Leased lines and dry pairs¶

Most modems dial through a telephone exchange. A leased line is different: it is a private circuit with no exchange at all, so there is no dial tone, no ringing, and nothing to dial. A dry pair (a bare pair of copper wires you own, run between two buildings for example) works the same way. D-Modem can use both.

For the full technical description, see the technical details.

Getting started¶

Choose the kind of circuit:

AT&L1 for a 2-wire circuit (one pair of wires, both directions share it, like an ordinary phone line).
AT&L2 for a 4-wire circuit (two pairs, one for each direction).

Then set one end to answer and the other to call:

AT&L1S0=1        the answering end, once
AT&L1            the calling end, then ATD

On a leased line ATD connects straight away; the number does not matter. If the connection drops, the modem reconnects by itself two seconds later. ATH, ATZ or AT&L0 stop it doing that.

What runs on each kind of line¶

2-wire (AT&L1): every ordinary modem standard D-Modem supports, from 300 bit/s up to V.34's 33600 bit/s. If the circuit changes in the middle of a call, V.34 and V.32 retrain to the new conditions.
4-wire (AT&L2): as well as the above, the classic leased-line modems: V.29 (up to 9600 bit/s), V.33 (up to 14400 bit/s), V.27ter (up to 4800 bit/s), V.26 and Bell 201 (2400 bit/s), Bell 208 (4800 bit/s) and Bell 202 (1200 bit/s). These keep their signal on for the whole call. If the line gets worse, V.29, V.33 and V.27ter step down to a slower speed by themselves. Two D-Modems connect with any of them; the old Bell and V.26 modems themselves start up differently, so D-Modem may not connect to one.
4-wire digital (AT&L2+MS=V91): on a digital circuit, such as a 64 kbit/s channel between two phone systems, two D-Modems can use V.91 to reach about 62000 bit/s in both directions.
A bare pair with the pico-serial WB stick: the modem uses its own much faster wideband mode instead. See V.wb.

Line tests¶

Leased lines are often checked with loop tests, where one end sends data and it comes back to it. D-Modem has the standard ones (V.54 and V.52):

Command	What it does
`AT&T1`	loop the signal back inside this modem
`AT&T6`	ask the far modem to send our data back
`AT&T7`, `AT&T8`	the same loops, sending a test pattern and counting errors
`AT&T4` / `AT&T5`	allow / refuse the far end's request for a loop (refuse is the default)
`AT&T0`	end the test (after a pattern test, it reports the error count)

The same loops also answer to V.250's standard test commands (AT+TRDL, AT+TAL, AT+TLDL), which add an error rate test you can run on a loop that is already up (AT+TTER, its counts with AT+TNUM?).

Simulated carrier (S48)¶

On a 4-wire line the signal never stops, so the far computer never sees the "carrier detect" light go off between bursts of data. Some equipment needs that. ATS48=1 turns on V.13's simulated carrier, which tells the far end when data starts and stops, so its carrier detect goes on and off as it would on an old half-duplex line. Both ends need it set.

Testing it¶

A test program runs two modems on a simulated circuit and checks each standard, the speed fallback, drop-outs, noise, the loop tests and the simulated carrier. See Building and testing.