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Leased line Mini HOWTO

What is a leased line

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

Modem

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:

• 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

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)

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).

Test

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

Originate:

ATL1&L1Q1&C1&D3S2=171\D1&W

Answer:

ATM0&L2Q1&C1&D3S0=1S2=171\D1&W

US Robotics Courier V-Everything

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

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

crtscts
mru 576
mtu 576
passive
Loc_Ip:Rem_Ip
-chap
modem
-pap
persist

crtscts
mru 576
mtu 576
passive
192.168.1.1:10.1.1.1
-chap
modem
-pap
persist

crtscts
mru 576
mtu 576
passive
10.1.1.1:192.168.1.1
-chap
modem
-pap
persist

Scripts

Starting the pppd and keeping it alive

#!/bin/bash
# Optional sleep
#/usr/bin/sleep 30
( /usr/sbin/pppd /dev/ttyS1 115200 ) &

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

#!/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

*/5 * * * *     /usr/local/sbin/test-Rem_Host-ppp

Some people run pppd from /etc/inittab, but I never tested this.

Setting the routes

#!/bin/bash
case $2 in
     /dev/ttyS1)
          /sbin/route add -net 0.0.0.0 gw Rem_Ip netmask 0.0.0.0
          ;;
esac

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

Test

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