[AVT] Issue with different timestamp frequencies

Dear experts,

I went through an issue that I am not confident to solve by myself, and
for which I would greatly appreciate the advice of this group.

The scenario is: VoIP + DTMF

The issue arose as soon as we introduced WideBand codecs. Indeed, with
SDP we can now offer WB (resolution 16000) as well as NB (resolution
8000) audio codecs, in conjunction with the "telephone-event".
All in a single RTP session.

The SDP offer looks something like:

m=audio 50000 RTP/AVP 97 98 99
a=rtpmap:97 AMR/8000
a=rtpmap:98 AMR-WB/16000
a=rtpmap:99 telephone-event/8000

To my knowledge, nothing prevents to negotiate codecs with different
timestamp frequency, thus, e.g. AMR-WB + DTMF
In such a case however we have a problem with RTCP reports, since they
use the RTP timestamp units (thus 8000 or 16000) for computing the
Interarrival Jitter (in SR and RR) and the RTP Timestamp (in SR) fields.

Since each RTCP report block is scoped by a single SSRC, a possible
solution I thougth about is to send data on the different PTs using
different SSRCs (and thus different SN and TS offsets). This is not
obvious (I haven't read about such a constraint anywhere), and also a
bit heavy in terms of RTCP overhead. I have then legacy implementation
using AMR and DTMF only, that share a single SSRC/TSoffset/SNoffset
space, thus I should probably better employ different sets of
SSRC/TSoffset/SNoffset only when multiple resolutions are negotiated.

Another approach that I considered is to use separate sockets for each
resolution, but this would create more issues with the legacy I have
(and is also a constraint that I have not seen anywhere)

Anyway, the usage of DTMF is just a detail, since the problem would
identically occur for any negotiation of audio codecs with different
sampling rates.

Did I miss some rule/constraint here and there?
Is there any other viable/advisable approach?

Thanks in advance
Guido Franceschini
Telecom Italia

Gruppo Telecom Italia - Direzione e coordinamento di Telecom Italia S.p.A.

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