The payload header consists of a 4 bit codec mode request.If octet aligned operation is used the payload header is padded to fill an octet and optionally an 8 bit interleaving header may extend the payload header. The bits in the header are specified as follows:
CMR (4 bits): Indicates Codec Mode Requested for the other communication direction. It is only allowed to request one of the speech modes of the used codec, frame type index 0..7 for AMR, see Table 1a in [2] or frame type index 0..8 for AMR-WB, see Table 1a in [4]. CMR value 15 indicates that no mode request is present, other values are for future use. It is RECOMMENDED that the encoder follows a received mode request, but if the encoder has reason for not follow the mode request, e.g. congestion control, it MAY use another mode. The codec mode request (CMR) MUST be set to 15 for packets sent to a multicast group. The encoder in the sender SHOULD ignore mode requests when sending to a multicast session but MAY use RTCP feedback information as a hint that a mode change is needed. The codec mode selection MAY be restricted by the mode set definition at session set up. If so, the selected codec mode MUST be in the signaled mode set.
R: Is a reserved bit that MUST be set to zero. All R bits MUST be ignored by the receiver.
If the use of interleaving is signaled out of band at session set up, octet aligned operation MUST be used. When interleaving is used the payload header is extended with two 4 bit fields, ILL and ILP, used to describe the interleaving scheme.
ILL (4 bits): OPTIONAL field that is present only if interleaving is signaled. The value of this field specifies the interleaving length used for frames in this payload.
ILP (4 bits): OPTIONAL field that is present only if interleaving is signaled. The value of this field indicates the interleaving index for frames in this payload. The value of ILP MUST be smaller than or equal to the value of ILL. Erroneous value of ILP SHOULD cause the payload to be discarded.
The value of the ILL field defines the length of an interleave group: ILL=L implies that frames in (L+1)-frame intervals are picked into the same interleaved payload, and the interleave group consists of L+1 payloads. The size of the interleaving group is the N*(L+1), if N is the number of frames per payload. The value of ILL MUST only be changed between interleave groups. The value of ILP=p in payloads belonging to the same group runs from 0 to L. The interleaving is meaningful only when the number of frames per payload (N) is greater than or equal to 2. All payloads in an interleave group MUST contain equally many speech frames. When N frames are transmitted in each payload of a group, the interleave group consists of payloads with sequence numbers s...s+L, and frames encapsulated into these payloads are f...f+N*(L+1)-1.
To put this in a form of an equation, assume that the first frame of an interleave group is n, the first payload of the group is s, number of frames per payload is N, ILL=L and ILP=p (p in range 0...L), the frames contained by the payload s+p are n + p + k*(L+1), where k runs from 0 to N-1. I.e.
The first packet of an interleave group: ILL=L, ILP=0
Payload: s
Frames: n, n+(L+1), n+2*(L+1), ..., n+(N-1)*(L+1)
The second packet of an interleave group: ILL=L, ILP=1
Payload: s+1
Frames: n+1, n+1+(L+1), n+1+2*(L+1), ..., n+1+(N-1)*(L+1)
The last packet of an interleave group: ILL=L, ILP=L
Payload: s+L
Frames: n+L, n+L+(L+1), n+L+2*(L+1), ..., n+L+(N-1)*(L+1)
references
https://www.ietf.org/proceedings/51/I-D/draft-ietf-avt-rtp-amr-10.txt
CMR (4 bits): Indicates Codec Mode Requested for the other communication direction. It is only allowed to request one of the speech modes of the used codec, frame type index 0..7 for AMR, see Table 1a in [2] or frame type index 0..8 for AMR-WB, see Table 1a in [4]. CMR value 15 indicates that no mode request is present, other values are for future use. It is RECOMMENDED that the encoder follows a received mode request, but if the encoder has reason for not follow the mode request, e.g. congestion control, it MAY use another mode. The codec mode request (CMR) MUST be set to 15 for packets sent to a multicast group. The encoder in the sender SHOULD ignore mode requests when sending to a multicast session but MAY use RTCP feedback information as a hint that a mode change is needed. The codec mode selection MAY be restricted by the mode set definition at session set up. If so, the selected codec mode MUST be in the signaled mode set.
R: Is a reserved bit that MUST be set to zero. All R bits MUST be ignored by the receiver.
If the use of interleaving is signaled out of band at session set up, octet aligned operation MUST be used. When interleaving is used the payload header is extended with two 4 bit fields, ILL and ILP, used to describe the interleaving scheme.
ILL (4 bits): OPTIONAL field that is present only if interleaving is signaled. The value of this field specifies the interleaving length used for frames in this payload.
ILP (4 bits): OPTIONAL field that is present only if interleaving is signaled. The value of this field indicates the interleaving index for frames in this payload. The value of ILP MUST be smaller than or equal to the value of ILL. Erroneous value of ILP SHOULD cause the payload to be discarded.
The value of the ILL field defines the length of an interleave group: ILL=L implies that frames in (L+1)-frame intervals are picked into the same interleaved payload, and the interleave group consists of L+1 payloads. The size of the interleaving group is the N*(L+1), if N is the number of frames per payload. The value of ILL MUST only be changed between interleave groups. The value of ILP=p in payloads belonging to the same group runs from 0 to L. The interleaving is meaningful only when the number of frames per payload (N) is greater than or equal to 2. All payloads in an interleave group MUST contain equally many speech frames. When N frames are transmitted in each payload of a group, the interleave group consists of payloads with sequence numbers s...s+L, and frames encapsulated into these payloads are f...f+N*(L+1)-1.
To put this in a form of an equation, assume that the first frame of an interleave group is n, the first payload of the group is s, number of frames per payload is N, ILL=L and ILP=p (p in range 0...L), the frames contained by the payload s+p are n + p + k*(L+1), where k runs from 0 to N-1. I.e.
The first packet of an interleave group: ILL=L, ILP=0
Payload: s
Frames: n, n+(L+1), n+2*(L+1), ..., n+(N-1)*(L+1)
The second packet of an interleave group: ILL=L, ILP=1
Payload: s+1
Frames: n+1, n+1+(L+1), n+1+2*(L+1), ..., n+1+(N-1)*(L+1)
The last packet of an interleave group: ILL=L, ILP=L
Payload: s+L
Frames: n+L, n+L+(L+1), n+L+2*(L+1), ..., n+L+(N-1)*(L+1)
references
https://www.ietf.org/proceedings/51/I-D/draft-ietf-avt-rtp-amr-10.txt
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