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• Call routes will default the Audio/Fax Stream Mode setting to DSP.  This mode should be used unless you have a specific reason for not using the SBC’s on-board DSP resources.
  
  
• Configure the call route Audio/Fax Stream Mode to Proxy if:
  • If the SBC does not have DSPs. 
  • The endpoints share a common codec that the SBC does not support. 
  • It may also be used for situations where the endpoints and the SBC all support a common codec, but for specific reasons, use of the on-board DSP resources is not desired. Be aware that the endpoints or upstream devices are responsible for providing ringback.
    
    
• Use thecall route Proxy Preferred over DSPconfiguration:
  • In an environment where the Signaling Groups have a mixture of Audio/Fax Stream Mode configurations and the RTP Proxy mode is the desired preference. This will permit the call route to prefer the Proxy mode over DSP mode based upon the combination of Stream Mode settings in the ingress and egress Signaling Groups.
    
    
• Use thecall route DSP Preferred over Proxy configuration:

  • In an environment where the Signaling Groups have a mixture of Audio/Fax Stream Mode configurations and DSP mode is the desired preference. This will permit the call route to prefer the DSP mode over  the Proxy mode based upon the combination of Stream Mode settings in the ingress and egress Signaling Groups.
    
    

    Info
    Note that if inbound and outbound 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, the call will be set to either DSP or RTP Proxy mode.  The media mode cannot be changed once it has been initially established.
  
  
• If RTP Proxy mode is desired, RTP Proxy Preferred mode should be used in configurations where a call route can be used by different call types (calls that are not exclusively SIP-to-SIP).

Limitations

• The voice metrics will be collected only in the DSP mode; audio metrics will not be collected for any call routed in the Proxy or Direct Media mode.
  
  
• Dynamic switching between DSP and Proxy modes is not supported. The RTP mode cannot be changed on a call once it has been selected.
  
  
• RTP Proxy does not provide transcoding. Codec mismatches between endpoints will result in call failure.
  
  
• All Forked responses are sent to the caller. Endpoints may not handle it very well.
  
  
• In RTP Proxy, the SRTP mandatory/optional, crypto keys, RTP payload packet size, order of codecs, are all negotiated by the endpoints.
  
  
• Calls will not work between endpoints that cannot create direct calls. (e.g., RTP Proxy does not work between Lync client and Tenor).
  
  
• Transferred calls are not supported.
  
  
• Endpoint-to-endpoint issues, such as one-way audio and no audio, usually manifest as call failures.
  
  
• In RTP Proxy, there is no support for local ring back, music on hold, etc. (these require a DSP).
  
  
• RTP Proxy does not support communications between RTP and SRTP endpoints. RTP to SRTP requires DSP transcoding.

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