LEASED LINE MINI HOWTO

  Rob van der Putten, [email protected]
  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:
    * 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

  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:
    * 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 DCD status (&D2 or &D3)

  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;
    * Reset on DTR toggle (&D3, this is sometimes a S register)
    * 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 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 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;
    * 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
-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.

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