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 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)
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).
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).
This is a rather vague `no name clone modem'. Its config string is however typical and should work on most modems.
This is what should work;
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.
The ESC char should be disabled by setting S2 > 127;
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:
OFF Disable result codes
ON Disable offline commands
ON For originate, OFF For answer
OFF Dumb mode