> channels (or timeslots), with one timeslot (0) reserved for
> framing. Many protocols, use timeslot 16 for higher level control;

R2 uses channel 16 as a bank of semaphore bits, for two peer exchanges (it
was designed as an exchange to exchange protocol) to communicate
sieze/ack/release/idle for each channel. Digits, CLID are exchanged inband
on each channel as required.

SS7 controlled IMT's (Inter Machine Trunks) can use channel 16 for calls
(as the signaling link is logically seperate), but some countries carry it
there for convenience. Over a lot of E1s, each one down one channel for
signaling adds up...

> the 64kbps "D" channel, which carries call control and optionally packet
> data, although I don't believe anyone uses this for IP.

AO/DI uses the D-channel to transport IP, in an X.25-like manner. Most
consumer TAs (cisco, Motorola, Lucent, etc) implement it, though naturally
it requires support in the PSTN. It is quite widely deployed in SE Asia.

--
Josh Bailey 
Local Vicar of the Regional Parish of the Cult of Telecoms Engineers
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