| Document: FSC-0075
| Version: 001
| Date: 23rd October 1993
| Author: Jan Ceuleers
ISDN capability flags in the Nodelist
A proposal
by Jan Ceuleers, 2:292/857
version 0.4, October 3rd 1993
1 Introduction
The Integrated Services Digital Network is a worldwide overlay
network, offering the same services as the PSTN (Public Switched
Telephone Network) and more. Its basic bearer capability is a
digital bit stream of 64000 bits/s, as opposed to the audio
channel with a 3.1 kHz bandwidth provided by the PSTN.
Transferring data across the ISDN can be done in one of two ways:
- by using the telephony services the ISDN provides. In this
mode, a standard modem can continue to be used. Some
modulation schemes currently in use are V.32bis, PEP, ZyXEL,
HST, V.32 etc. We already have nodelist flags for these
cases.
- by using ISDN's 'native' mode. In this case, a number of
protocols (either or not ISDN-specific) are used, such as
V.110, V.120, X.75 etc.
This document aims to define the way in which nodes are to
advertise their ISDN capabilities in the nodelist, irrespective
of whether or not ISDN-only nodes should be in the nodelist in
the first place. This latter problem is to be solved by the
politicians.
Descriptions of ISDN equipment and compatibility with POTS (Plain
Old Telephone Service) equipment are beyond the scope of this
document. More detailed information on ISDN is also not provided;
readers are referred to the literature (e.g. 'Computer Networks'
by Andrew Tanenbaum).
2 Different flavors of the same protocol
Some ISDN protocols have different flavors, some of which differ
only marginally, but others are quite distinct.
For the techies among the readership: examples of both
cases can be found in the 1988 definition of V.110.
There are two variants of the frame structure used in
the 56kbps synchronous mode (marginal difference),
while there is quite a major difference between the
synchronous and asynchronous versions of V.110.
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ISDN capability flags in the nodelist
Since FidoNet applications are essentially character-based, the
asynchronous versions of protocols will be preferred over the
synchronous-ones(1). This applies to V.110 and V.120 and to
any other such protocol.
If there is an option, 8 data bits, no parity and 1 stop bit will
be used in preference over all other possible combinations. (This
is in line with the FOSSIL spec).
3 Speeds
Some protocols (such as V.110) can be used at different speeds.
Certain implementations of these protocols may not support some
of these speeds.
The baud rate field in the nodelist should not be used for
indicating the maximum speed an ISDN node is capable of, since
ISDN capability flags could (technically) co-exist with normal
modem capability flags(2).
4 Nodelist flags
ISDN-related nodelist flags consist of a prefix, a protocol
indicator and an optional (set of) suffixes.
The prefix is the capital letter I (for ISDN).
The protocol indicator is one of the strings defined in paragraph
5 below.
The suffix indicates the way in which the implementation deviates
from the preferred implementation, as indicated in paragraphs 2
and 3. The possible suffixes are:
Onnn The only bit rate supported = nnn * 100 (e.g.
IV110O384 means that this node only supports V.110
asynchronous at 38400 bps and at no other speeds.
1. As a matter of fact, there are no provisions for
advertising the synchronous versions of such protocols.
2. ISDN technology indeed allows for the possibility of both
modem and ISDN-specific datacom connectivity on the same
'telephone' number.
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ISDN capability flags in the nodelist
Pnnnnn Maximum packet size supported in bytes. This is a
layer 2 packet protocol parameter. Communication
between two nodes is only possible if either end's
maximum packet size is not exceeded. Leading
zeroes are to be omitted.
Rnnn Highest bit rate supported = nnn * 100 (e.g.
IV110R192 means that this node does not support
V.110 asynchronous at 38400 bps, but does support
all other standardised rates up to and including
19200 bps)
Wn Window size. The window size must be less than the
modulo value (i.e. in modulo 8, the maximum window
size is 7).
If more than one suffix is used, the suffixes will be sorted in
ascending order.
5 Protocols
This section defines the meaning of the base protocol indicators.
The aim is to have this base protocol indicator cover the
majority of cases, so that suffixes will only rarely be required.
5.1 V.110
The protocol indicator is V110. When specified without
suffixes, the IV110 nodelist flag indicates V.110
asynchronous capability at bit rates up to and including
38400 bps.
5.2 V.120
The protocol indicator is V120. When specified without
suffixes, the IV120 nodelist flag indicates V.120
asynchronous capability. Due to the nature of the protocol,
the O and R suffixes are irrelevant.
There is no explicit mention of frame size in the V.120
specifications. However, since Q.921 is the layer-2 protocol
of V.120, one might assume the frame size of Q.921, which is
260 bytes. Frame sizes larger than that should be negotiated
between sysops.
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ISDN capability flags in the nodelist
5.3 T.90 (X.75)
The protocol indicator is T90. Base protocol parameters are:
modulo: 8
window size: 2
packet size: 2048 bytes
Currently, there is no standardized method for negotiation
of the modulo mode (Recommendation ITU-TS T.90 reserves this
subject for further study), all T.90-capable nodes should
answer in modulo-8 mode. It is therefore useless to
advertise modulo-128 capability. This also limits the
maximum window size to 7.
Some implementations have a maximum frame length of 130
bytes and a maximum window size of 1. This would be
documented as IT90P130W1. The 1992 version of the T.90
standard specifies a method for in-band negotiation of frame
length and window size.
5.4 Other protocols
Additional protocols can be added to this document (and thus
assigned a nodelist flag) if sufficient technical
information is made available.
Neither X.25 on B nor on D have been added, because there is
no room in the nodelist for the X.25 address.
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ISDN capability flags in the nodelist
6 Conversion from old to new
The ISDNA, ISDNB and ISDNC nodelist flags are already in use in
zone 2. The table below shows the relationship between old and
new.
