling capacity of a link by some 8.5%.

(h) Fixed position of ACU in block: While the unique ACU has significant dis-
advantage in regard to unrequested retransmissions on ACU corruption, it
has advantage when it is in a fixed position in a block. The SU will
always be known as being an ACU and thus corruption and undetected errors
can never be interpreted as being a nommal SU. On the other hand, the
fixed position of the ACU introduces complication when it splits up the
SUs of a MUM. If a retransmission is requested for any of the message SUs
comprising a MUM, then the whole MUM must be retransmitted. Thus even
though the correct receipt of the first part of a MUM in a particular
block may be acknowledged by the appropriate ACU, the possibility of a
retransmission cannot be rulcd out until the receipt of the subsequent
ACU in the next block which acknowledges the SUs forming the remainder
of the MUM.

(i) Delays: These are relatively long and are

(i) up to almost two blocks delay when requesting retransmission
(ii) up to one or two blocks delay in retransmitting.

It is concluded that the system 6 error-control method is complex, and that
far too much requirement is placed on the processor to enable the error
control to function. It will be understood of course that the system 6 error
control was formulated on the basis that the processor would be involved.

=��=
11.5.5 Synchronisation
=��=

The transmit and receive terminals on a c.c.s. signalling channel must be in
synchronism to enable transmitted information to be correctly received. It is
not essential that the two signalling channels of a link be synchronised to
each other and are usually not in telephony c.c.s. Four levels of synchronism
are necessary in system 6: bit, unit, block and multiblock. The number of
blocks (and multiblocks) in the error control loop at any one time will depend
upon the signalling bit rate, the traffic load and the error control loop
delay of the signalling link.

(a) Initial synchronisation

In analogue 6, bit synchronisation is provided by the data modem (modulator
and demodulator) which uses a 4-phase, synchronous, differentially-coherent
modulation method. The four phases each represent two bits of information
(00, 01, 10, 11) called dibits. The modem then presents a continuous serial
bit stream to the signalling equipment which then must identify units, blocks
and multiblocks. In digital 6, modems not being required, bit synchronism is
assured by the synchronised digital transmission system.

The block completed and block acknowledgment counters are set to zero during
the initial synchronisation procedure and are checked periodically using the
multiblock monitoring procedure.

On start-up of a link, each signalling terminal transmits blocks consisting
of 11 SYUs and an ACU. The SYU (Fig. 11.5b) includes a 16-bit fixed pattern
(for bit - analogue - and unit synchronisation) and a 4-bit field (for block
synchronisation). The coding of the 4-bit field identifies the location of the
SYU within the block (i.e. 1st, 2nd....or 11th unit). The particular 16-bit
pattern of the SYU presents an easily recognisable pattern with a variation
at the end to aid in its detection, and because the pattern provides six dibit
transitions, to aid the attainment of bit synchronisation by the modems in
analogue c.c.s. Bit synchronisation is maintained by the transition between
dibits.

The ACUs are transmitted initially with the 11 acknowledgment indicators set
to 1, and the block-completed and block-acknowledged 3-bit sequence number
fields set to 000.

After bit-synchronisation has been established (in the demodulator of the
modem for analogue c.c.s. and by the transmission system in digital c.c.s.)
the incoming bit stream is monitored to find an SYU pattern. When found, and
thus an SYU verified, its position within the block is known and the ACU posi-
tion located. In due course, the ACU on the incoming channel should be cor-
rectly received with its block number. At this time, the acknowledgment indi-
cators in the next ACU sent back are set to reflect any detected errors in the
SYUs of the associated receive block. Both block-sequence number fields in the
ACU sent back remain at 000.

The reception of at least two ACUs at the transmit terminal which check cor-
rectly and acknowledge one (or more) SYU as having been received correct at
the other end indicates that both terminals are in bit, unit and block syn-
chronism.

Block-sequence numbering is initiated by the block-completed counter, and
the block-completed sequence number in the next outgoing ACU from the trans-
mit terminal being set to 1. Thereafter the blockcompleted counter and the
block-completed sequence number are incremented by 1 each time an ACU is
transmitted.

When the terminal receives an ACU having a block-acknowledged bit-field
other than 000, the block-acknowledged counter at that terminal is set to this
number received. Thereafter the counter is updated by the appropriate block-
acknowledged number each time an ACU is received.

When the block-acknowledged counter is advanced for the first time, the
number of blocks in the error-control loop may be determined by subtracting
the contents of the block-acknowledged counter from the contents of the block-
completed counter. This gives the maximum number of blocks which may apply in
the error-control loop of the signalling link concerned at the signalling bit-
rate used. If the initial synchronisation procedure has indicated more than
eight blocks in the error-control loop, the multiblock monitoring procedure is
used once every cycle of the block-completed counter. In this case, the multi-
block procedure is used for block synchronisation. On receipt of a multiblock
acknowledgment signal, the multiblock and block numbers are compared With the
contents of the block-acknowledgment counter. Multiblock synchronism is
assumed to exist if the received number is within -4 to + 3 of the contents of
the block-acknowledgment counter.

(b) Resynchronisation

Unit resynchronisation: Loss of unit synchronism results in continuous
failure of SUs to check correctly, and when this occurs a signalling terminal
takes unilateral action to resynchronise to the incoming bit stream. In any
ACUs transmitted during this procedure, all the 11 acknowledgment-indicator
bits are set to 1 and the block-acknowledged and block-completed numbers in-
cremented as in normal operation. When synchronisation is re-established on
the incoming channel, which is detected by SUs being checked correctly, the
acknowledgment indicators are set according to the incoming SUs, i.e. normal
operation is resumed. The SU error rate monitor continues to count SUs in
error during this procedure, and changeover results if synchronisation is not
re-established before the changeover criteria becomes operative.

Block resynchronisation: Loss of block synchronism is recognised when

(i) a valid SU, which is not an ACU, is received in the 12th position in a
block
(ii) an ACU is received in other than the 12th position in a block
(iii) the block-completed number is not the one expected.

On recognition of any one of the above, the terminal will stop sending
message signals and send only SYUs and repeated ACUs, the latter meaning that
the acknowledgment indicators and the block-acknowledged number from the
previous blocks are repeated.

When the terminal has identified the unit position in a block, either by
recognising the SYU number or by identifying an ACU in the 12th position, and
has also received an ACU which checks correctly, resynchronisation has been
achieved. After successful block resynchronisation, the block being trans-
mitted is completed with SYUs and an ACU. At least one complete block of 11
SYUs is sent before normal operation is resumed.

The first ACU sent after resynchronisation has been achieved has:

(i) the 11 acknowledgment indicators all set to 1
(ii) the block completed number set to the next in sequence
(iii) the block acknowledged number corresponding to the latest ACU received

After the completion of block resynchronisation, multiblock synchronism
should be checked if the multiblock condition is applicable, i.e. more than
eight blocks in the error-control loop.

Multiblock resynchronisation: If the multiblock numbers in a multiblock ack-
nowledgment SU are not within -4 to +3 of the contents of the block-acknow-
ledged counter, a new multiblock monitoring signal is sent. If the result of
the second measurement is not within the above limit, multiblock synchronism
has been lost, and can be regained by updating the contents of the block-
acknowledged counter to the obtained result. When the second multiblock
monitoring signal is sent, the terminal will send only SYUs and ACUs for three
blocks. Normal operation is then resumed and all messages transmitted in the
interval between the two multiblock monitoring signals are retransmitted. If
multiblock sychronism cannot be regained, changeover is initiated.

=��=
11.5.6 System 6 analogue and digital versions
=��=

Signalling in the digital environment, either point-to-point p.c.m. or inte-
grated digital-switching and transmission, can be achieved by the built-in,
channel-associated, p.c.m. signalling technique (Sections 6.2 and 6.3). Such
signalling is underutilised in the telephony service. Improved utilisation can
be achieved by treating the signalling facility on speech-circuit group rather
than on a per-p.c.m. system basis and c.c.s. is preferred when s.p.c. applies.

The c.c.s. technique has potential for rationalised signalling for analogue
and digital application and this concept is adopted. The basic features of the
c.c.s. system apply on a common basis, the analogue and digital differences
being limited to the signalling bit-rates. Ideally, the common signalling
features should be optimum for both analogue and digital, but as this is
difficult to achieve in all the feature areas, some penalty may well arise in
either application.

The pioneering CCITT system 6 was produced primarily for analogue applica-
tion and its digital versions are far from optimum. The evolution of networks
to integrated digital has shown the need for a new rationalised c.c.s. system
optimised for digital rather than analogue, and this new system is currently
being studied by the CCITT (Section 11.11).

Derivation of signalling data-bit streams

2.4 kbit/s (analogue version) and 4 kbit/s and 56 kbit/s (digital versions)
signalling bit-rates are presently specified for system 6. Optional 4.8 kbit/s
analogue is presently being considered, The data-bit streams are derived from
the modem (analogue) and from the multiplex (CCITT specified p.c.m. systems)
for digital, as follows:

(a) Analogue 2.4 kbit/s. A 2.4 kbit/s modem (modulator/demodulator) is used.
The modulation technique uses phase-shift-keying to transmit serial binary
data over the analogue signalling channel. The binary-data signal is
encoded by first grouping it into bit pairs (dibits), each dibit being
represented by one of four possible carrier phase shifts. Thus, the output
from the phase modulator consists of phase-shifted carrier pulses at half
the data bit-rate. The phase shift between two consecutive modulation
elements contains the information to be transmitted, The principal
requirements of the modem used for system 6 are:
(i) use of differential 4-phase modulation
(ii) use of differential coherent 4-phase demodulation
(iii) full duplex operation over a 4-wire signalling link
(iv) a modulation rate of 1200 bauds
(v) a bit rate of 2.4 kbit/s.

(b) 1.544 Mbit/s digital transmission 4 kbit/s signalling bit rate. As now
specified, 24-channel p.c.m. systems (1.544 Mbit/s) do not permit a 64
kbit/s channel bearer for signalling purposes. Here, for signalling
purposes, a channel is derived over which a stream of pulses is trans-
mitted at 4kbit/s. The binary data from the signalling terminal is trans-
ferred serially at a data transmission rate of 4kbit/s to the primary
multiplex. Here, each bit of the data stream is successively inserted into
the S-position (Section 6.2.3).
In the receive direction, the primary multiplex extracts the bits from
the S-position and transfers them serially to the signalling terminal.

(c) 2.048 Mbit/s digital transmission. A 64 kbit/s channel bearer is available
for signalling in 2.048 Mbit/s 30-channel p.c.m. systems (Section 6.3) and
signalling bit-rates of 4 kbit/s or 56 kbit/s may apply. A digital inter-
face-adapter is provided between the multiplex and the signalling
terminal.

4 kbit/s signalling bit rate: The binary data stream from the signalling
terminal is transferred serially to the interface-adapter. Here, the 4 kbit/s
data stream is modulated on a 64 kbit/s bearer channel such that 16 bits of
bearer channel correspond to one bit of the 4 kbit/s channel. The 64 kbit/s
data stream is transferred serially to the 2.048 Mbit/s primary multiplex in
alignment with an 8kHz clock (octet timing). At the primary multiplex, the 16
bits corresponding to one signalling information bit are inserted into the
designated channel time slot of two successive frames.

In the receive direction, the primary multiplex extracts the bits from the
designated time slot and transfers them serially at 64 kbit/s to the inter-
face-adapter. Here, the 16 bits corresponding to one signalling information
bit are extracted after detection and the binary data is transferred serially
to the signalling terminal at 4 kbit/s.

56 kbit/s signalling bit rate: The system 6 signal unit is 28 bits. As this
is not a multiple of the 8bit octet it has weaknesses in digital transmission.
Four bits are added to the signal unit to increase its size from 28 to 32
bits, for the reasons of:

(a) octet timing The signal unit and padding bits together occupy exactly 4
octets. This simplifies the synchronisation and resynchronisation proce-
dures.

(b) slip detection: With appropriate coding, the four padding bits may be used
to detect octet slips.

No signalling information is carried by the four padding bits. Thus with the
system 6 28-bit signal unit, the signalling bit-rate is 56 kbit/s on the 64
kbit/s bearer. The bit pattern for the padding bits is 1010...1010, which
allows for detection of octet slips. As the pattern and location of the
padding bits interfere with check-encoding, the padding bits are added after
encoding and removed prior to decoding.

The binary data from the signalling terminal is transferred serially to the
interface-adapter. Here, the 28 bits of the signal unit are placed in bit
positions 1-7 of four 8-bit bytes. These four bytes are transferred serially
at the data transmission rate of 64 kbit/s to the 2.048 Mbit/s primary multi-
plex, in alignment with an 8kHz clock (byte timing). At the primary multiplex,
the four bytes are inserted into the designated time slot of four successive
frames.

In the receive direction, the primary multiplex extracts the bits from the
designated channel-time slot and transfers them serially at the data trans-
mission rate of 64 kbit/s to the interface-adapter, in alignment with an 8
kHZ clock. In the interface-adapter, the bits 1-7 of each 8-bit byte are
transferred serially to the signalling terminal at the 56 kbit/s rate.

The padding bits do not apply for the 4 kbit/s digital version. They would
not be necessary in signalling systems having a signal unit size a multiple
of the 8-bit octet.

=��=
11.6 Common channel signalling loading
=��=

With c.c.s., the signalling is time shared and a speech circuit does not
have exclusive use of a signalling facility, thus a queue is built up from
which signals are transmitted in order of their time of arrival and of their
priority. Thus the loading in terms of number of speech circuits served per
c.c.s. link must reflect acceptable queueing delays. This loading will be
influenced by a number of factors, in particular:

(a) the number of busy hour calls per speech circuit
(b) the number of signals involved per call
(c) the mix of LSUs and MUMs; the greater the number of MUMs for a given total
signal information transfer per call, the less the signalling load
(d) the signalling bit rate: the faster the bit rate, the less the signal
emission time and the greater the permissible number of speech circuits
served.
(e) the error-control method: typically, a noncompelled error control method
permits more speech circuits served per c.c.s. link than a compelled
method, other factors being equal.

It is thus not practicable to specify a general maximum limit of speech
circuits served per c.c.s. link. An adopted limit must take account of the
various conditions applying so that the total signalling load per link is held
to a level which will maintain an acceptable queueing delay. It is generally
accepted that the signalling load per link should not exceed of the order 0.4
Erlang in normal operation. This allows temporary signalling overload in
normal operation and for increased load in abnommal conditions such as
security backup for a failed regular signalling link, when the load of the
failed link is transferred to the backup link, the queueing delay increasing
in the abnormal condition.

It is also generally accepted that the maximum number of circuits served per
c.c.s. link adopted in practice for a particular network should reflect the
probable size of the speech circuit groups applying, and the signalling
network security philosophy adopted, for that network. Typically, one admi-
nistration may adopt a maximum loading of, say, 1500 circuits per c.c.s. link
in normal operation, whereas another may wish to adopt a smaller number, say
500 circuits, which reflects the different network conditions.

Indications of the maximum number of speech circuits served per CCITT system
6 c.c.s. link in the telephony service with acceptable queueing delays, assu-
ming 15 busy hour calls per speech circuit, 0.4 Erlang signalling link
loading, and representative conditions, are:

2.4 kbit/s signalling bit rate 1500 circuits served
4 kbit/s 2500
56 kbit/s 20000

It is stressed that these values must be regarded with some reserve in view
of the various assumptions adopted, nevertheless the broad indication is
reasonable and is of interest. The values for the new CCITT optimised digital
c.c.s. system with signalling bit rates 2.4, 4 and 64 kbit/s (Section 11.11)
will be different as much will depend upon the finally resolved method of
transferring the signal information - by messages of variable bit length or by
fixed bit length SUs. In either event however, there are indications that the
number of signalling bits transferred per telephone call will be somewhat
greater than in system 6, and with noncompelled error control the maximum
number of speech circuits served per c.c.s. link may well be somewhat less
than for system 6 at the same signalling bit rate and for the same assumed
conditions. Nevertheless, at 64 kbit/s, it is thought that some 20000 speech
circuits would apply. Compelled error control, should this apply, will signi-
ficantly reduce the values relative to non compelled.

While in some circumstances some 2500 or 20000 speech circuits could be
served per c.c.s. link in the telephony service in normal operation, it is
extremely unlikely, particularly for signalling network security reasons, that
such high values would be adopted in practice. It is thought that a more
realistic maximum adopted would be of the order 1500 circuits in normal
operation. This implies that a c.c.s. link would be very lightly loaded in the
telephony service in normal operation at the higher signalling bit rates 56
kbit/s and 64 kbit/s. The maximum number of circuits served per c.c.s. link
for the switched circuit data service would very likely be significantly less
than for the telephony service in view of the shorter call durations.

Circuit label capacity
The size of the circuit label bit field should allow for greater possibili-
ties than the requirement for the maximum number of circuits served per c.c.s.
link in normal operation. This is to allow for the increased load in the ab-
normal condition when a link functions as security backup for a failed link.
The CCITT specification for system 6 provides for an 11-bit (7-bit band, 4-bit
circuit number) circuit label bit field (potential to identify 2048 speech
circuits). This may be varied in particular national applications of the
system, typically, the Bell system national version of system 6 proposes a
13-bit (9-bit band, 4-bit circuit number) circuit label field (potential to
identify 8192 circuits), which reflects the large size speech circuit groups
applying in the N. American network.

It is considered advisable that the circuit label bit field be of size
giving flexibility to cater for possible future requirements not yet defined.
Thus the potential for a larger field than that provided by the CCITT speci-
fied system 6 is preferred, which approach is proposed for the new optimised
digital c.c.s. system (Section 11.11). A national network not requiring the
large number would have possibility to use spare circuit label bits for other
suitable national purposes.

The circuit label bit field is divided into band number bits and circuit
number bits. The band number identifies a group of circuits (typically a
p.c.m. system), the circuit number identifies a particular circuit in a group.
The banding concept minimises the memory required at a signal transfer point
(s.t.p.) in the quasi- and dissociated-c.c.s. signalling modes. The s.t.p.
processor translates on only the band number bit field and not on the complete
circuit label field, as signalling routing only is required at s.t.ps. A dis-
advantage of banding is the loss of some label capacity since all speech
circuit group sizes are not an integer multiple of band size. Usually the
circuit number field is 4 bits, which identifies a circuit within a group
(band) of up to 16 circuits. Should spare bits from a large circuit label
field be used for other national purposes, these should be spared from the
band, and not from the circuit number, bit-field.

=��=
11.7 Signalling link security and load sharing
=��=

Various security and load-sharing arrangements are possible within a parti-
cular c.c.s. system specification and in national application would be matters
of individual administration's choice. Bilateral agreement on particular
arrangements may apply in international application.

=��=
11.7.1 Security
=��=

The basic concept of securing a signalling route is to supplement its regu-
lar signalling facility with a readily accessible alternative brought in by
automatic procedures. This requires the provision of at least two signalling
facilities, termed 'regular' and 'backup' respectively for convenience.

The backup may be:

(a) A standby nonsynchronised link switched into service when required. (e.g.
a speech circuit nominated as the signalling backup).
(b) A synchronised signalling link in the same signalling relationship (e.g.
regular and backup signalling links on an associated signalling route).
(c) A separate signalling relationship (e.g. quasi-associated signalling as a
backup for associated signalling).

There are other possibilities.

Further signalling security can be achieved by fabricating a backup under
the control of network management. As this would not be an automatic proce-
dure, it would not be a function of the signalling system.

It is clearly preferable that the backup link(s) should have immediate
signalling capability on failure of the regular link. Thus while (a) is a
valid approach, the procedures required to prove the signal carrying capabi-
lity of the backup takes time and adds to the complexity. Approaches (b) and
(c) are preferred.

Inbuilt associated signalling security: This implies that an associated-
signalling route has inbuilt features to achieve, by automatic procedures, the
degree of signalling security required. The concept requires the provision of
at least two signalling links in that associated signalling relationship,
preferably diversely routed. The provision of more than one signalling link to
achieve the security can be justified on the relative cost insensitivity of
providing additional link(s) due to:

(i) the inherent 'common equipment' feature of c.c.s.
(ii) the application of such signalling to concentrated traffic
(iii) in the digital environment, the low cost of signalling time slots in
digital transmission systems due to the cost sharing in multiplex trans-
mission.

Associated signalling module security: There is an interplay between the
maximum number of speech circuits to be served per c.c s. link in normal ope-
ration and the size of the circuit label bit field to determine the number of
signalling links which may apply in an associated signalling relationship. The
term 'signalling module' is adopted in this regard. Typically, with a 6000
circuit label capacity, the maximum size signalling module would be four links
for a maximum loading of 1500 speech circuits per c.c.s. link, and six links
for 1000 circuits per link. In practice, not all modules in a network would be
of maximum size, they would be of size dependent upon the sizes of the speech
circuit groups served. Many associated signalling routes in many networks
would consist of two signalling links only (1 + 1), which could be smaller
than the maximum size signal module. The larger speech circuit groups would
require more than two signalling links and the very large groups would require
more than one signalling module.

Various security approaches may apply with the signalling module concept,
but the following are thought logical:

(a) Any one signalling link in the module should be capable of being a backup
for any other links in the module. This precludes a particular link being
the single nominated backup link.

(b) When the module is bigger than 1 + 1, the potential to allow for more than
one failed link in the module should be admitted in the automatic security
procedures. In theory, failure could be allowed to the limit of one sig-
nalling link carrying the full load of the module, which approach could
result in significant increase in queueing delay. In practice, an admini-
stration may wish to limit the extent of permitted failure within a module
to be covered by the automatic security procedures. In a two-link module
of course, one link must carry the total signalling load on failure of one
link.

(c) When a number of signalling modules are provided on a signalling route,
each module should be autonomous, being self-contained in the sense that
no traffic assigned to a module would ever be handled by another module.
Each module would be inbuilt secured and would not rely on another module
for signalling security.

The circuit label bit field of the international system 6 is of modest size
(11 bits, 2048 circuits). This is mainly suitable for a 1 + 1 signalling
concept and would restrict the module size in a module concept. If necessary
of course, national variants of the international system 6 could arrange for
additional circuit label bit(s) within the 6 SU size, as instanced by the
13-bit circuit label field of the Bell system version of system 6 for
application in N. America.

Quasiassociated signalling security: A number of possibilities arise,
typically

(a) Assuming the quasiassociated signalling route to have associated signal-
ling on the relevant constituent routes (Fig. 11.2b), and the associated
signalling modules to have inbuilt security, the quasiassociated signal-
ling route is then securcd in the inbuilt concept without the necessity
for further arrangements.

(b) Quasiassociated signalling without associated signalling on the consti-
tuent routes (Fig. 11.2c). Assuming quasiassociated signalling to be the
principal signalling mode in a network, situations arise where it would
not be of the type having associated signalling on the constituent routes
of the quasisignalling relationship. A signalling module concept could
apply for quasiassociated signalling of this type. The module size would
be determined by the same considerations as apply for an associated
signalling module, the module would have inbuilt security and all the
considerations postulated for the associated signalling module would
apply. The smallest quasisignalling module would be two links 1 + 1.

Unlike the associated signalling module case, s.t.p. security considerations
arise in quasiassociated signalling of the type being considered and the
s.t.ps. may or may not be duplicated depending upon an administration's
policy. If duplicated, the operation of the quasiassociated signalling module
would need to take account of the duplicated s.t.ps., but the basic
philosophy, and the basic working arrangements of the quasiassociated signal-
ling module would not be disturbed with duplicated s.t.ps. An example of this
type module is the proposed Bell system quasiassociated signalling in the
N. American network (Fig. 11.8). Here it is proposed that the complete network
comprises a number of signalling regions, quasiassociated signalling being
applied between the regions and thus in an organised manner on a complete
network basis, as distinct from quasiassociated signalling applied on parti-
cular signalling route situations and thus not as the principal mode for the
network. Quasiassociated signalling is proposed as the main c.c.s. mode in the
N. American network. In some situations, associated signalling may apply on
particular speech circuit groups and when applied, the basic capacity of an
associated signalling link is 1500 speech circuits maximum in normal operation
at the 2.4 kbit/s signalling. Under fault conditions, an associated signalling
link may carry twice this load, but with an increase in signalling delay.

This same capacity, 1500 speech circuits maximum, applies to the quasi-
associated A-links (Fig. 11.8). A toll office of 3000 speech circuits or less
would connect with the quasiassociated signalling network by providing a
single pair of A-links, one link to each regional s.t.p. An additional A-link
pair would be provided for each increment of 3000 speech circuits terminated
in the toll office. Each A-link pair functions as an autonomous pair, having
no operational interaction (such as failure backup) with other A-link pairs.

The interregional quasisignalling quad (four-signalling link module fully
inbuilt secured) forms a functional unit serving up to 6000 speech circuits
between a pair of signalling regions. Additional autonomous quads would be
provided for each increment of 6000 interregional speech circuits. As
mentioned, this arrangement requires a 13-bit circuit label field (9 bit band,
4-bit circuit number).

Exploitation of c.c.s. modes for signalling security: Signalling security
can be achieved by employing a backup signalling mode of a different type, or
another mode of the same type, from the regular signalling mode. Both inbuilt
security and regular modes secured by backup modes can be applied in the one
network. The following are possibilities:

(a) When it is not possible, or not economic, to inbuilt secure an associated
signalling route (i.e. when a single associated signalling link applies),
such a route could rely on a quasiassociated signalling backup for its
security.

(b) A quasiassociated signalling backup for a regular quasiassociated signal-
ling route, the associated signalling constituent routes of the regular
route not being inbuilt secured.

------------------------[See fig11-8.pcx in tadxf005]------------------------
Fig. 11.8 Bell system c.c.s network configuration
-----------------------------------------------------------------------------

(a) is liable to arise in practice. (b) introduces circuit labelling problems,
and routing complexities could arise should the quasiassociated backup be a
regular quasiassociated signalling route in anothcr relationship, with
possible further quasiassociated backup. While not preferred, (b) could be
applied under strictly controlled conditions within the circuit labelling
capacity applying.

It is thought that the associated and quasiassociated signalling modes would
be suitable for most national networks. Secured quasiassociated signalling
could be applied in particular situations where associated signalling is not
justified, or as in (a) above.

=��=
11.7.2 Load sharing
=��=

The technique of distributing the total module signalling traffic over the
individual links is termed 'load sharing', it being understood that all the
links, regular and backup, carry signalling traffic in normal operation.

There are two main categories of load-sharing mode:

(i) random mode
This may be either:
(a) messages distributed randomly on the links of the module
(b) all messages for a call transferred over the same signalling link,
which link is randomly selected from the links in the module.

(ii) Predetermined mode
This may be either
(c) predetermined on a call basis. There are a number of variants of this
basic approach, but all aim in principle that all messages for a call
be transferred over the same signalling link. There is little dif-
ference between this and (b) above.
(d) predetermined on a speech circuit basis. All the speech circuits
served by a module are distributed over all the signalling links in
the module on a predetermined basis in normal operation. The circuit
labels of a number of speech circuits are allocated to a particular
link, and all messages for all calls carried by the allocated speech
circuits are transferred over the same signalling link.

With (a), SUs or messages may get out of correct sequence on message
transfer. With (b)-(d), all messages for a particular call are transferred
over the same signalling link, which has potential to avoid out of sequence
due to the load-sharing technique. SUs or messages may get out of correct
sequence with certain error-control methods (e.g. system 6). With such
methods, there is no compelling reason to adopt load-sharing techniques which
aim to avoid out of sequence. Thus any one of (a)-(d) could be applied. With
error control methods purposely designed to avoid out of sequence on message
transfer, which is preferred, it would be illogical to adopt load-sharing
technique (a), and here it is considered that the choice rests between (b)
and (d).

Technique (b) has merits:

(i) The warking links in a module would automatically take the load on
signalling link(s) failure. Similarly when failed link(s) are restored
to service.
(ii) Would automatically achieve the limit of one signalling link taking the
whole load of a module.
(iii) Avoids a predetermined nominated link for security.
(iv) Meets the preferred principles of module signalling link security as
discussed without requiring procedures beyond busying out failed link(s)
and automatically diverting traffic from these link(s).
(v) Avoids a set program to allocate speech circuits to signalling links on
a predetermined basis, which also avoids updating the allocation when
speech circuit groups are extended.
(vi) Has the possibility of equalised distribution of signalling load on the
working links in a module on signalling link(s) failure.

On the other hand (b):

(i) Necessitates an arrangement to ensure that all messages for a call are
transferred over a randomly selected signalling link. This could be some-
what complex with certain approaches to processor system architecture.
(ii) On signalling link failure, requires a link to be nominated for the
retrieval procedure if messages are to maintain correct sequence, with
consequent complexity. For practical arrangements, it is often preferred
to avoid nominated retrieval links, and retrieval on the link changed
over to is considered preferable.

The features and merits of(d) are:

(i) Changeover to another link in a module on signalling link failure. The
circuit labels of a failed link are transferred in total to another link
(or alternatively, distributed over the remaining working links).
(ii) On restoration of a failed link back to service, all the circuit labels
allocated to the failed link prior to the failure are transferred back.
(iii) Would automatically achieve, in potential, the limit of one signalling
link carrying the whole module load on multilink failure.
(iv) Avoids requiring a nominated link for retrieval.
(v) Meets the principles of module signalling link security as discussed
under the control of the changeover and changeback procedures.
(vi) Avoids any further arrangement in the processor to ensure that all
messages for a call are transferred over the same signalling link. This
feature is assured by the predetermined allocation of circuits labels
to signalling links, which arrangement may be more capable of accom-
modating various approaches to processor function system architecture.

On the other hand (d):

(i) Necessitates an administrative procedure to allocate speech circuits to
particular signalling links and to update the allocation on speech
circuit group extension.
(ii) Compared with random selection, could result in a signalling link in a
module carrying a significantly heavier load than others on signalling
link(s) failure when all the circuit labels of a failed link are trans-
ferred to one working link. Depending upon the signalling load in normal
operation, this could result in queueing delay problems in the abnormal
condition of signalling link(s) failure, particularly at slow signalling
bit rates.

On balanced assessment, there is probably little between load sharing
techniques (b) and (d) and either could be applied when it is required to
avoid out of sequence, which is preferred. Should simple arrangements be the
main consideration, it is thought that this would be achieved by (d) in
combination with the transfer of all the circuit labels of a failed link to
one backup link. This could result in a significant increase in queueing
delay in certain circumstances with certain size signalling modules depending
upon the extent of multilink failure in a module permitted by the automatic
security procedures.

=��=
11.8 Changeover, retrieval and changeback
=��=

The changeover, retrieval and changeback procedures consequent on signalling
link failure are detailed and the principle only will be discussed. At this
time, the procedures for international system 6 are the only ones available.
These could form the basis for procedures required for any future c.c.s.
system(s). Administrations are free to adopt the CCITT international proce-
dures for national application of international c.c.s. system(s), or, if
desired, could adopt procedures more suited to the conditions of individual
national networks, which approach would not disturb the basic specification of
international c.c.s system(s).

Changeover

Signalling traffic is diverted from a regular link to an alternative signal-
ling backup when the regular link is recognised as having failed. Failure may
be caused by (Section 11.5.3):

(a) loss ot the analogue data carrier or loss of the digital frame alignment
(b) continuous failure of SUs (or messages) to check correctly
(c) unacceptable intermittent failure of SUs, or messages, to check correctly
(d) loss of relevant synchronism for a certain duration.

Each signalling terminal in c.c.s. is equipped with appropriate monitoring
equipment to recognise failure due to any one of the above causes. In system
6, the error rate monitor initiates changeover when:

(i) X consecutive SUs are received in error for 350ms,

or

(ii) 2% of SUs are received in error out of Y SUs received.

X and Y are assessed as follows for the telephony service:

Signalling bit rate Number of SUs
(kbit/s) X Y
-----------------------------------------------------------
2.4 31+/-1 2500
4 50 4200
56 700 60000
64 800 68500

On detection of failure, each terminal starts sending 'faulty link infor-
mation' of appropriate form on the link just failed. Typically, in system 6,
this could consist of alternate blocks of 11 changeover and 11 SYU signals
plus ACU. If the terminal has lost synchronism, the normal synchronising
procedure is started. Where applicable, the synchronising procedure on the
backup signalling link is initiated.

retrieval

Messages and acknowledgments may be lost in the transfer process on signal-
ling link failure. Thus a terminal would not be aware whether or not all or
some of the messages in the retransmission store had been received correct at
the receive terminal. Retrieval is the process of recovering from this situa-
tion to avoid loss of messages on changeover. For simplicity, and also to
avoid out of sequence messages, retrieval should be performed over one link,
and for further simplicity, preferably over the backup link.

There are a number of retrieval possibilities, typically:

(a) Retransmission of all the contents of the retransmission store. This could
result in duplicated messages, as arises in system 6.

(b) Retransmission only of messages known to have been received corrupted.
This requires the receive terminal to inform the transmit terminal, by
backward signalling over the retrieval link, as to the last message
received correct over the failed link should cyclic retransmission be used
(not in system 6).

(c) With SU (or message) sequence numbering, the retrieval retransmission
carrying its original sequence numbering, (a) or (b) could apply and
retrieval start and stop indicators would be necessary if retrieval is on
a working link due to the differences in number sequences on the failed
and backup links.

(d) With sequence numbering, the retrieval retransmission carrying the
sequence numbering of the working backup link over which retrieval is
performed. Assuming SU (or message) sequence numbering, (b) would apply
and retrieval start and stop indicators would not be essential if
retrieval is on a message, LSU, or MUM basis. A start retrieval indicator
could be of value if retrieval is on an SU basis to cover the situation of
a partial MUM having been received correct over the failed link.

There are other possibilities. The retrieval method adopted would depend upon
preferences and circumstances.

Changeback

Once the backup facility has been taken into service, the regular signalling
link should not be brought back into service for signalling traffic until it
has been checked to give satisfactory performance for a period of about one
minute. The proving period begins when either terminal has regained synchro-
nism on the failed link.

End-of-failure monitoring equipment is provided at each terminal. In system
6, the failed link is not restored to service until a SU error rate of 0.2% or
less has been achieved in the proving period. In the telephony service, the
end-of-failure is assessed to have been achieved when not more than:

10 SUs at 2.4 kbit/s
16 SUs at 4 kbit/s
240 SUs at 56 kbit/s
266 SUs at 64 kbit/s

are received in error in the one minute proving period.

On end-of-failure recognised, a terminal will cease sending faulty link
information. In system 6:

(a) The faulty link information is replaced by continuous blocks of SYUs
(plus ACUs).

(b) To return to the regular link, an exchange, say A, initiating the change-
back sends two load transfer signals on the regular link. Exchange B
responds with a load transfer acknowledgment signal on the regular link
and transfers signalling traffic from the backup to the regular link.
Receipt of one load transfer acknowledgment causes A to transfer signal-
ling traffic from the backup to the regular link.

=��=
11.9 Continuity check of the speech path
=��=

Unlike speech path signalling, c.c.s. does not monitor the speech path and
other arrangements must be made to assure speech path continuity (line plus
switching), otherwise there could be possibility of a connection being set up
on the signalling path but without the speech transmission facility. Speech
path continuity assurance may be by:

(a) a per-call continuity check, or alternatively

(b) a statistical method of routine testing of idle transmission circuits and
idle switching paths (and no per-call continuity check).

The per-call continuity check is made prior to the conversation. It may be
loop link-by-link or end-to-end and consists of the transmission and detec-
tion of receipt of an audio frequency on the speech path. Typically, in
system 6, the per-call check is line loop link-by-link with a 2000Hz check
tone frequency. A separate cross-office check is made.

To admit the possibility of 2-wire circuits, which may arise in national
networks, the per-call check could consist of different forward and backward
check frequencies.

On 'check not OK' a repeat attempt is made to set up the call on an alter-
native speech path. To cater for intermittent faults, the original speech
path is rechecked and passed to maintenance attention if still found to be
faulty.

=��=
11.10 Signal priority
=��=

Transmission (queueing) delays can arise with c.c.s. As c.c.s. may have a
considerable signal transfer requirement, embracing a number of different
services and signals within the different service categories, delay to time
sensitive signals or to time sensitive service categories could give rise to
system problems and complexity. For this reason, some signals and some
categories may be given priority over others for transfer purposes and the
processor function is so programmed. The priority program will depend upon
the characteristics of the c.c.s. system, typically, with representative
loadings a high speed 64 kbit/s c.c.s. system would transfer information at a
faster rate and the queueing delay would tend to be less, relative to a slower
speed system, and thus the extent of the priority requirement would tend to be
less. As the arrangements for signal priority complicate c.c.s., it is logical
that all efforts be made to minimise the priority requirement.

The following priority rules apply for international system 6:

(a) ACUs (12th position signal unit of each block) have absolute priority for
emission at their fixed predetermined position.
(b) Faulty link information has priority over all signals except ACUs.
(c) The answer signal has priority over other waiting telephone signals and
signalling system control signals except those in (a) and (b).
(d) All other telephone signals (LSUs and MUMs) and all other system control
signals, except synchronisation signal units, have priority over manage-
ment or other signals concerned with the bulk handling of information.
(e) Any signal which is to be retransmitted will take precedence over other
waiting signals in the same priority category.
(f) Management signals have priority over synchronisation signal units.
(g) Synchronisation signal units have no priority.

It is the intention to minimise the priority requirement in the new opti-
mised digital 64 kbit/s c.c.s. system. Typically, it could be reasoned with
some logic that due to the high speed of information transfer, there would be
no need to give priority to the answer signal. In slower systems, answer
signal priority has been considered desirable to avoid speech clipping due to
speech transmission path line splits on interworking v.f. signalling systems.
Signal priority may be associated with the 'break-in' feature, which allows a
priority message to break into the emission of a lower priority MUM to avoid
delay in the emission of the higher priority message. The following break-in
rules apply for international system 6:

(a) Potential for a priority LSU to break into a MUM is provided for in the
specification of the system, but initially this will not apply except for
ACUs.

(b) All telephone signals to have potential to break into a network management
MUM or other MUMs concerned with the handling of bulk information.

The break-in feature gives rise to significant complication in c.c.s.
systems and it is not proposed to adopt it for the new optimised 64 kbit/s
digital c.c.s. system. To facilitate this, the maximum bit length of messages
in the new system may well require to be limited.

=��=
11.11 CCITT No. 7 optimised digital common channel signalling system
=��=

=��=
11.11.1 Basic concepts
=��=

The pioneer CCITT 6 c.c.s. system was produced primarily for analogue appli-
cation, the digital versions included in the specification being produced by
subsequent study, but retaining the basic features of the analogue system. In
the result, system 6 has limitations in the digital environment. In the back-
ground of experience gained in the production of system 6, and with recogni-
tion of the evolution of many networks from analogue to digital (integrated
switching and transmission), the CCITT is presently engaged on the specifi-
cation of a new c.c.s. system. This system will be optimised for the 64 kbit/s
digital environment, will include other digital signalling bit rate(s), and
analogue bit rate(s). Particular regard is being paid to national network
requirements, and as it is aimed to produce a simpler and more flexible system
than 6, a main desire for national application, there is little doubt that
many administrations, programming c.c.s. application, will adopt this new
system for their national networks in the s.p.c. digital (or analogue)
environment. This CCITT study is not yet concluded, but it is clear from the
approaches emerging that many features of the new system will be different
from those of system 6, which, in part, reflects further knowledge in the
c.c.s. art since system 6 was specified.

The c.c.s. technique has significant potential for signalling rationalisa-
tion and an objective of the new system is to use the same basic system for
international, regional and national for a variety of services, digital and
analogue. This would allow the potential, from the signalling aspect, for
dedicated digital service networks (telephony, data, etc.) to evolve to a
single multipurpose network, should this ever be a future requirement. While
the new system will be optimised for digital, there is little doubt that it
will be a superior system to 6 for analogue, as well as for digital. UK
pioneered much of the new system (CCITT No. 7) and has adopted it for its
System X digital network.

Rationalisation implies a flexible signalling system and it is proposed that
the new system be based on a number of concepts to achieve this (Fig. 11.9).
These concepts cater for various services and applications without mutual
interaction, and give potential to cater for future requirements not yet
defined.

Commonality concept: Options incorporated in the same signalling system
would permit reasonably optimum arrangements for different services and appli-
cations (international, regional and national) of the same service. A single
c.c.s. system for various services within a particular network could also be
realised by appropriate choice of options incorporated. Desired simplicity of
national network c.c.s. is an example of the merit of the option approach, as
here advantage could be taken of national network conditions, not present in,
say, international, which could contribute to signalling simplicity. Typical
options concern the signalling bit rate, the error control method, and message
bit length, formats and codings for different services.

Functional concept: A functional concept embracing processor function, user
function, and message transfer function, allows change to any one function
without significant impact on the others. It involves, for example, flexible
arrangements in the message transfer function to enable an error control
option to be exercised without involving the processor. This implies that the
error control should be associated with the signalling terminals, the
processor never being involved.

User subsystem concept: This is part of the functional concept. User sub-
systems (separated parts of the processor) are functional parts which control
the message requirements for particular services, typically, telephony call
processing, data call processing, network management, network maintenance,
centralised call accounting, etc. Such subsystems (Fig. 11.9) may be regarded
as being the service dependent parts of the signalling system. They define
message, signalling and call control procedures, and the function and coding
of the signalling information for different services. Each user subsystem
module deals with a particular service, and the concept allows the system to
evolve in that additional services may be catered for by the addition of
appropriate subsystem modules.

The message transfer function (the c.c.s. system) allows user subsystem
modules held on different processors to communicate with each other by means
of addressed messages. This processor subsystem addressing (type of service)
directs the message to the appropriate subsystem module, the subsystem
addressing identifying the service for which the message is intended. Should
corresponding user subsystems communicate with each other for the interchange
of information not related to call handling, such messages will be conveyed
by the message transfer function.

------------------------[See fig11-9.pcx in tadxf005]------------------------
fig 11.9 Generalised functional arrangements common channel signalling
A = Telephony call processing ---|
B = Data call processing |
C = Viewphone |-> Typical User Subsystems
D = Network Management |
E = Network Maintenance |
F = Call Accounting ---|
----------------------------------------------------------------------------

=��=
11.11.2 Signalling bit-rate
=��=

A 64 kbit/s bearer is conveniently available for c.c.s. in 30-channel digi-
tal transmission, and with a message bit length a multiple of the 8-bit octet,
the signalling bit rate can be this bearer bit rate. There is no reason to
depart from this rate for telephony c.c.s., indeed, at this rate the message
signalling activity will be very light with the speech circuits per c.c.s.
link loading likely to apply. Commonality c.c.s. for data services introduces
further considerations as such services wish a faster connection set-up time
than that usually acceptable for telephony. Relative to telephony, the data
service may require a greater signalling load in terms of calls per second
handled per c.c.s. link, which could slow data call connection set-up. Study
concluded however that signalling bit rates greater than 64 kbit/s would not
significantly reduce queueing delay, nor connection set-up time in the data
service, and the new c.c.s. system will be optimised for 64 kbit/s. Optional
signalling rates 4 kbit/s (for 24-channel digital) and 24 kbit/s (for
analogue) will be used. Other rates could apply as appropriate (typically
optional 4.8 kbit/s analogue).

=��=
11.11.3 Error control
=��=

The combination of the error control method and the various consequential
procedures constitute perhaps the most complex part of c.c.s., and as a simple
system was desired, the error-control problem was reassessed.

Forward error correction: Here, the receive end performs both error detection
and correction, and error correcting codes ('Hamming' codes) used, sufficient
check bits being required to enable the original message to be reconstituted
on error detected. Retransmission of messages does not arise, and as the
method has some potential for simple arrangements, the following analysis is
made in regard to c.c.s.:

(a) It is independent of the propagation time of signalling links.
(b) The procedure is simple; acknowledgment signalling and sequence
numbering of messages not being required.
(c) Messages cannot get out of correct sequence, nor be duplicated.
(d) Error correction is reasonably rapid relative to correction by retrans-
mission, which has merit on long propagation time signalling links.
(e) No retransmission store required.
(f) The automatic changeover procedure is simple and retrieval not required.
This, however, is valid only when the possibility of loss of messages is
acceptable when diverting signal traffic to an alternative signalling
facility.

On the other hand, forward error correction:

(g) Requires far more redundancy in check bits relative to that required for
error detection only (or for a given number of check bits it is possible
to detect more errors than it is possible to correct). The coding and
decoding systems for the check bits are relatively complex.
(h) Cannot request retransmission of a message which has been error detected
but not corrected.
(j) Complicates the receiving terminal.

The following points are made in the consideration of the above:

(i) As c.c.s. is error-free for most of the time, penalties (g) and (j) in
particular are not justified.
(ii) Error-free messages are slowed.
(iii) In any c.c.s. system there is the need to safeguard against major break-
down of a signalling link. Correction by retransmission has a signi-
ficant merit relative to forward error correction in that it has an
inherent ability to direct signalling traffic to an altemative signal-
ling facility without loss of signal information.
(iv) With reasonable arrangements, correction by retransmission is superior
to forward error correction in both throughput and undetected error
rate.

It was concluded that forward error correction would have limitations in
c.c.s. application and that correction by retransmission be adopted for the
new c.c.s. system. This supported a previous conclusion in regard to system 6,
but not the system 6 errorcontrol method, which would not be suitable for the
various reasons discussed in Section 11.5.4.

Guidelines for a preferred error-control method: From the analysis Section
11.5.4, it is considered that the main guidelines are:

(a) Messages should not get out of correct sequence in any working situations
of the error control.
(b) Duplicated messages should not arise, but if they do they should be detec-
ted and dealt with by simple means at the signalling terminal, the
processor function never being involved.
(c) Unnecessary and unrequested retransmissions should not arise.
(d) The processor function should not be involved in the error control
function.

Guideline (d) requires all messages passed from the message transfer
function to the processor function to be valid in all respects, except for
errors undetected by the error control. Ideally, to achieve this, the error
control method itself should be such that neither out of sequence nor dupli-
cated messages occur, but this may not be conveniently realisable.

It would be illogical to require a signalling terminal to detect and correct
for messages in incorrect sequence (a) as this would necessitate a measure of
reasonableness checking and correction logic at the signalling terminal, to
complicate the system. It is thought that the errorcontrol method itself
should be such that out of sequence messages do not occur. With sequence
numbering (or the equivalent) of messages, it would be a simple matter for a
signalling terminal to detect for duplicated messages (b) and to discard
messages already received. Thus, if necessary, duplicated messages could be
admitted on the message transfer function.

A philosophy of 'ignoring corruptions' in the error control could eliminate
both unnecessary and unrequested retransmissions (c). Here, no action is taken
on error detected, the system merely awaits the receipt of the next error free
SU (or message) before taking decision as to the appropriate action to be
taken. With this philosophy:

(a) corrupted SYUs (and corrupted messages which would have been unacceptable
if error free) would not cause unnecessary retransmissions.

(b) to eliminate unrequested retransmissions, the acknowledgment indications
are required to be in a continuous stream.

An error-control method based on the above philosophy precludes adoption of
the system 6 error-control method because of the block transmission and the
single acknowledgment (ACU).

'Ignore corruptions' error-control method: The error-control method for the
new CCITT No. 7 c.c.s. system is not yet finalised by the CCITT, but present
indications are that it will be based on the ignore corruptions philosophy.
The following is a possible realisation of the philosophy (Fig. 11.10), is
presented to demonstrate the principle, and should not be regarded as being
the final agreement in the detail. Basic features are:

(a) Cyclic retransmission on error detected, meaning that the corrupted
message SU (or message) and all the message SUs (or messages) following
it in the retransmission store are retransmitted, Corrupted SYUs are not
corrected and do not cause retransmission.
(b) Each SU (or message) contains both a forward sequence number (FSN) and a
backward sequence number (BSN).
(c) SYUs transmitted between message SUs (or messages) all carry the same
forward sequence number of the next message SU (or message) to be trans-
mitted.
(d) Each SU (or message) contains a forward indicator bit (FIB) and a backward
indicator bit (BIB), used to control retransmissions.

The philosophy may be applied with message SUs (LSUs and constituent SUs of
MUMs) of fixed bit length or with messages of variable bit length. The FSN,
BSN, FIB, BIB and check bits are 'housekeeping' bits, concerned with the con-
trol of message transfer, and are not passed to the processor. Drift problems
do not arise.

------------------------[See fig11-10.pcx in tadxf005]-----------------------
Fig. 11.10 Typical realisation ignore corruptions error control
-----------------------------------------------------------------------------

FSN: Message SUs (or messages) are numbered sequentially to enable the
correct sequence to be preserved during the message-transfer process, and to
identify message SUs (or messages) when retransmission is required. The range
of sequence numbers must be such that a retransmission store never contains
two message SUs (or messages) bearing the same FSN and reflects the maximum
number of message SUs (or messages) liable to exist in the error-control loop.

BSN. The BSN contained in a unit (or message) transmitted from a terminal
indicates the FSN of the next message SU (or message) that terminal is pre-
pared to accept. If a terminal has just accepted a message SU (or message)
with a FSN of n, the next unit (or message) to be transmitted from that ter-
minal will have a BSN of n + 1. When accompanied by a retransmission request
(change in polarity of the BIB), the BSN indicates the point in the transmit-
ted sequence at which the retransmission should start.

BIB: Is used by a receive terminal to signal back to the transmit terminal
that a retransmission is required, the request being made by reversing the
polarity of the next BIB to be transmitted. Once a receive terminal has
requested a retransmission, no further reversals of polarity are made until
a new retransmission is required. No action is taken by a terminal when a
received unit (or message) is corrupted. The system waits (ignores corrupt-
ions) until an error-free unit (or message) is received. The polarity of the
BIB is reversed to request retransmission when the FSN of this error free
received unit (or message) is not one greater (cyclically) than the last
correctly received, and accepted, message SU (or message). After requesting
a retransmission, a terminal takes no further action until a change in polari-
ty of a received FIB indicates the retransmission, the terminal then rever-
ting to normal operation.

FIB: The polarity of the FIB in any particular unit (or message) transmitted
by a terminal is always the same as that of the BIB contained in the last unit
(or message) correctly received by that terminal. Since a change of polarity
of the BIB is used to request a retransmission, a change of polarity of the
FIB will indicate that the retransmission is taking place. The polarity of the
FIB is reversed when a retransmission is started, and after a reversal the
polarity is unchanged until the next retransmission is started. The FIB and
BIB thus perform a 'handshaking' procedure.

Operation

Information is transferred in the two directions of a signalling link, but
as the two directions are independent from the error-control point of view,
it is only necessary to consider one of them (A-B).

(a) Error-free: A message SU (or message) is accepted at B provided it has a
FSN which is one greater (cyclically) than the previous one accepted.
The BSNs returned to A signal the progress made in the process of accep-
ting correct message SUs (or messages). When received at A they enable the
message SUs (or message) to be cleared from the retransmission store.

(b) Error: The corrupted unit (or message) is ignored. The system waits the
receipt of the next error free unit (or message), the FSN of which will
serve to indicate whether or not the corruption was a message SU (or
message) or a SYU. If the FSN is the next in the sequence expected by B,
the corruption must have been a SYU, in which case no further action is
necessary and normal operation applies. The receipt of any other FSN
serves to indicate that a message SU (or message) was corrupted and a
retransmission necessary. B requests this by reversing the polarity of
the BIBs sent back B to A and does not accept further message SUs (or
messages) until it has detected that a retransmission has taken place.

The retransmission takes place when the BlB polarity is reversed and the
reversal recognised at A, the first message SU (or message) of the retrans-
mission being identified by the latest BSN received at A. The fact that the
retransmission has taken place is indicated by reversed polarity of the FIBs
transmitted A to B, commencing with the first message SU (or message) of the
retransmission. B detects this FIB reversal to recognise the retransmission.
If there are no further errors, the first message SU (or message) of the
retransmission is accepted by B, which then reverts to normal operation.

If the first one (or more) message SU (or message) of the retransmission is
corrupted, when B eventually receives an error free SU (or message) it will
detect from the FIB reversal that a retransmission has taken place, but the
FSN will be different from that B will accept. A second request for retrans-
mission is made by again reversing the polarity of the BIBs sent back. B then
rejects all further message SUs (or messages) until the next retransmission
is detected by the reversals of the FIBs received.

Noncompelled error-control mode

The ignore-corruptions error control as described above is an example of
this mode, messages being transmitted in a continuous stream. A number of
messages may be present on the signalling link at any one time and the error-
control method is required to cater for this, which accounts for the sequence
numbering in the example described. The mode is not dependent upon the magni-
tude of the error control loop delay. With noncompelled, the c.c.s. link will
be very lightly loaded at high signalling bit rates in the telephony service,
which gives possibility of repeat transmission of the information (typically
by consecutive multiple transmission of a message SU (or message), or by
repeated transmissions of the contents of the retransmission store) to mini-
mize, but not replace, correction by retransmission.

Compelled error-control mode

Here, one message only is transmitted at a time, a following message not
being transmitted until the previous message had been correctly received and
the transmit end is aware of this, either implicitly or explicitly. The error
control loop delay of the signalling link is occupied for the complete trans-
mission of a single message, and relative to noncompelled, the c.c.s. link
occupancy is significantly increased for a given signalling requirement.
Compelled signalling is interleaved in the two directions. The mode reduces
the number of speech circuits which may be served per c.c.s. link relative to
noncompelled under otherwise equal conditions, the loading being dependent
upon the error control loop delay and thus on the propagation time of the
signalling link.

The compelled mode has potential for simplicity (one message only dealt with
at a time, no sequence numbering, retransmission store limited to one
message), and has possibility of being optional to noncompelled in the common-
ality concept of the c.c.s. system. While the mode may be suitable for the
telephony service in certain restricted national network conditions, it is not
thought suitable for the switched circuit data service in any conditions due
to the excessive queueing delays which would arise, the data service having a
high proportion of short duration calls, with consequential high c.c.s. link
signalling requirement. It is thought that the application flexibility merit
of the noncompelled mode in any service may well outweigh any merit which may
be realisable with the compelled mode in restricted application.

=��=
11.11.4 Message structure
=��=

The message size, formatting and codings for the new No. 7 system are not
yet finalised by the CCITT. As the system is to be optimised for 64 kbit/s,
and thus a c.c.s. link very lightly loaded in the telephony service, the bit
length of the constituent bit fields of a message, and the total message bit
length, can be reasonably long, and significantly longer than those in system
6, without penalty. The flexibility of the new c.c.s. system will permit,
optionally, messages of fixed bit-length SUs, or messages of variable bit-
length, as desired. For compatibility with the digital environment it is
logical that the message length should be a multiple of the 8-bit Octet, and
to simplify the processing procedures, a byte structure adopted.

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References
=��=

1. LUCAS, P., LEGARE, R., and DONDOUX, J.: 'Principes nouveaux, pour la
signalisation telephonique', Commun. & Electron., 1967, 18, pp. 7-23
2. CCITT: Green Book, 6, Pt. 3, Recommendations Q252-295 'Specification of
signalling system No. 6', ITU, Geneva, 1973, pp. 427-521
3. CREW, G.L.: 'CCITT system No. 6 - a common channel signalling scheme',
Telecommun. J. Australia, 1968, 18, 3, pp. 251-256
4. AKIMARU, H., TEKEDA, H., and ABU, M.: 'Common channel signalling system
for DEX2 electronic switching system', Rev. Electr. Commun. Lab. (Japan),
1969, 17, p. 11
5. DAHLBOM, C.A.: 'Common channel signalling - a new flexible interoffice
signalling technique'. lEEE International Switching Symposium Record,
Boston, USA, 1972
6. PETERSON, W.W., and BROWN, D.T.: 'Cyclic codes for error detection', Proc.
Inst. Radio Eng., 1961, 49, Pt. 1, pp. 228-235
7. CCITT: Green Book, 6, Pt. 3, Recommendation Q277 'System No. 6 error
control', ITU, Geneva, 1973, pp. 497-499
8. CCITT: Green Book, 6, Pt. 3, Recommendation Q267 'Unreasonable and super-
fluous messages', ITU, Geneva, 1973, pp, 477-480
9. CCITT: Green Book, 6, Pt. 3, Recommendation Q274 'System No. 6 modulation
method and modem requrement~', ITU, Geneva, 1973, pp. 491-495
10.CCITT: Green Book, 6, Pt. 3, Recommendations Q291 and Q293 'Security
arrangements', ITU, Geneva, 1973, pp. 508-510 and 512-518
11.CCITT: Green Book,6, Pt. 3, Recommendation Q271 'System No. 6 continuity
check of the speech path', ITU, Geneva, 1973, pp. 484-487
12.WELCH, S.: 'Common channel signalling - a flexible approach'. International
Switching Symposium Record, Kyoto, Japan, 1976
13.HAMMING, R.W.: 'Error detecting and error correcting codes', Bell Syst.
Tech. J., 1950, 29, p. 147

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