[Asterisk-Users] Re: call progress - what are the sticking points?

[Joe Greco]

> Joe Greco wrote:
> >>Stephen David wrote:
> >>>i don't have a specific bug in mind, i was just wondering WHY call progress doesn't work so well -- in particular, on analog lines.  ie. is it a hardware or software problem (or both).  with more info, i'd like to help to work out the kinks, for myself and everyone.  :)  
> >>>      
> >>>
> >>Back in the days of Stowger exchanges you knew when the called party 
> >>answered, by a reversal of the DC voltage on your analogue line. With 
> >>digital exchanges that stopped, and no solid feedback is given to the 
> >>caller on ordinary analogue lines. You have to infer that someone has 
> >>answered, and the reliability of that can be poor. Digital lines, like 
> >>ISDNand SS7, and protocols like MFC/R2 tell you positively that someone 
> >>has answered.
> >
> >That's a good explanation.  I'll expand upon it a bit by saying that even
> >with reversal, there's a limited amount of information you can represent
> >with that.  POTS was always intended to be cheap basic phone service, and
> >keeping it simple was not considered a downside by the phone company.
> >
> >As it is, you run into an information representation issue with the
> >existing technology:  the entire "traditionally used" bandwidth of the
> >channel during a call is used for audio data (that is, to say, that they 
> >send an analog signal).  As a call originator, you really can not tell 
> >the difference between a ringing signal generated by the phone company
> >and a ringing signal caused by the called party picking up the phone and
> >playing an identical sound.  Reversal fixed that, but was largely made
> >obsolete by out of band supervision - since the real purpose of reversal
> >was for the telephone company to be able to bill correctly for completed 
> >calls (IIRC, ICBW).
>
> Actually it was not really intentional. The reversal back to the calling 
> party was just a byproduct of the way a Strowger exchange worked. Within 
> the network it was used for billing purposes.

Okay, I remembered that it had been used for billing, and started to
disappear with the advent of digital...  sometimes hard to remember which
came first, the chicken or the egg.  ;-)

> >More difficult is the problem of knowing when the remote end has gone
> >away.  Reversal, loop break, dial tone, and just plain silence are not
> >all that unusual as methods of detection.  In some cases, you do actually
> >need to infer that the remote has gone away.
>
> Hangup is relatively easy. Most lines now give a strong distinct beeping 
> either the moment the phone is dropped, or a short time after. The 
> problem in * is its detector is not very good, or very voice immune. I 
> have a much better one in my spandsp library, but it isn't integrated 
> with * right now. Detecting answer is the tough one. There is nothing 
> unambiguous about it.

Yup.  In fact, I just picked up one of these Sipura 3000's, and it's really
kind of interesting, the phone rings twice (to gather CID) and then the
Sipura answers and continues to generate ringing tones while it does SIP
stuff.  Weird.

> >There's no real excuse for us to be using this technology anymore, with
> >the availability of things like ISDN BRI, which allows for digital
> >signalling of call progress.  However, we continue to use it because the
> >ILEC's have done such a fab job of making ISDN a dead technology.  Funny
> >thing is, it'll end up biting them where it hurts, as customers drift to
> >VoIP to gain the features that ISDN promised, at a fraction of the cost.
>
> As someone whose colleagues built one of the first ISDN muxes in the 
> 80's, I can tell you attitudes made it dead from day one.

I know, I know.  What a damn shame.  They've been trying to kill DSL too,
but I think the realization has finally dawned that the cable company (and
soon the power company, and maybe next year the water company) are all
working to bring high speed access.  With that will inevitably come VoIP.
It won't be a serious contender in the short term, but at the point where
the broadband technology is stable and 98% reliable, and VoIP has mature
E911 support, many people will feel fine giving up their land lines in
exchange for a cell phone plus VoIP.

In the meantime, I'm looking for a cheap but good BRI-to-SIP gateway.
Just ran across the Patton SmartNode 1200.  Anyone know anything about
this?  :-)

... JG
-- 
Joe Greco - sol.net Network Services - Milwaukee, WI - http://www.sol.net
"We call it the 'one bite at the apple' rule. Give me one chance [and] then I
won't contact you again." - Direct Marketing Ass'n position on e-mail spam(CNN)
With 24 million small businesses in the US alone, that's way too many apples.