Leased line Mini HOWTO
Rob van der Putten, [1]
[email protected]
v1.7, 29 October 1999
The most recent (beta) version of this HOWTO can be found at:
[2]www.sput.signature.nl
What is a leased line
Any fixed, that is permanent, point to point data communications link,
which is leased from a telco or similar organisation.
The leased line involves cables, such as twisted pair, coax or fiber
optic, and may involve all sorts of other hardware such as (pupin)
coils, transformers, amplifiers and regenerators.
This document deals with:
Configuring your modem and pppd to use a 2 wire twisted pair
leased line.
This document does NOT deal with:
SLIP, getting or installing pppd, synchronous data
communication, baseband modems.
Assumptions
You should already have a working pppd on your system.
You also need Minicom or a similar program to configure your modems.
Modem
A leased line is not connected to a telephone exchange and does not
provide DC power, dial tone, busy tone or ring signal. This means that
your modems are on their own and have to be able to deal with this
situation.
You should have 2 identical (including firmware version) external
modems supporting both leased line and dumb mode. Make sure your
modems can actually do this! Also make sure your modem is properly
documented.
You also need:
* 2 fully wired shielded RS232 cables. The shield should be
connected to the connector shell (not pin 1) at both ends (not at
one end).
* A RS232 test plug may be handy for test purposes.
* 2 RJ11 cords, one for each end of the leased line.
* A basic understanding of 'AT' commands.
Modem Configuration
A note on modem configuration and init strings in general:
Configure your modem software such as minicom or (m)getty to use the
highest possible speed; 57600 bps for 14k4 and 115200 bps for 28k8 or
faster modems.
Lots of people use very long and complicated init strings, often
starting with AT&F and containing lots of modem brand and -type
specific commands. This however is needlessly complicated.
Most programs feel happy with the same modem settings, so why not
write these settings in the non volatile memory of all your modems,
and only use 'ATZ' as an init string in all your programs. This way
you can swap or upgrade your modems without ever having to reconfigure
any of your software.
Most programs require you to use the following settings;
* Fixed baud rate (no auto baud)
* Hardware bidirectional RTS-CTS flow control (no x-on/x-off)
* 8 Bits, no parity, 1 stopbit
* The modem should produce the TRUE DCD status (&C1)
* The modem should NOT ignore the DTR status (&D2 or &D3)
Check this with AT&V or AT&Ix (consult your modem documentation)
These settings are not necessarily the same as the default factory
profile (&F), so starting an init string with AT&F is probably not a
good idea in the first place. The smart thing to do is probably to use
AT&F only when you have reason to believe that the modem setup stored
in the non volatile memory is really screwed up.
If you think you have found the right setup for your modems, write it
to non volatile memory with AT&W and test it thoroughly with Z-modem
file transfers of both ASCII text and binary files.
Only if all of this works perfectly should you configure your modems
for leased line.
Find out how to put your modem into dumb mode and, more importantly,
how to get it out of dumb mode; The modem can only be reconfigured
when it is not in dumb mode.
Make sure you actually configure your modems at the highest possible
speed. Once in dumb mode it will ignore all 'AT' commands and
consequently will not adjust its speed to that of the COM port, but
will use the speed at which it was configured instead (this speed is
stored in a S-register by the AT&W command).
Now configure your modem as follows;
* Reset on DTR toggle (&D3, this is sometimes a S register). This
setting is required by some ISP's!
* Leased line mode (&L1 or &L2, consult your modem documentation)
* The remote modem auto answer (S0=1), the local originate (S0=0)
* Disable result codes (Q1, sometimes the dumb mode does this for
you)
* Dumb mode (\D1 or %D1, this is sometimes a jumper)
In dumb mode the modem will ignore all AT commands (sometimes you
need to disable the ESC char as well).
Write the configuration to non-volatile memory (&W).
Test
Now connect the modems to 2 computers using the RS232 cables and
connect the modems to each other using a RJ11 lead. Use a modem
program such as Minicom (Linux), procom or telix (DOS) on both
computers to test the modems.
You should be able to type text from one computer to the other and
vice versa. If the screen produces garbage check your COM port speed
and other settings.
Now disconnect and reconnect the RJ11 cord. Wait for the connection to
reestablish itself. Disconnect and reconnect the RS232 cables, switch
the modems on and off, stop and restart Minicom.
The modems should always reconnect at the highest possible speed (some
modems have speed indicator leds).
Check whether the modems actually ignores the ESC (+++) character. If
necessary disable the ESC character.
If all of this works you may want to reconfigure your modems;
Switch off the sound at the remote modem (M0) and put the local modem
at low volume (L1).
Examples
Hi-Tech
This is a rather vague 'no name clone modem'. Its config string is
however typical and should work on most modems.
Originate (local):
ATL1&C1&D3&L2%D1&W&W1
Answer (remote):
ATM0L1&C1&D3&L2%D1S0=1&W&W1
Tornado FM 228 E
This is what should work;
Originate (local):
ATB15L1Q1&C1&D3&L2&W&W1
Answer (remote):
ATM0B15M0Q1&C1&D3&L2S0=1&W&W1
Move the dumb jumper from position 2-3 to 1-2.
Due to a firmware bug, the modems will only connect after being hard
reset (power off and on) while DTR is high. I designed a [3]cirquit
which hard resets the modem on the low to high transition of DTR.
The FreeBSD pppd however, isn't very happy about this. By combining
the setting &D0 with a [4]cirquit which resets on the high to low
transition instead, this problem can be avoided.
Tron DF
The ESC char should be disabled by setting S2 > 127;
Originate:
ATL1&L1Q1&C1&D3S2=171\D1&W
Answer:
ATM0&L2Q1&C1&D3S0=1S2=171\D1&W
US Robotics Courier V-Everything
The following is based on information supplied by [5]Rolf Raar.
The USR Sportster and USR Courier-I do not support leased line. You
need the Courier V-everything version for this job.
There is a webpage on the USR site 'explaining' how to set-up your
Courier for leased line. However, if you follow these instructions you
will end up with a completely brain dead modem, which can not be
controlled or monitored by your pppd.
The USR Courier can be configured with dip switches, however you need
to feed it the config string first.
First make sure it uses the right factory profile. Unlike most other
modems it has three; &F0, &F1 and &F2. The default, which is also the
one you should use, is &F1. If you send it an AT&F, however it will
load the factory profile &F0!
For the reset on DTR toggle you set bit 0 of S register 13. This means
you have to set S13 to 1. Furthermore you need set it to leased line
mode with &L1;
ATS13=1&L1&W
The dip switches are all default except for the following:
3
OFF Disable result codes
4
ON Disable offline commands
5
ON For originate, OFF For answer
8
OFF Dumb mode
PPPD
You need a pppd (Point to Point Protocol Daemon) and a reasonable
knowledge of how it works. Consult the relevant RFC's or the [6]Linux
PPP HOWTO if necessary.
Since you are not going to use a login procedure, you don't use
(m)getty and you do not need a (fake) user associated with the pppd
controlling your link. You are not going to dial so you don't need any
chat scripts either.
In fact, the modem circuit and configuration you have just build, are
rather like a fully wired null modem cable. This means you have to
configure your pppd the same way as you would with a null modem cable.
For a reliable link, your setup should meet the following criteria;
* Shortly after booting your system, pppd should raise the DTR
signal in your RS232 port, wait for DCD to go up, and negotiate
the link.
* If the remote system is down, pppd should wait until it is up
again.
* If the link is up and then goes down, pppd should reset the modem
(it does this by dropping and then raising DTR), and then try to
reconnect.
* If the quality of the link deteriorates too much, pppd should
reset the modem and then reestablish the link.
* If the process controlling the link, that is the pppd, dies, a
watchdog should restart the pppd.
Configuration
Suppose the modem is connected to COM2, the local IP address is
'Loc_Ip' and the remote IP address is 'Rem_Ip'. We want to use 576 as
our MTU.
The /etc/ppp/options.ttyS1 would now be:
crtscts
mru 576
mtu 576
passive
Loc_Ip:Rem_Ip
-chap
modem
#noauth
-pap
persist
So, if the local system is 192.168.1.1 and the remote system is
10.1.1.1, then /etc/options.ttyS1 on the local system would be:
crtscts
mru 576
mtu 576
passive
192.168.1.1:10.1.1.1
-chap
modem
#noauth
-pap
persist
The options.ttyS1 on the remote system would be:
crtscts
mru 576
mtu 576
passive
10.1.1.1:192.168.1.1
-chap
modem
#noauth
-pap
persist
The passive option limits the number of (re)connection attempts.
The persist option will keep pppd alive in case of a disconnect or
when it can't connect in the first place.
If you telnet a lot while doing filetransfers (FTP or webbrowsing) at
the same time, you might want to use a smaller MTU and MRU such as
296. This will make the remote system more responsive.
If you don't care much about telnetting during FTP, you could set the
MTU and MRU to 1500.
The noauth option may be necessary with some newer distributions.
Scripts
Starting the pppd and keeping it alive
The script /usr/local/sbin/PRem_Host.sh shown below starts the pppd.
You have to start it after the COM ports are configured (substitute
Rem_Host with the remote host's name).
#!/bin/bash
# Optional sleep
#/usr/bin/sleep 30
( /usr/sbin/pppd /dev/ttyS1 115200 ) &
It could be called from the script which configures your networkcard
(/etc/init.d/network on Debian systems), in which case you have to
uncomment the sleep, to make sure the COM ports are configured first.
If you use setserial, you could call it from there. An other way is to
call it from rc.local.
On Debian systems, there is a directory /etc/rc.boot/. According to
[7]Rolf Raar, the scripts in this Directory are executed in
alphabetical order. The setserial script is called 0setserial. If you
call your script from a script called 2leased-lines, it will be
executed after 0setserial.
Some older systems will not accept the speed 115200. In this case you
will have to set the speed to 38400 en set the 'spd_vhi' flag with
setserial.
Some systems expect you to use a 'cua' instead of 'ttyS' sevice.
Though the persist option should make this superfluous, the pppd can
also be restarted using ip-down;
#!/bin/bash
case $2 in
/dev/ttyS1)
/usr/bin/sleep 30
/usr/local/sbin/PRem_Host.sh &
;;
esac
If the pppd dies it should be restarted by the persist option or the
ip-up script (optional). If this doesn't happen the script
/usr/local/sbin/test-Rem_Host-ppp restarts it for you. This script
checks whether the remote interface exists and will try to start the
pppd if it doesn't;
#!/bin/bash
if ! ( /sbin/ifconfig | grep Rem_Ip > /dev/null )
then
if [ ! -f /var/lock/LCK..ttyS1 ]
then
# PPP gone
logger "Rem_Host PPP gone ; restarted"
/usr/local/sbin/PRem_Host.sh &
fi
fi
The following line in crontab runs the above script every 5 minutes.
You could run the script more often if you like but I would not run it
more often then once every two minutes.
*/5 * * * * /usr/local/sbin/test-Rem_Host-ppp
You can edit the crontab with 'crontab -e'. This will restart cron
after editing. It uses the editor specified by the 'EDITOR'
environment variable, which may be set by /etc/profile or ~/.profile
(export EDITOR=Your_Favourite_Editor).
Some people run pppd from /etc/inittab, but I never tested this.
Setting the routes
The default route can be set with the defaultroute option or with the
/etc/ppp/ip-up script;
#!/bin/bash
case $2 in
/dev/ttyS1)
/sbin/route add -net 0.0.0.0 gw Rem_Ip netmask 0.0.0.0
;;
esac
Ip-up can also be used to sync your clock using netdate.
Of course the route set in ip-up is not necessarily the default route.
Your ip-up sets the route to the remote network while the ip-up script
on the remote system sets the route to your network. If your network
is 192.168.1.0 and your ppp interface 192.168.1.1, the ip-up script on
the remote machine looks like this;
#!/bin/bash
case $2 in
/dev/ttyS1)
/sbin/route add -net 192.168.1.0 gw 192.168.1.1 netmask 255.255.255.0
;;
esac
The 'case $2' and '/dev/ttyS1)' bits are there in case you use more
than one ppp link. Ip-up will run each time a link comes up, but only
the part between '/dev/ttySx)' and ';;' will be executed, setting the
right route for the right ttyS.
You can find more about routing in the [8]NET-3-HOWTO section on
routing.
Test
Test the whole thing just like the modem test.
If it works, get on your bike and bring the remote modem to the remote
side of your link.
If it doesn't work, one of the things you should check is the COM port
speed;
Apparently, a common mistake is to configure the modems with Minicom
using one speed and then configure the pppd to use an other. This will
NOT work! You have to use the same speed all of the time!
References
1. mailto:
[email protected]
2.
http://www.sput.signature.nl/software/leased-line.html
3.
http://www.sput.signature.nl/hardware/modem-reset.html#l2h
4.
http://www.sput.signature.nl/hardware/modem-reset.html#h2l
5. mailto:
[email protected]
6.
http://metalab.unc.edu/linux/HOWTO/PPP-HOWTO.html
7. mailto:
[email protected]
8.
http://metalab.unc.edu/linux/HOWTO/NET-3-HOWTO.html
