I'm in US West land.
Dave Keal
SIERRA INTERNET
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On Sat, 17 May 1997, Jay Hennigan wrote:
> On Sun, 11 May 1997, Charles Scott wrote:
>
> > 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 --
> WestNet: Internet service to Santa Barbara, Ventura and the world.
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