
                            Leased line Mini HOWTO
                                       
   Rob van der Putten, [1]rob@sput.signature.nl
   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 &ampD0 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:rob@sput.signature.nl
   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:rolf@tardis.webster.nl
   6. http://metalab.unc.edu/linux/HOWTO/PPP-HOWTO.html
   7. mailto:rolf@tardis.webster.nl
   8. http://metalab.unc.edu/linux/HOWTO/NET-3-HOWTO.html
