Leased line Mini HOWTO

Rob van der Putten, rob@sput.webster.nl
v1.4, 5 July 1998
The most recent (beta) version of this HOWTO can be found at: www.sput.webster.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 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:

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;

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;

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

Originate (local):
ATL1&C1&D3&L2%D1&W&W1
Answer (remote):
ATM0&C1&D3&L2%D1S0=1&W&W1

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 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 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;

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
-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
-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
-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.

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 (subsitute 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 comports 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 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.

Though the persist option should make this superfluous, the pppd can also be restarted using ip-down;

#!/bin/bash
case $s 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
     # PPP gone
     logger "Rem_Host PPP gone ; restarted"
     /usr/local/sbin/PRem_Host.sh &
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 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.