From: Minh N. Hoang
Date: Thursday, 30 May 1985

Recent messages indicated that quite a few modems are getting into
remote digital loops inadvertently so I poked around to find out how
the modems intentionally get into this.  The following comes from
CCITT V.22bis recommendation which specifies the 2400/1200 bps modem.
Except for the answer back tone, the 1200 mode is 212-compatible,
including the loop.

For reference, modem A is the instigator and modem B the one that
loops back data. (All the modems that see data echoed back are
instigators.) Here it goes:

*** Entering remote loopback ***

When modem A is instructed to instigate a remote loop, it shall
transmit an initiation signal of unscrambled binary one at the current
operating speed.  (If you happen to listen to this, you'd hear a tone
instead of the usual hiss-like noise.)

Modem B shall detect 154 - 231 ms. of the initiation signal, and then
transmit to modem A scrambled alternating binary 1 and 0. (And your
modem faithfully passes this to your terminal - hence the UUUUs).

Modem A shall detect 231 - 308 ms. of scrambled reversals, cease
transmission of the initiation signal, and then transmit scrambled
binary 1.

Modem B shall detect the loss of initiation signal and activate the
loopback.  (Now, everything modem A transmits will be "echoed").

Modem A after receiving 231 - 308 ms. of scrambled binary 1 (which it
sent), shall indicate to the terminal that it may begin sending test
messages.  (Modem A will now send terminal's data as usual so you can
see all you type).

*** Exiting remote loopback ***

When modem A is instructed to terminate the remote loop, the line
signal shall be suppressed for 77 +- 10 ms, after which transmission
shall be restored. (A very short simulated carrier loss).

Modem B detects the loss of signal in 40 - 65 and its reappearance
within 155 +- 50 ms, after which modem B returns to normal operation.
(For most modems, carrier loss of >300 ms may cause disconnect).

*** -+- ***

Thus, if you're inadvertently dumped into this loop and your modem
doesn't have a remote loop switch, it's kinda tough to get out without
losing the connection.

How does your modem initiate this loop?  (It's usually not modem B's
fault.)  A common cause is called scrambler lock-up.  These modems use
a scrambler of the form D_tx[n] = D_in[n] .XOR. D_tx[n-14] .XOR.
D_tx[n-17] and transmit the scrambled output.  If the scrambler's
delay line happens to be filled with 1's, then the modem effectively
sends unscrambled 1's, the initiation signal.  The other modem
responds with UUUUs. You type a few characters in panic and end the
lock-up.  The other modem senses the end of the initiation signal and
voila... you're looping.

To prevent this, once the scrambler outputs 64 consecutive 1's, it
should invert the next input bit.  Sometimes, the modem doesn't
incorporate this detector-inverter because the situation is relatively
rare, supposedly.

Cheers,