Missed call communication protocol

Alexandre Ghisoli alex at ghisoli.ch
Thu Feb 8 15:06:52 CET 2007

Le jeudi 08 février 2007 à 11:16 +0100, Richard Bennett a écrit :

> > 2/ For ISDN network (Q.931 signaling), use of the UserInfo field, on the
> > signaling session (yes, you can send some byte when the phone is
> > ringing, without charge)
> Ok... Are you sure you don't mean the 'Display' field in the ISDN SETUP 
> message? I can't find references to a UserInfo field in q931.
> http://www.cotse.com/CIE/Topics/126.htm

Yes, this is the DISPLAY (sorry for the confusion, INFORMATION is a
message itself).

> Apparently the Q.931 protocol limits the maximum length of the Display Field 
> information element to 44 octets, that should mean one can use 44 characters 
> for a message without opening the line, which would be fine.

ITU spec say 2 bytes minimum, up to 34 or 82 bytes, depending of the
network (should be national specific ?)

> It seems that GSM should support the full range of ISDN messages,
> http://kbs.cs.tu-berlin.de/~jutta/gsm/js-intro.html
> so I'm wondering, why is no one using this for enhanced caller-ID, or similar 
> applications. Where's the catch?

I was working on a project, maybe 15 years ago, where we developped a
"modem" to transfer data :
1/ during setup, for free
2/ during the blanks of a conversation (approx 80% of the time, because
only one speaks at time, so you got a free 64kbps channel :))

This takes not very long until telco notice the D channel trafic, and
since then, they charge for alerting, even if no answer (maybe 10% of
unanswered call) to cover this trafic costs. 

Not sure about now, if this still a used (i'm not working anymore on low
level communications).

So, second part was to cut additional messages (SETUP DISPLAY) between
network (aka roaming or international call). This need a more deeper
test, but IMOO, this *could* be working on some networks, and probably
not on some other.

Now, about GSM networks and Gateways, I really dont know if this is a
1:1 passtrough .. 


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