Leased lines and dry pairs: technical details¶

This is the technical companion to Leased lines and dry pairs, the plain-English overview. Start there; this page keeps the full detail: the code's names, the Recommendations' clauses, the numbers and the test lists.

A leased line (a private circuit, or a dry copper pair of your own) has no exchange: no dial tone, no ringing and no hook. D-Modem runs on one with AT&L1 (a 2-wire circuit) or AT&L2 (a 4-wire one). The setting is kept in S47 (AT&L0 is a dialled line).

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

How a call starts and ends¶

• Calling. ATD (with any number, or none) calls at once. Call progress runs only to listen for the answer tone; it does not wait for a dial tone, listen for busy, or dial a number.
• Answering. ATA answers. With S0 above 0, the modem answers by itself, with no ring, whenever it is idle.
• Starting again. When the line ends a call (NO CARRIER), the modem starts the call again by itself 2 s later, dialling or answering as before. ATH, DTR, ATZ or AT&L0 stop this until the next command line.
• Wideband. At 48000 Hz and above (a bare pair driven by pico-serial's WB stick) the modem is always a leased-line modem, in its wideband mode V.wb (dsp/vwb.md).

2-wire: AT&L1¶

Every voice band modulation of +MS runs on AT&L1: V.34, V.32bis, V.32, V.22bis, V.22, Bell 212A, V.23, V.21 and Bell 103, with V.8 and automode as on a dialled call. Both directions share the pair, so the echo cancellers of V.34 and V.32 do the same work they do on a dialled line.

A circuit can change under a call: a reroute on a carrier's network brings a new echo path or delay.

• V.34 retrains when its receiver's error grows (11.5/V.34).
• V.32 and V.32bis retrain too (5.5/V.32). The receiver asks for a retrain when its mean square error stays above its rate's limit for 2 s while the far end's signal is still there. The limit is the rate's least SNR less 4 dB (7 dB at 4800 bit/s). DSP_V32_LOST=0 turns this off.

4-wire: AT&L2¶

On a 4-wire circuit each end sends on its own pair, and the carrier can stay on for the whole call. AT&L2 with one of the 4-wire modulations runs them as duplex data pumps on a continuous carrier (dsp/dp_4wire.cpp):

V27TC is V.250's name; V29, V33, V26 and the Bell ones are D-Modem's. The other modulations run on AT&L2 as they do on AT&L1.

• V.26 and Bell 201 use V.27ter's 2400 bit/s symbols (1200 baud, carrier 1800 Hz), with the data unscrambled as those modems sent it. The dibits' phase changes are those of V.26's alternative A (0, 90, 180, 270 degrees) or B (45, 135, 225, 315: Bell 201). Their marks are phase reversals and their zeros an unmodulated carrier, so in the data neither means the far end is synchronizing: only the loss of the carrier or of the equalization does.
• Bell 208 is V.27ter's 4800 bit/s signal (8-phase at 1600 baud, scrambled with 1 + x^-6 + x^-7), which V.27 took from it, at that rate alone.
• Bell 202 runs on the FSK pump (dsp/dp_fsk.cpp): 1200 Hz mark, 2200 Hz space, 1200 bit/s from each end on its own pair. Each end sends its mark at once, and the data starts once the far end's mark has been steady for 150 ms.

• The DPSK ones start with V.27ter's training as their synchronizing signal. That is D-Modem's own for V.26 and Bell 201, which had none, and Bell 208's own training is not V.27ter's: another D-Modem finds it, but an old modem of these kinds does not.

• Start-up. Each end sends its synchronizing signal at once (10/V.29, 9/V.33). It sends it again 1.2 s after its last one if the far end's has not come. CONNECT comes when this end's receiver has trained on the far end's signal and its own signal has gone out.

• Rates. +MS's highest rate sets the rate an end sends at, and its lowest rate the lowest the end comes down to. Each receiver finds the far end's rate by itself:
• V.29 and V.27ter have no rate signal. Parallel receivers try each rate of the family at once, and the one with the least training error wins.
• V.33 sends its rate in its rate signal (segment 3, 8.3/V.33).

An end that sends faster than the far end comes down to the far end's rate. For V.33 this is the Recommendation's rule (9/V.33); for V.29 and V.27ter it is D-Modem's own. - Resynchronizing. A receiver that loses the far end's signal or its equalization (its error above the rate's limit for 0.3 s) sends its own synchronizing signal, and the far end answers with its own. While this goes on, the DTE's data waits and the received data is held at ones. - Falling back. An end that loses its equalization twice within 20 s at its own rate takes the next rate down: V.33 14400 to 12000, V.29 9600 to 7200 to 4800, V.27ter 4800 to 2400. It never goes below +MS's lowest rate. - Carrier loss. S10 is the time without the far end's signal before NO CARRIER. The call then starts again by itself, as on AT&L1.

V.91 on a 4-wire digital circuit: AT&L2+MS=V91¶

A digital leased circuit (a 64 kbit/s DS0 or E1 time slot, or a G.711 trunk between two PBXs) carries PCM codes each way unchanged. Between two D-Modems on such a circuit, AT&L2+MS=V91 runs V.91, the symmetric PCM modem: up to 62667 bit/s both ways (about 58700 bit/s of data through V.42).

Both ends need a digital line at 8000 Hz: libdmodem with its G.711 law, dmodemd on a PBX or SIP trunk, or the Asterisk and FreeSWITCH modules. On an analogue line +MS=V91 runs V.34.

• Start-up (8.8/V.91). There is no V.8 and no Ez. Each end sends INFO at once (8.2.1.2), and goes on sending it until the far end's comes, however late that end comes up. The rest is V.91's own start-up: INFO', the DIL, CP and B1. CONNECT comes when data mode is up both ways. The rate is the same each way unless the circuit treats the two directions differently.
• Rates. As on a switched call (dsp/v91.md): 62667 on a clean circuit in either law, and less where the circuit robs bits (61333) or pads.
• Retrains and carrier loss. A receiver retrains (sends INFO again) when it decodes poorly, or when the far end's signal is gone (the same value for 6 ms: a cut circuit's idle code). The far end answers its INFO, and LAPM sends again what was lost. With no INFO back within 10 s, the call ends with NO CARRIER and starts again by itself, as on AT&L1.
• No V.34 escape. There is no escape to V.34: 8.3/V.91 is for calls that began with V.8.

Loop tests: AT&T (V.54, V.52)¶

Loop 3¶

Loop 3 (&T1, &T8) runs offline, on a leased line (&L1 or &L2) with a 4-wire modulation (+MS=V29, V33, V27TC, V26, B201 or B208). The pump's signal goes back into its own receiver, and the line is silent.

• &T1 answers CONNECT, and the DTE's data comes back to it through the modulator and demodulator.
• &T8 answers OK and stays in command state.

Loop 2¶

Loop 2 (&T6, &T7, and &T3 at the looping end) is asked for in band from online command state (after +++), on a call with no error correction (\N0). It uses V.54 clause 6's simplified signalling for point-to-point circuits. The patterns are scrambled with 1 + x^-4 + x^-7 and sent at the data rate.

1. This end sends PREP: 2048 bits of scrambled 0s.
2. The far end, if it allows it (&T4), sends ACK (1948 bits of scrambled 1s) and loops what it receives.
3. This end starts the test 2148 bits after it recognises the ACK. &T6 answers CONNECT and the DTE's data comes back to it; &T7 answers OK.
4. AT&T0 sends TERM (8192 bits of scrambled 1s, then 64 1s), after which the far end leaves its loop. ATO then goes back to the call.

With no ACK within 6144 bits after the PREP, the command gives ERROR. The far end also leaves its loop when the carrier goes for 1 s.

The self-test¶

The self-test (&T7, &T8) sends V.52's 511-bit pattern (x^9 + x^5 + 1) in place of the DTE's data. The checker synchronises on what comes back (32 bits in a row that follow the pattern), counts the bits in error, and synchronises again after a slip (64 errors in 256 bits).

Recognising the patterns¶

The recognition criteria are left to the DCE (V.54 5.1). Here, a pattern is present in a 256-bit block when:

• at least 240 of its bits descramble to the pattern's bit, and
• the raw bits have a run of seven of the other value. (Seven 1s keep HDLC flags from passing for PREP.)

PREP is recognised after 4 such blocks in a row, ACK after 3 and TERM after 16.

The code is core/v54.h and core/v54.cpp. It sits between the link and the data pump, in Modem::get_bits() and put_bits().

V.250's test commands (V.58)¶

V.250 6.7 has AT commands for V.58's test objects. They drive the same loops (core/v58.cpp):

The error rate test counts once the checker has synchronised; a block is errored when one of its bits is. The pattern is V.52's 511-bit one only (pattern 1): +TTER's 63-bit, 2047-bit and fixed patterns give ERROR.

Simulated carrier: S48 (V.13)¶

On a continuous carrier the far end's carrier detect never drops. With ATS48=1, on a call with no error correction, V.13's simulated carrier control signals it in band:

• Between the DTE's bursts of data, this end sends the idle pattern: scrambled 1s (1 + x^-3 + x^-7). Before each burst it sends the ON pattern, 8 scrambled 0s, and then the data.
• The far end turns its circuit 109 (carrier detect) OFF after 56 bits of the idle pattern, and ON again after the ON pattern. While 109 is OFF its DTE gets nothing. A 109 left OFF for 2048 bits with no idle pattern comes ON by itself.

A pty has no RTS line, so circuit 105 (request to send) follows the DTE's data. It turns ON with the data's first start bit, and OFF after 64 bits with nothing to send; at least 128 bits of the idle pattern go out before it comes ON again.

The receiver's bits reach the link 64 bits late, so that none of the idle pattern seen before 109 goes OFF reaches the DTE. Without S48 on the far end, the idle pattern reaches its DTE as characters.

The code is core/v13.h and core/v13.cpp.

Tests¶

dsp/test/leased_call_test.cpp runs two of libdmodem's modems at 9600 Hz on a simulated circuit: each way's delay and loss, white noise, and, on a 2-wire circuit, each end's own signal through its hybrid.

V.91 runs on a digital circuit instead (G.711 octets each way at 8000 Hz):

See testing.md for how to run them.