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Overview

When RTP proxy Proxy Relay is enabled and properly configured, SIP signaling is processed by the SBC 1000/2000 the 

The RTP Proxy Relay function enables endpoints on either side of the SBC to negotiate codecs and media capabilities directly between themselves. In RTP Proxy Relay, the SBC is unaware and does not need to support the communications codecs.

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How Proxy Mode Works

In RTP Proxy Relay Mode, RTP packets between endpoints are switched internally within the SBC: the SBC updates IP address and port numbers, but the remainder of the RTP packet is switched unmodified from the input leg to the output leg.

A combination of the Signaling Group and Call Route configuration determines whether DSP or RTP Proxy Relay mode will be used. As an example:

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Overall, the configuration of the Signaling Group’s Audio/Fax Stream Proxy and DSP Modes controls which modes are available to the endpoints.  The call route controls which modes can be used for calls between specific endpoints.

When to Use RTP Proxy Relay Mode

Advantages of Using RTP Mode

• Allows the SBC to establish calls for codecs that it does not support.
  
  
• Improves the calls per second (CPS) of SBC 1000/2000.
  
  
• RTP Proxy Relay calls do not use use no DSPs or SDP licensestranscoding licenses (Enhanced Media Sessions with Transcoding). RTP Proxy Relay calls only use non-transcoding licenses (Enhanced Media Sessions without Transcoding).
  
  
• Improves call quality.
  
  
• Reduces internal call processing.
  

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DSP vs. Proxy in the Call Route

• 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 Relay 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.
    
    

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    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 Relay mode.  The media mode cannot be changed once it has been initially established.
  
  
• If RTP Proxy Relay 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 Relay 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 Relay, 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 Relay 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 Relay, there is no support for local ring back, music on hold, etc. (these require a DSP).
  
  
• RTP Proxy Relay does not support communications between RTP and SRTP endpoints. RTP to SRTP requires DSP transcodingyou to configure the Audio Stream Mode as DSP.