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Anchor, bypass, transcode, and DTMF

Anchoring terminates RTP on the SBC. Bypass lets endpoints exchange media. Transcoding and DTMF interworking need the media path.

Learn in 5 minutes

  1. Separate anchoring from bypass
  2. Tie transcoding to an anchored path
  3. Treat DTMF interworking as a media function

Visual

Anchored answer SDP · study drawing · cyphermonkeylabs.com AliceSBCCarrier INVITE INVITE 200 200

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Capacity for transcoding is platform-specific.

Worked example

Negotiate an anchored answer Alice offers PCMU and PCMA. The anchored answer keeps PCMU and telephone-event.

    Fault

    The carrier is supposed to send RTP to the SBC, but the INVITE still advertises the phone.

    c=IN IP4 203.0.113.10 tells the carrier to send RTP to the phone.

    Quiz

    Media anchoring means


    Media bypass means


    Transcoding requires


    Where do you read how many transcoded sessions a box supports?


    RFC 4733 telephone-event is


    In the anchored ladder, Alice’s 200 OK uses c=


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