2.2.5.1.1 "AS" Bandwidth Modifier

A "b=" field with the "AS" (Application-Specific) bandwidth modifier MUST be specified for each media description that corresponds to a stream in the ASF content. The bandwidth that is specified by the "b=" field MUST correspond to the peak bit rate of the ASF stream, if the ASF stream has a peak bit rate that is different from the average bit rate. Otherwise, the bandwidth that is specified by the "b=" field MUST correspond to the average bit rate of the ASF stream.

How a server determines the peak and average bit rates is implementation-specific. The ASF specification [ASF] provides various alternatives which might be used to determine the bit rates. For example, see [ASF] sections 3.12 and 4.1.

Note What is referred to as "Average Bitrate" in [ASF] section 3.12 and "Alternate Data Bitrate" in [ASF] section 4.1 is actually the peak bit rate. What is referred to as "Data Bitrate" in [ASF] section 4.1 is the average bit rate.

A "b=" field with the "AS" bandwidth modifier MUST also be specified once at the SDP session level. In this situation, the value of the attribute MUST be set to the peak bit rate required to stream all of the ASF streams. If an ASF stream does not have an explicitly defined peak bit rate, its average bit rate MUST be used instead.

The following example shows the "a=control" attribute: a=control:rtsp://MS-WMSP-L-S1/OnDemand/"AS" bandwidth modifier:

 b=AS:107