Leased line Mini HOWTO
v2.1, 3 August 2000
Configuring your modem and pppd to use a 2 wire twisted pair leased line.
The most recent (beta) version of this HOWTO can be found at:
This document is distributed under the terms of the GNU Free Documentation License.
You should have received a copy along with it. If not, it is available from
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
- This document does NOT deal with:
SLIP, getting or installing pppd, synchronous data communication,
baseband modems, xDSL.
You should already have a working pppd on your system.
You also need Minicom or a similar program to configure your modems.
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.
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
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
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
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).
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
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
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).
This is a rather vague `no name clone modem'. Its config string is however
typical and should work on most modems.
- Originate (local):
- Answer (remote):
Tornado FM 228 E
This is what should work;
- Originate (local):
- Answer (remote):
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
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
which resets on the high to low transition instead, this problem can be
The ESC char should be disabled by setting S2 > 127;
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
The dip switches are all default except for the following:
OFF Disable result codes
ON Disable offline commands
ON For originate, OFF For answer
OFF Dumb mode
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
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
- 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.
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:
Stuff like `asyncmap 0', `lock', `modem' and `-detach' are probably already in
/etc/ppp/options. If not, add them to your
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:
The options.ttyS1 on the remote system would be:
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.
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.
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
Some systems expect you to use a `cua' instead of `ttyS' device.
Setting the routes
The default route can be set with the defaultroute option or with the
case $2 in
/sbin/route add -net 0.0.0.0 gw Rem_Ip netmask 0.0.0.0
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;
case $2 in
/sbin/route add -net 192.168.1.0 gw 192.168.1.1 netmask 255.255.255.0
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.
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
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!