[asterisk-ss7] chan_ss7 SAM and overlap signaling

[Kai Militzer]

Hello Ercan,

On 01.03.2007 11:59, Ercan Yücebas wrote:
> I just find out that the number of SAM's are depending on from what type
> of line (analog or isdn) you are making the calls
> 
>>From an isdn line, all pending digits are sended in one SAM
> But
>>From an analog line, every pending digit are sended in a separated SAM 

It seems to you you have some problems understanding how SAMs work (should
work) with chan_ss7, so I will try to shed some light on it.

At first you need to know that it depends on the phone on the originating
side of the call and the involved switches on the way to you asterisk if
and how digits are submitted in subsequent address messages. For example
with modern digital or analog DECT phones that dialed number is in most
cases submitted in one block (digital phones do this directly on the ISDN
layer, analog phones because they dial the digits very "closely" to each
other), with an old analog phone you can easily trigger the generation of
SAMs on the originating switch by dialing slowly.


On the asterisk side the handling of SAMs depends on what you are doing and
how your dialplan looks. If you have a "real" dialplan on the asterisk that
terminates the SS7 connection, e.g. because you use it for a call-center,
you may want to handle the matching there. For this case the handling of
SAMs implemented in chan_ss7 works just fine (at least that's what the
source looks like, I have never tested it myself).

If your dialplan is not able to handle SAMs, e.g. because you use your
asterisk only as a "dumb" gateway from SS7 to IAX2 or SIP, it gets
trickier. In this case you only have knowledge if the number is complete,
if the switch you are connected to tells you so by attaching a 0xf after
the last digit to the last SAM. The problem is, that can only be the case
in some defined situations, for example if you reach the maximum of allowed
digits. In all other cases you have to find a way to detect by yourself if
the number is complete. The only way to do this is to use a timer, in this
case the t35 timer. Simply put, after each received IAM or SAM you wait a
defined amount of time if a(nother) SAM arrives. If you receive one, the
timer is stopped and started again to wait for another SAM and the
number(s) in the SAM are put at the end of the already received digits
(from the IAM and/or previous SAMs). If the timer times out without
receiving another SAM the dialed number is complete and you can transmit it
to asterisk, that then matches it with the dialplan.

In chan_ss7 you can define the timeout for t35 and if a received 0xf in an
IAM or SAM should stop the timer, the only problem is, that at least in
chan_ss7-0.8.4 this is broken and works not as it should. I attach a patch
that I think fixes this behavior, but I cannot guarantee that it works, and
won't break any other wanted behavior handling SAMs and t35.

I hope I could make you understand the handling of SAMs in asterisk and
chan_ss7 a bit better, if not, feel free to ask.

Regards,
Kai

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