More on IPCS and Silence Supression

Further to my previous post, Genesys have confirmed that it is not possible to disable silence supression with GVP / IPCS 7.6. I had already found this out in the following post (,4811.0.html) which Genesys support then usefully quoted back to me as the solution!

Regardless, I still think that we have an Avaya MEDPRO problem relating to firmware FW49 and plan to test the latest FW51 as soon as possible.

According to RFC 3389, since the ability to suppress silence is one of the primary motivations for using packets to transmit voice, the RTP header carries both a sequence number and a timestamp to allow a receiver to distinguish between lost packets and periods of time when no data was transmitted. Discontiguous transmission (silence suppression) MAY be used with any audio payload format. Receivers MUST assume that senders may suppress silence unless this is restricted by signaling specified elsewhere. (Even if the transmitter does not suppress silence, the receiver should be prepared to handle periods when no data is present since packets may be lost.)

A Silence suppression attribute is specified in RFC 3108 – the Session Description Protocol (SDP):


RFC 3108 ATM SDP May 2001 The ‘silenceSupp’ attribute

When present, the ‘silenceSupp’ attribute is used to indicate the use
or non-use of silence suppression. The format of the ‘silenceSupp’
media attribute line is as follows:

a=silenceSupp: <silenceSuppEnable> <silenceTimer> <suppPref> <sidUse>
If any of the parameters in the silenceSupp media attribute line is
not specified, is inapplicable or is implied, then it is set to “-“.

m=audio 8804 RTP/AVP 18 0 101
a=rtpmap:0 PCMU/8000
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=yes
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=silenceSupp:off – – – -\r\n

It is not quite clear from clear from RFC3108 if silenceSupp has to be negotiated for unicast connections or it can be different in each direction? For example, can caller say “silenceSupp:on” and this will turn on silence suppression on called side and then called can say “silenceSupp:off” to receive voice without silence suppression?

Regardless, we do not see silenceSupp being specified in the SIP INVITE message from Avaya CM:



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