  Leased line Mini HOWTO
  Rob van der Putten, rob@sput.nl
  v2.1, 3 August 2000

  Configuring your modem and pppd to use a 2 wire twisted pair leased
  line.
  ______________________________________________________________________

  Table of Contents


  1. Introduction

     1.1 What is a leased line
     1.2 Assumptions

  2. Modem

     2.1 Modem Configuration
     2.2 Test
     2.3 Examples
        2.3.1 Hi-Tech
        2.3.2 Tornado FM 228 E
        2.3.3 Tron DF
        2.3.4 US Robotics Courier V-Everything

  3. PPPD

     3.1 Configuration
     3.2 Scripts
        3.2.1 Starting the pppd and keeping it alive
        3.2.2 Setting the routes
     3.3 Test


  ______________________________________________________________________

  The most recent (beta) version of this HOWTO can be found at:
  http://www.sput.nl/software/leased-line/


  1.  Introduction

  1.1.  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, xDSL.



  1.2.  Assumptions

  You should already have a working pppd on your system.  You also need
  Minicom or a similar program to configure your modems.

  2.  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.


  2.1.  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).

  2.2.  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).

  2.3.  Examples

  2.3.1.  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


  2.3.2.  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 circuit 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 &D0 with a circuit which resets on the high to low transition
  instead, this problem can be avoided.

  2.3.3.  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


  2.3.4.  US Robotics Courier V-Everything

  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


  3.  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;


  ·  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.


  3.1.  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



  Stuff like `asyncmap 0', `lock', `modem' and `-detach' are probably
  already in /etc/ppp/options. If not, add them to your
  /etc/ppp/options.ttyS1.  So, if the local system is 192.168.1.1 and
  the remote system is 10.1.1.1, then /etc/ppp/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 sys­
  tem more responsive.  If you don't care much about telnetting during
  FTP, you could set the MTU and MRU to 1500.  Keep in mind though, that
  UDP cannot be fragmented. Speakfreely for instance uses 512 byte UDP
  packets. So the minimum MTU for speakfreely is 552 bytes.  The noauth
  option may be necessary with some newer distributions.

  3.2.  Scripts

  3.2.1.  Starting the pppd and keeping it alive

  You could start the pppd form a boot (rc) script. However, if you do
  this, and the pppd dies, you are without a link.  A more stable
  solution, is to start the pppd from /etc/inittab;



       s1:23:respawn:/usr/sbin/pppd /dev/ttyS1 115200


  This way, the pppd will be restarted if it dies.  Make sure you have a
  `-detach' option (nodetach on newer systems) though, otherwise inittab
  will start numerous instances of pppd, will complaining about
  `respawning too fast'.

  Note: 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' device.

  3.2.2.  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 Linux Networking HOWTO section on routing.

  3.3.  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!



