[SIPForum-discussion] Decoding G711 audio

Thu May 22 09:10:17 UTC 2008

Well Rajesh, I'm afraid it may not be as simple as that.

It all rather depends on what the other end you want to communicate also
wants to talk. Some one more up with all the intricacies of the different
SIP RFCs will be better qualified to comment than me, but there is no way,
as I understand it, to negotiate that you are going to transfer DTMF via SIP
INFO.

You might get it, you might not.

The only one of the 3 methods that you actually negotiate is RFC2833 in the
SDP.

It all depends on your final application, so if you have a setup where you
know exactly what the other end will be setup to do, and you can rely on it
handling SIP INFO, then great. If you want your application to talk to more
generic SIP phones, then you might not be so lucky, as it will depend on how
they are configured.

For example, one of our test phones is a Grandstream
(http://www.voiptalk.org/products/Video+Phones/Grandstream+GXV-3000+IP+Video
phone), and the setting on whether it will send its DTMF in SIP INFO is in
its config, and I have yet to see any indication of the setting in any of
the SIP call setup messaging coming out of it.

Welcome to the wonderful, dubiously defined, world of SIP :-)

Regards

Malcolm Howlett

  _____  

From: mfc at aol.in [mailto:mfc at aol.in] 
Sent: 22 May 2008 09:48
To: malcolm at c3.co.uk
Cc: discussion at sipforum.org
Subject: Re: [SIPForum-discussion] Decoding G711 audio

I thank you for your valuable information and time, Mr. Malcom. I've
selected the SIP INFO message way of doing this. Thanks again and I really
appreciate your help. There's just the third question left unanswered and
I'm hoping that someone would answer that too. :-)

Rajesh R.

-----Original Message-----
From: Malcolm Howlett <malcolm at c3.co.uk>
To: discussion at sipforum.org
Sent: Thu, 22 May 2008 1:33 pm
Subject: Re: [SIPForum-discussion] Decoding G711 audio

  _____  

From: mfc at aol.in [mailto:mfc at aol.in] 
Sent: 22 May 2008 06:26
To: discussion at sipforum.org
Subject: [SIPForum-discussion] Decoding G711 audio

Hello all,

I have a few queries and I am totally new to VoIP.

|1. Is there a way to convert G711 (both alaw and mulaw) back to PCM? I
searched through the internet and am still |searching, but I thought I will
put up this query to you people as someone might have experienced this
problem |already.

Well at a minimum there are lots of people who do hardware cards with DSPs
that do it. Aculab, Pika technologies are 2 companies that spring to mind,
not a cheap alternative

2. How are DTMF tones transmitted over a SIP call? If I dial say 34 when a
call is mature, how is this tone sent to the other end? What are the
commands used? Is it sent as a MESSAGE?

Any one of (and indeed combination of) 3 ways

In band audio - dependant on the audio codec as some of them can't carry it

RFC2833 - negotiated in SDP

SIP INFO messages

3. Is there a way to retrieve all the registered client information (the
client extension and ID/name) from the SIP Server/Registrar? Is this
possible? Can you provide me some leads to a program or some API/Functions?

I thank you all for your time. I appreciate it.

Warm Regards,
Rajesh R.

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