Leased line Mini HOWTO

Rob van der Putten, rob@sput.dsl.nl
v1.2, 3 March 1998

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 FDDI, 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

Configure your modem at the highest possible speed; 57600 bps for 14k4 and 115200 bps for 28k8 or faster modems. Once the modem is in dumb mode it will use the speed at witch it was configured.
Configure both your modem software (Minicom) and your modem to use the following settings: Check this with AT&V or AT&Ix (consult your modem documentation)

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.

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 one of the screens 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

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 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

>From the script which configures your networkcard (/etc/init.d/network on Debian systems) the script /usr/local/sbin/test-Rem_Host-ppp is called (substitute 'Rem_Host' with the remote hosts name). This script checks whether the remote interface exists and will try to start the pppd if it doesn't. It starts with a sleep, making sure the boot process configures the COM ports first.
#!/bin/bash
/usr/bin/sleep 30

while true
do
     if ! ( /sbin/ifconfig | grep Rem_Ip > /dev/null )
     then
          # PPP gone
               logger "Rem_Host PPP gone ; restarted"
               /usr/local/sbin/PRem_Host.sh &
     fi
     sleep 300
done
You could of course, remove the 'sleep 300' and the do-done loop and start the whole thing from cron instead.
Some people run pppd from /etc/inittab, but I never tested this.

The default route can be set with the defaultroute option or with a /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

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
The pppd is started using the /usr/local/sbin/PRem_Host.sh script
#!/bin/bash
( /usr/sbin/pppd /dev/ttyS1 115200 crtscts Loc_Ip:Rem_Ip persist ) &
I suppose some of the command line options make some of the options statements superfluous. Well, better safe then sorry.

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.