The SDP includes an optional bandwidth attribute with the following syntax
b=:
where is single alpha numeric word giving the meaning of the bandwidth figure, and where the default units of
bandwidth value is kilobits per second. This attribute specifies the proposed bandwidth to be used by the session or media.
The typical use is with the modifier “AS” (Application specific Maximum) which may be used to specify the total bandwidth for a
single media stream from one site (source)
another modifier value is CT. If the bandwidth of a session or a media in a session is different from the bandwidth implicit from the scope,
a b=“CT” line should be supplied for the session giving the proposed upper limit to the bandwidth used (the “conference total” bandwidth)
The primary purpose of this is to give an approximate idea as to whether two or more sessions can co-exist simultaneously. when using CT
modifier with RTP, if several RTP sessions are part of the conference, the conference total refers to the total bandwidth of all the RTP sessions.
References:
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