Cisco IP Phone 7841 Codec Negotiation
Question
A Cisco IP Phone 7841 that is registered to a Cisco UCM with default configuration receives a call setup message.
Which codec is negotiated when the SDP offer includes this line of text? m=audio 49181 RTP/AVP 0 8 97
Answers
Explanations
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A. B. C. A. E. A. G.D.
Those are the codecs.
They are RTP payload types, and in preference order.
Eg 8=g711alaw, 0=g711ulaw, 18=g729, 97=this one is special, it's actually what they want to Mark RTP-NTP (RFC2833) DTMF relay as.
https://community.cisco.com/t5/ip-telephony-and-phones/sip-sdp-no-codec-specifed/td-p/3913990 https://en.m.wikipedia.org/wiki/RTP_payload_formatsBased on the given SDP offer line "m=audio 49181 RTP/AVP 0 8 97", the negotiated codec will be one of the codecs mentioned in the RTP/AVP field, which represents the audio codecs that are supported by the endpoints. The three codec numbers mentioned in the RTP/AVP field are 0, 8, and 97.
To determine the actual codec being negotiated, we need to refer to the mapping between codec numbers and codec names. This mapping is defined in the RTP Payload Types registry maintained by the Internet Assigned Numbers Authority (IANA).
According to the RTP Payload Types registry, codec number 0 corresponds to the PCMU (G.711ulaw) codec, codec number 8 corresponds to the PCMA (G.711alaw) codec, and codec number 97 corresponds to the iLBC codec. The G.722 codec is not mentioned in the RTP/AVP field, so it is not being negotiated in this case.
Therefore, the correct answer to the question is option B, which is G.711alaw. This means that the call will be set up using the G.711alaw codec, which is a high-quality, uncompressed audio codec that is commonly used in enterprise VoIP deployments.
