SBC 1000/2000 4.1.x : Working with RTP Proxies


When RTP proxy is enabled and properly configured, SIP signaling is processed by the SBC 1000/2000 and the audio packets are switched without being processed through one of the DSPs. This enables endpoints on either side of the SBC to negotiate codecs and media capabilities without the need for the SBC to know about or support them. This feature is customizable through SIP Signaling Groups and Call Routing.

When to Use RTP Mode

Endpoints have some common capabilities, that would negotiate the Offer/Answer (codec negotiation) either direction.  In other words, should both endpoints make a direct call without SBC 1000/2000 it should work

Advantages of Using RTP Mode

  • It allows the SBC to establish calls that it otherwise cannot.
  • This improves the CPS of SBC1k/2K.
  • RTP Proxy calls use use no DSPs or SDP licenses.
  • Improved call quality.
  • Less internal call processing.

Call Routing Notes

  • Default mode of operation is the DSP mode.
  • RTP Proxy mode shall be used if the SBC doesn’t have DSPs or the endpoints on both sides of SBC1K/2K are the same or have similar capabilities.
  • RTP Proxy preferred mode guarantees RTP proxy mode if the inbound and outbound SGs are SIP and allow proxy mode otherwise the call will be connected in DSP mode.
  • Note that if inbound and out bound SG don’t have common media mode the call will be rejected in any media mode of operation defined in the routing entry.
  • Once the media mode is determined by CCC, SIP and MSC will be set to either DSP or RTP proxy mode and the call cannot transition to another mode until it gets released.
  • RTP Proxy preferred mode is mode known only by the Call Router and CCC. This should be used when routing entries can be used by different (TDM, SIP). 


  • Dynamic switching between DSP and Proxy modes is not supported in the same call.
  • Does not provide capability mismatch resolution.
  • All Forked responses are sent to caller.  Endpoints may not handle it very well.
  • SRTP mandatory/optional, crypto keys, RTP payload packet size, order of codecs, all endpoints negotiated. 
  • Calls can fail should endpoints fail. (e.g.,  RTP proxy does not work between Lync client and Tenor).
  • Transferred calls are not supported.
  • Endpoint-to-endpoint issues, such as on-way audio and no audio, usually manifest as call failures.
  • No support for local ring back, music on hold, etc. (these require the DSP).






What are the SDP licenses? Do you mean SBC licenses?

Posted by ukabir at Mar 11, 2015 11:56

Session Description Protocol (SDP) is a set of rules defining how multimedia sessions are set up to allow all end points to effectively participate in the session.

Pulled from this page:

Posted by jasonw at Mar 11, 2015 13:05

I am commenting on this phrase: "RTP Proxy calls use use no DSPs or SDP licenses."

What are SDP licenses? I believe you meant SBC? plus can you fix the sentence? Thx Umar

Posted by ukabir at Mar 11, 2015 13:11