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How Each Media Mode Works

The SBC Edge and SBC SWe Lite support the following media modes:

RTP Media Manipulation Mode. A media session that requires media manipulation; these flows may be common with endpoints that communicate across enterprise and public boundaries. Configuration option includes: 

    • DSP. The RTP media packets are processed through the DSP; the SBC bridges any interoperability issues with the peer devices.

RTP Proxy Media Mode. A media session that flows through the SBC but does not require media manipulation. Configuration options include:

    • Proxy Relay. The RTP media packets flow through the SBC's network processor and do not use the DSP; the codecs and SRTP are passed through. For additional details, refer to Working with RTP Proxies.
    • Proxy with local SRTP. Media packets flow the same as Proxy Relay, except encryption services (including SRTP ↔ RTP, authentication and hash algorithms, etc.) are negotiated and processed.

RTP Direct Media Mode. A media session that does not flow through the SBC; these flows are common with endpoints that reside within the same physical premises. Configuration option includes:

    • Direct. The media path does not flow through the SBC. Instead, peer devices have to exchange the media directly with each other while only the SIP messages go through the SBC.

Table : Media Modes Supported

Media Mode and CapabilitySBC EdgeSBC SWe LiteCodec NegotiationSRTP Negotiation
DSP(tick)(tick)SBCSBC
Proxy Relay(tick)(tick)End to EndEnd to End
Proxy with Local SRTP(error)(tick)End to EndSBC
Direct(tick)(tick)End to EndEnd to End

How Multiple Media Streams work

The SBC enables multiple media streams, which includes Video Call Passthrough support. Video codecs are passed transparently through the SBC and are negotiated end to end.

Specific configuration allows multiple media streams to be bundled in the Proxy Relay Media Mode or Direct Media Mode (through Audio/Fax Stream Mode and Video/Application Stream Mode). As a result, all media streams (Audio/Video/Application) are supported by the SBC. The media streams can have mixed (hybrid) modes, which enables audio to be transcoded and video to be in Proxy Relay mode.

  • Proxy Relay Mode enables multiple media streams to be bundled in Proxy Relay Media Mode; all media streams (Audio/Video/Application) are passed through the SBC.
  • Mixed Media (Hybrid) mode allows the SBC to handle multiple streams with different media modes (DSP and Proxy Relay) in the same call; audio is transcoded and video/application is in Proxy Relay Media Mode.
  • Direct Mode enables multiple media streams to be bundled in Direct Media mode; all media streams (Audio/Video/Application) are not passed through the SBC, but the reachable SIP devices are allowed to exchange media directly with each other.

Sonus recommends using Mixed Media (Hybrid) mode over Proxy Relay mode. Mixed Media (Hybrid) mode allows the SBC to handle multiple streams with different media modes (DSP and Proxy) in the same call; audio is transcoded and video/application is in Proxy Relay Media Mode.

Proxy with local SRTP mode introduced in the 7.0 release supports audio stream only, and does not support video.

Scenarios

  • The media streams can be a mixed (Hybrid) mode, allowing audio to be transcoded and video to be in Proxy Relay Media Mode for SIP to SIP calls.
  • The media streams can be an all-proxy mode, allowing both audio and video to be in the Proxy Relay Media Mode for SIP to SIP calls.
  • The media streams can be an all-direct mode, allowing both audio and video to be in the Direct Media Mode for SIP to SIP calls between UAs.

While the audio media codec parameters are configurable in the SBC, there is no video media codec parameter configuration provided by the SBC, and any type of video codec is passed transparently through end to end.

Various modes and call outcomes due to end-to-end compatibility are listed below:

Table : Media Modes and Call Outcomes

Inbound/Outbound peer
Media compatibility
SBC Audio/Video
Mode Configuration
Call OutcomeComments
Audio-codec Compatible & SRTP IncompatibleProxy with local SRTP/DisabledAudioSRTP is negotiated per leg basis, and end-to-end audio call succeeds.
Video-Disabled
Audio-codec incompatible irrespective of SRTP compatibilityProxy with local SRTP/DisabledCall FailureEnd-to-end codec compatibility does not exist.
Video-Disabled
Audio-CompatibleProxy Relay/Proxy Rely or Direct/DirectAudio/video callEnd-to-end compatibility exists for audio and video.
Video-Compatible
Audio-CompatibleDSP/Proxy RelayAudio/video callEnd-to-end compatibility exists for audio and video, and Audio media uses a DSP in the SBC.
Video-Compatible
Audio-CompatibleProxy Relay/Proxy Relay or Direct/DirectAudio callEnd-to-end compatibility does not exist for video.
Video-Incompatible
Audio-CompatibleDSP/Proxy RelayAudio callEnd-to-end compatibility does not exist for video, and Audio media uses a DSP in the SBC.
Video-Incompatible
Audio-IncompatibleProxy Relay/Proxy Relay or Direct/DirectVideo only callEnd-to-end compatibility does not exist for audio, and video only call works provided end-to-end compatibility.
Video-Compatible
Audio-IncompatibleDSP/Proxy RelayAudio/video callEnd-to-end compatibility does not exist for audio and audio media requiring a DSP in the SBC.
Video-Compatible
Audio-IncompatibleProxy Relay/Proxy Relay or Direct/DirectCall failsEnd-to-end compatibility does not exist for audio and video.
Video-Incompatible
Audio-IncompatibleDSP Proxy RelayAudio CallEnd-to-end compatibility does not exist for audio and video and audio media requiring a DSP in the SBC.
Video Incompatible
Audio-IncompatibleProxy Relay/proxy Relay or Direct/DirectCall failsEnd-to-end compatibility does not exist for audio and video.
Video-Incompatible

For Direct Media calls, the SIP peer devices should be reachable by each other without the SBC being in the middle. The media would not go through the SBC, and the immediate peer devices are allowed to send/receive media to each other.

Configuration

The following steps outline the basic configuration for using a Video Call. See Usage Examples below for specific configuration information. For detailed field configuration, see Creating and Modifying SIP Signaling Groups and Creating and Modifying Entries to Call Routing Tables.

Step 1: Configure Media Stream Type

Through the SIP Signaling group's Media Information, a mode could be selected for each media stream type. See Creating and Modifying SIP Signaling Groups.

Step 2: Configure Priority for Media Stream Type

Through the Call Route Table, a media priority can be selected for each of these media stream types (Audio/Fax Stream, Video/Application Stream). See Creating and Modifying Entries to Call Routing Tables.

Usage Examples

Calls in RTP Proxy Mode that do not require DSP

Calls in RTP Proxy Mode that do not require DSP: any codec not suppored by DSP (i.e., Opus) is able to establish an audio path in RTP Proxy mode.

Audio Stream Only - Video Disabled (Default configuration)

 

ComponentHow to Configure
Inbound Signaling Group Entry


  • Audio/Fax Stream Mode - DSP/Proxy Relay/Proxy with local SRTP/Direct
  • Video/Application Stream Mode - Proxy/Direct
Outbound Signaling Group Entry
  • Audio/Fax Stream Mode - DSP/Proxy Relay/Proxy with Local SRTP/Direct
  • Video/Application Stream Mode - Proxy/Direct
Call Route Entry
  • Audio/Fax Stream Mode - DSP
  • Video/Application Stream Mode - Disabled

Proxy Relay Mode - Video Enabled

Proxy Mode enables multiple media streams to be bundled in Proxy Relay Media Mode; all media streams (audio/video/application) are passed through the SBC.

For this configuration there are two outcomes:

  • For Audio Stream only: Audio stream on Proxy Relay Mode.
  • For Multiple Streams: All streams on Proxy Relay Mode.

 

ComponentHow to Configure
Inbound Signaling Group Entry
  • Audio/Fax Stream Mode - DSP/Proxy Relay//Proxy with local SRTP/Direct
  • Video/Application Stream Mode - Proxy/Direct
Outbound Signaling Group Entry
  • Audio/Fax Stream Mode - DSP/Proxy Relay/ Proxy with local SRTP/Direct
  • Video/Application Stream Mode - Proxy/Direct
Call Route Entry
  • Audio: Audio/Fax Stream Mode - Proxy Preferred over DSP
  • Video: Video/Application Stream Mode - Proxy

 

Direct Mode - Video Enabled

Direct Mode enables multiple media streams to be bundled in Direct Media Mode; all media streams (audio/video/application) are bypassed by the SBC.

For Direct Media Calls, the SIP peer devices should be reachable by each other without the SBC being in the middle. The media would not go through the SBC and the immediate peer devices are allowed to send/receive media to each other.

For this configuration, there are two outcomes:

  • For Audio Stream only: Audio Stream on Direct Mode.
  • For Multiple Streams: All Streams on Direct Mode.

 

ComponentHow to Configure
Inbound Signaling Group Entry
  • Audio/Fax Stream Mode - DSP/Proxy Relay/Proxy with local SRTP/Direct
  • Video/Application Stream Mode - Proxy/Direct
Outbound Signaling Group Entry
  • Audio/Fax Stream Mode - DSP/Proxy Relay/Proxy with local SRTP/Direct
  • Video/Application Stream Mode - Proxy/Direct
Call Route Entry
  • Audio/Fax Stream Mode - Direct
  • Video/Application Stream Mode - Direct

Mixed Media (Hybrid) Mode - Video Enabled

Mixed Media (Hybrid) mode allows the SBC to handle multiple streams with different media modes (DSP & Proxy) in the same call; audio is transcoded and video/application is in Proxy Media Mode.

For this configuration, there are two outcomes:

  • For Audio Stream only: Audio stream on DSP Mode.
  • For Multiple Streams: Hybrid with DSP/Proxy Relay.

 

ComponentHow to Configure
Inbound Signaling Group Entry
  • Audio/Fax Stream Mode - DSP/Proxy Relay/Proxy with local SRTP/Direct
  • Video/Application Stream Mode - Proxy/Direct
Outbound Signaling Group  Entry
  • Audio/Fax Stream Mode - DSP/Proxy Relay/Proxy with local SRTP/Direct
  • Video/Application Stream Mode - Proxy/Direct
Call Route Entry
  • Audio/Fax Stream Mode - DSP preferred over Proxy
  • Video/Application Stream Mode - Proxy

Recommendations

Recommendations when using the video mode to avoid seeing video call failures:

  • Disable video mode for E911 configuration.
  • Before enabling video, review peers compatibility end-to-end to handle the offer/answer exchange reliably. In other words, no capability mismatch resolution is provided by the SBC.
  • Any SMM or Call Router level message manipulation should be done carefully.
  • If there is a potential mismatch of capabilities between two Routes, disable Video.

Limitations

Limitations Due to End-to-End Compatibility

  • Call un-hold, might have a period of no video depending upon the the video-fast-update in its SIP-INFO and/or RTCP. After observing different phone end points, this could range from 0 (GrandStream) to 30 seconds(Bria).

  • Early 183 does not function since the SBC does not negotiate codecs with remote peers.

  • Ring-back generation is not available for all-proxy call.

  • Dead call detection does not work for all-proxy calls since the SBC cannot analyze RTP/RTCP streams.

Limitations Due to the Video Call Pass-through Feature

  • Local transfer is not supported for video calls. Sonus recommends configuring the Access Mode as Broadsoft Extension in order to support the transfer feature.
  • Sonus recommends disabling the SIP Session Timer, since it requires SDP and the SBC cannot guarantee media parameter change.

Frequently Asked Questions

What is hybrid mode?
Audio media is processed through the DSPs and Video media is passed through the Network Processor level without involving the DSPs. 

 

When will you use hybrid mode?
This configuration is best suitable for the call routes that need to have audio transcoded, and where the same route needs to handle video calls as well.

 

How does it compare with Proxy mode?
In Proxy Mode, both the audio and video media are passed through the Network Processor level without involving the DSPs. 

 

What comes extra compared with 3.0 release?
For audio calls, the call handling with respect to Offer/Answer remains identical.

 

What comes extra compared with 3.1 release?
For audio calls, the call handling with respect to Offer/Answer remains identical.

 

What is taken away enabling this? (transfer issues, etc.)
There is no difference in audio call processing. However, if the call is video, then transferring the call is supported only in Broadsoft like deployments, wherein it requires configuring the SBC as Broadsoft extension.

This configuration is best suitable for call routes that need to have audio transcoded, and the same route needs to handle video calls as well. See How Sonus SBC Edge and SBC SWe Lite Transfers an RTP Stream.