How exactly does the SCCP protocol work on the UC520 with 79 series phones. How many different codes are there? What is the default codec used? Can you change the codec used to one of lower bandwidth? What is the CLI config area to do this?
This overview helps alot thanks, I think I understand most of this but still a few questions and confirmations.
1) First you define voice class codec 1 for the codecs you want to use and in what priority for calls origonationg from the UC520 to a genaric sip providor?
2) Then you assing the voice class codec to dial peer 1002 and 3009. I presume 1002 is for UC520 to generic sip providor only and 3009 is for generic sip providor to UC520 only and not for internal phones to UC520?
Now is where it is a little more confusing.
1) What is this is the transcoding used for codecs other than 711? If a DID call comes in on dial peer 3009 and it is 729 as nefotiated and prefernced by the assigned voice class codec then does this mean you need transcodeing becouse the phones only support 711? Or is there always a logical break at the UC520 in that irrespective of what codec calls to and from the genric sip providor are in (dial peer 1002 and 3009) the UC520 converts those calls to 711 to go to the phones?
2) In order to force the cisco phones to use a specifc codec to and from the UC520 would you simply removed the unwanted codecs from the dspfarm profile?
The voice class codec 1 is actually there if you configured SIP trunking in CCA and it also appears in the UC500 default configuration file. However, CCA does not yet have an option to adjust codec negotiation.
Remember that Cisco Unity Express can only handle G.711 mu-law audio. It cannot handle G.711 A-law, G.729, G.722, or iLBC audio streams. Transcoding is the process of converting an audio stream from one codec to another codec, and a transcoded call gets transcodec in both directions. In a transcodec call, audio packets from endpoint A to endpoint B are transcoded from codec A to codec B, and audio packets from endpoint B to endpoint A are transcoded from codec B to codec A.
Other situations where transcoding might occur:
Call is transferred to voicemail, auto attendant, teleworker telephone, or is a call using the iLBC or G.722 codec that is transferred to a phone that does not support that codec
Calls from a teleworker phone to a ITSP that does not support the G.729 codec
Calls with G.722 audio that are answered by phones that do not support the G.722 codec
Calls with iLBC audio that are answered by phones that do not support the iLBC codec
Remember that the codecs allowed for a particular leg of the call are the codecs that are listed in all of the following:
the session descriptors within the SIP signaling
the voice class codec list of the inbound dial peer, for inbound calls from non-SCCP endpoints
the voice class codec list of the outbound dial peer, for outbound calls to non-SCCP endpoints
If none of the codecs allowed for the leg of the call between the ITSP and the UC520 unit are supported on the endpoint that is answering the call, the UC520 will either transcode the call or disconnect the call. However, if there is at least one supported codec that is supported by the initiating endpoint, supported by the answering endpoint, listed in the voice class codec list of the inbound dial peer, and listed in the voice class codec list of the outbound dial peer, transcoding will usually not occur, and a codec supported by both endpoints is usually negotiated.
1002 and 3009 were provided as examples from a typical UC520 configuration. There will be dial peers on a UC500 system with tag numbers other than 1002 and 3009, but the format of these dial peers will be similar. And a standard UC520 configuration will usually have at least 5 dial peers for external calls.
I only shown two dial peers in the example, but UC520 systems that use SIP usually have many more dial peers than that. Here are the details about SIP dial peers:
voice class codec , where is substituted with the actual voice class codec tag number, is actually used to determine the codecs to be used for a call coming to or from a particular dial peer, in order from most preferred to least preferred. Codec preferences with lower numbers are more preferred and codec preferences with higher numbers are less preferred. The voice class codec list used for codec negotiation on a particular dial peer is configured with the voice-class codec line in the dial peer configuration (notice that it is voice-class in the dial peer configuration to avoid confusion).
SIP dial peers are in the format dial-peer voicevoip, where is substituted with the actual dial peer number
There are separate dial peers for inbound and outbound calls
Inbound dial peers always have a incoming called-number line
Outbound dial peers always have a destination-pattern line
The UC520 will either negotiate a codec that is supported by both call endpoints, transcode a call, or drop the call if a compatible codec cannot be negotiated. This is one reason why transcoding needs to be properly configured on the UC520.
Dial peers to the ITSP have the session target sip-server line
Additional parameters for the ITSP are specified in the voice service voip and sip-ua sections of the UC500 configuration. There are actually parameters that are not yet exposed in the CCA SIP Trunk window, but might actually get added to the CCA SIP trunk window in the future.
Remember that voice-class codec is not used in CUE VM and AA dial peers. CUE VM and AA dial peers have the codec g711ulaw line, which guarantees that calls to and from CUE are actually using the G.711 mu-law codec.
The dspfarm profile is actually the wrong place to remove unwanted codecs, even though you do not need to have iLBC or G.722 in that list if they are not being used by your ITSP and you have phones that do not support the G.722 or iLBC codecs. The correct place to remove unwanted codecs is in the voice class codec section. Remember that if a codec is not listed in the voice class codec section, it will not be negotiated.
To answer your question about G.729 calls, all of the Cisco IP phone models have support for G.711 mu-law, G.711 A-law, and G.729. The UC520 does not have to transcode G.729 calls, as Cisco Unified IP phones can handle the G.729 codec. Since all Cisco IP phones and Cisco Unity Express support the G.711 mu-law codec, calls do not have to be transcoded if answered by Cisco Unity Express or by on-site IP phones. However, G.711 mu-law might get transcoded to G.729 if answered by a teleworker telephone, depending on how the teleworker phone is configured on the UC520. Since all Cisco IP phones support G.711 A-law codec, calls do not have to be transcoded if answered by on-site IP phones. However, G.711 A-law might get transcoded to G.711 mu-law if answered by Cisco Unity Express, or G.711 A-law might get transcoded to G.729 if answered by a teleworker phone.
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