goldstein@delni.DEC.COM (Fred R. Goldstein dtn226-7388) (11/03/87)
re: V8I21 question about the 7405,
No, you shouldn't unplug it. The 7405, like its forebears the Rolm ETS,
the SL-1set, the AT&T ECTS and other electronic sets, is electrically not
much at all like a Plain Black Phone. There are two major differences.
One, the 7405 (like other AT&T 74- and 75-series sets, but not the
71-, 72- and 73-series) uses digital transmission. There's a codec in
the set which converts the analog microphone signal into a 64kbps stream,
and vice versa to the earpiece.
More important is the way all of these electronic sets do their signaling.
They don't use conventional analog techniques (relays, HV ringers, etc.);
instead, there's bandwidth muxed into the digital link which carries signaling
messages. The 74-series uses AT&T's proprietary link format, the newer
75-series is an ISDN-based format (more standarized). In the 7405, when
you pick up (or hang up) or press a button or whatever, a message is sent
down the signaling channel. When the switch wants to signal you, it sends
a message up the signaling channel which the set translates to mean
things like "ring" and "display FWD 6399".
Now even when the set is idle, there's a constant stream of bits on the
wire. If you unplug the set, the PBX line card won't see the bits coming
from your set, and will assume that something's wrong, sound an alarm
(probablay write it to a maintenance log in memory) and possibly shut down
your line card (I don't know if it actually does that but it might).
And if it does shut down your line card, your set won't work when you
plug it back in. Either way, the maintenance folks will be peeved.
So the circuit cards in the set do voice digitization, message-oriented
signaling (rather primitive actually), multiplexing and line driving of
the digital link back to the switch. No wonder these cost more than
2500 sets.
fred