rfc3267.txt

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         this case at the other end of the IP path.  Feedback about
         congestion SHOULD be provided to that IP sender through RTCP or
         other means, and then the sender can choose to avoid congestion
         using the most appropriate mechanism.  That may include
         adjusting the codec mode, but also includes adjusting the level
         of redundancy or number of frames per packet.

   The encoder SHOULD follow a received mode request, but MAY change to
   a lower-numbered mode if it so chooses, for example to control
   congestion.

   The CMR field MUST be set to 15 for packets sent to a multicast
   group.  The encoder in the speech sender SHOULD ignore mode requests
   when sending speech 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 a session parameter to
   a subset of the available modes.  If so, the requested mode MUST be
   among the signalled subset (see Section 8).

4.3.2. The Payload Table of Contents

   The table of contents (ToC) consists of a list of ToC entries, each
   representing a speech frame.





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RFC 3267        RTP Payload Format for AMR and AMR-WB          June 2002


   In bandwidth-efficient mode, a ToC entry takes the following format:

    0 1 2 3 4 5
   +-+-+-+-+-+-+
   |F|  FT   |Q|
   +-+-+-+-+-+-+

   F (1 bit): If set to 1, indicates that this frame is followed by
      another speech frame in this payload; if set to 0, indicates that
      this frame is the last frame in this payload.

   FT (4 bits): Frame type index, indicating either the AMR or AMR-WB
      speech coding mode or comfort noise (SID) mode of the
      corresponding frame carried in this payload.

   The value of FT is defined in Table 1a in [2] for AMR and in Table 1a
   in [4] for AMR-WB.  FT=14 (SPEECH_LOST, only available for AMR-WB)
   and FT=15 (NO_DATA) are used to indicate frames that are either lost
   or not being transmitted in this payload, respectively.

   NO_DATA (FT=15) frame could mean either that there is no data
   produced by the speech encoder for that frame or that no data for
   that frame is transmitted in the current payload (i.e., valid data
   for that frame could be sent in either an earlier or later packet).

   If receiving a ToC entry with a FT value in the range 9-14 for AMR or
   10-13 for AMR-WB the whole packet SHOULD be discarded.  This is to
   avoid the loss of data synchronization in the depacketization
   process, which can result in a huge degradation in speech quality.

   Note that packets containing only NO_DATA frames SHOULD NOT be
   transmitted.  Also, frame-blocks containing only NO_DATA frames at
   the end of a packet SHOULD NOT be transmitted, except in the case of
   interleaving.  The AMR SCR/DTX is described in [6] and AMR-WB SCR/DTX
   in [7].

   The extra comfort noise frame types specified in table 1a in [2]
   (i.e., GSM-EFR CN, IS-641 CN, and PDC-EFR CN) MUST NOT be used in
   this payload format because the standardized AMR codec is only
   required to implement the general AMR SID frame type and not those
   that are native to the incorporated encodings.

   Q (1 bit): Frame quality indicator.  If set to 0, indicates the
      corresponding frame is severely damaged and the receiver should
      set the RX_TYPE (see [6]) to either SPEECH_BAD or SID_BAD
      depending on the frame type (FT).





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RFC 3267        RTP Payload Format for AMR and AMR-WB          June 2002


   The frame quality indicator is included for interoperability with the
   ATM payload format described in ITU-T I.366.2, the UMTS Iu interface
   [16], as well as other transport formats.  The frame quality
   indicator enables damaged frames to be forwarded to the speech
   decoder for error concealment.  This can improve the speech quality
   comparing to dropping the damaged frames.  See Section 4.4.2.1 for
   more details.

   For multi-channel sessions, the ToC entries of all frames from a
   frame-block are placed in the ToC in consecutive order as defined in
   Section 4.1 in [24].  When multiple frame-blocks are present in a
   packet in bandwidth-efficient mode, they will be placed in the packet
   in order of their creation time.

   Therefore, with N channels and K speech frame-blocks in a packet,
   there MUST be N*K entries in the ToC, and the first N entries will be
   from the first frame-block, the second N entries will be from the
   second frame-block, and so on.

   The following figure shows an example of a ToC of three entries in a
   single channel session using bandwidth efficient mode.

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|  FT   |Q|1|  FT   |Q|0|  FT   |Q|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Below is an example of how the ToC entries will appear in the ToC of
   a packet carrying 3 consecutive frame-blocks in a session with two
   channels (L and R).

   +----+----+----+----+----+----+
   | 1L | 1R | 2L | 2R | 3L | 3R |
   +----+----+----+----+----+----+
   |<------->|<------->|<------->|
     Frame-    Frame-    Frame-
     Block 1   Block 2   Block 3

4.3.3. Speech Data

   Speech data of a payload contains one or more speech frames or
   comfort noise frames, as described in the ToC of the payload.

      Note, for ToC entries with FT=14 or 15, there will be no
      corresponding speech frame present in the speech data.





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RFC 3267        RTP Payload Format for AMR and AMR-WB          June 2002


   Each speech frame represents 20 ms of speech encoded with the mode
   indicated in the FT field of the corresponding ToC entry.  The length
   of the speech frame is implicitly defined by the mode indicated in
   the FT field.  The order and numbering notation of the bits are as
   specified for Interface Format 1 (IF1) in [2] for AMR and [4] for
   AMR-WB.  As specified there, the bits of speech frames have been
   rearranged in order of decreasing sensitivity, while the bits of
   comfort noise frames are in the order produced by the encoder.  The
   resulting bit sequence for a frame of length K bits is denoted d(0),
   d(1), ..., d(K-1).

4.3.4. Algorithm for Forming the Payload

   The complete RTP payload in bandwidth-efficient mode is formed by
   packing bits from the payload header, table of contents, and speech
   frames, in order as defined by their corresponding ToC entries in the
   ToC list, contiguously into octets beginning with the most
   significant bits of the fields and the octets.

   To be precise, the four-bit payload header is packed into the first
   octet of the payload with bit 0 of the payload header in the most
   significant bit of the octet.  The four most significant bits
   (numbered 0-3) of the first ToC entry are packed into the least
   significant bits of the octet, ending with bit 3 in the least
   significant bit.  Packing continues in the second octet with bit 4 of
   the first ToC entry in the most significant bit of the octet.  If
   more than one frame is contained in the payload, then packing
   continues with the second and successive ToC entries.  Bit 0 of the
   first data frame follows immediately after the last ToC bit,
   proceeding through all the bits of the frame in numerical order.
   Bits from any successive frames follow contiguously in numerical
   order for each frame and in consecutive order of the frames.

   If speech data is missing for one or more speech frame within the
   sequence, because of, for example, DTX, a ToC entry with FT set to
   NO_DATA SHALL be included in the ToC for each of the missing frames,
   but no data bits are included in the payload for the missing frame
   (see Section 4.3.5.2 for an example).













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RFC 3267        RTP Payload Format for AMR and AMR-WB          June 2002


4.3.5 Payload Examples

4.3.5.1. Single Channel Payload Carrying a Single Frame

   The following diagram shows a bandwidth-efficient AMR payload from a
   single channel session carrying a single speech frame-block.

   In the payload, no specific mode is requested (CMR=15), the speech
   frame is not damaged at the IP origin (Q=1), and the coding mode is
   AMR 7.4 kbps (FT=4).  The encoded speech bits, d(0) to d(147), are
   arranged in descending sensitivity order according to [2].  Finally,
   two zero bits are added to the end as padding to make the payload
   octet aligned.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | CMR=15|0| FT=4  |1|d(0)                                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                     d(147)|P|P|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
























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4.3.5.2. Single Channel Payload Carrying Multiple Frames

   The following diagram shows a single channel, bandwidth efficient
   compound AMR-WB payload that contains four frames, of which one has
   no speech data.  The first frame is a speech frame at 6.6 kbps mode
   (FT=0) that is composed of speech bits d(0) to d(131).  The second
   frame is an AMR-WB SID frame (FT=9), consisting of bits g(0) to
   g(39).  The third frame is NO_DATA frame and does not carry any
   speech information, it is represented in the payload by its ToC
   entry.  The fourth frame in the payload is a speech frame at 8.85
   kpbs mode (FT=1), it consists of speech bits h(0) to h(176).

   As shown below, the payload carries a mode request for the encoder on
   the receiver's side to change its future coding mode to AMR-WB 8.85
   kbps (CMR=1).  None of the frames is damaged at IP origin (Q=1).  The
   encoded speech and SID bits, d(0) to d(131), g(0) to g(39) and h(0)
   to h(176), are arranged in the payload in descending sensitivity
   order according to [4]. (Note, no speech bits are present for the
   third frame).  Finally, seven 0s are padded to the end to make the
   payload octet aligned.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | CMR=1 |1| FT=0  |1|1| FT=9  |1|1| FT=15 |1|0| FT=1  |1|d(0)   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                         d(131)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |g(0)                                                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          g(39)|h(0)                                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                           h(176)|P|P|P|P|P|P|P|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+