> Our channelized T-1's came in bahaving as you describe below. We
> didn't know that initially and what was happening was that the PM3 would
> answer the line and attach a modem, they it would never handshake with
> the other end. We saw what was happening when we put a test set on the
> T-1 and broke out a channel to listen to it. The dialed number came in
> as DTMF after the PM3 accepted the call, but then nothing--you could
> hear the carrier from the PM3 modem but not the other end even though we
> knew the other modem was sending. At that point I knew that the switch
> (5ESS) wasn't completing the call.
Actually, the call was set up, but the PM3 wasn't supervising (completing)
it.
> I ran into an amazing amount aggrivation when I tried to describe this
> to the Ameritech switch people until I got some guy on the line who asked
> if I wanted to "delete all digits" (huh?). I said "sure, do it" and it
> all started working. The switch was apparently waiting for us to signal
> that the call had been setup on the desired extention but when they
> deleted all the digits so that it didn't send us any of the called number
> it just went ahead and completed the calls to the PM3.
Sort of.
> I'm sure that someone has a much more elegant explanation of all this
> and how we probably asked for the wrong type of service. In any case it's
> working!
Here's a somewhat elegant explanation of how DID is supposed to work, and
what I suspect is happening.
How DID is supposed to work:
1. Call comes in. Telco sends a seizure to you to signal that there's a
call. This seizure remains for the duration of the call.
2. Your equipment should send [1] a momentary wink of supervision to
indicate that it's ready to accept digits.
3. Telco sends digits by DTMF to indicate last portion of dialed number
(extension).
4. Your equipment decodes those digits [2], routes the call, and rings the
extension.
5. Telco cuts through outbound audio so that caller hears ringing, busy,
etc. generated by your equipment. Inbound audio to you is muted. [3]
6. When your extension (modem) answers, you return supervision seizure on
the line for the duration of the call.
7. When either end drops supervision (seizure), the call is torn down.
Back to step one.
My suspicion is that the PM3 is really not designed to work with DID at
all, and is operating in the same fashion as a conventional trunk. When a
call comes in, the telco seizes to ring, the PM3 seizes to answer.
If this is the case, then when the telco set DID for no digits, you got
lucky because that particular switch and software revision no longer
expects the wink - send (zero, in your case) digits - supervise
handshaking routine. The PM3 may not wink at all, just answer and
supervise. The problem is that, if this is the case, and telco later
changes something to expect a wink - pause (no digits) - seizure, or if
other switches configured as DID expect this, then the PM3 will not
connect. It makes _no sense_ to configure DID trunks for zero digits, as
the whole purpose of DID is for the called party to route the call based
on the digits passed by telco. It also makes little sense to order DID
lines for the PM3 if the PM3 has no means of dealing with the DTMF digits
passed. Straight trunks should work just as well, allow outbound calls,
and be easier to set up.
----
[1] Does the PM3 really send a wink, or just answer and return hard
supervision? For DID lines, a wink is specified. My suspicion is that
if this is working at all without a true wink, it's because of a glitch
in the sending switch.
[2] Does the PM3 actually have either DSP or hardware DTMF receivers?
Is it capable of decoding DTMF? Obviously not necessary for standard
operation but will be for DNIS or "real" DID over CT-1.
[3] The telco needs to pass audio back to the caller before the call is
supervised as answered so that the calling party can hear audible ringing
tone or busy signals generated by your equipment. However, because the
call is not officially "answered" until you return supervision, at this
step in the process the call is not yet billable. Telco blocks audio _to_
you to prevent you from defrauding them by suppressing supervision and
conversing two-way without the calling party being billed. The call is
officially answered for billing purposes at step six above, and at this
point the audio (or 56k bitstream) is full duplex.
-- Jay Hennigan jay@west.net --
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