The AMR and AMR-WB payload format supports transmission of multiple frames per payload, the use of fast codec mode adaptation, and robustness against packet loss and bit errors.
The payload format consists of one payload header with an optional interleaving extension, a table of contents, optionally one CRC per payload frame and zero or more payload frames.
The payload format is either bandwidth efficient or octet aligned, the mode of operation to use has to be signalled at session establishment. Only the octet aligned format has the possibility to use the robust sorting, interleaving and CRC to make it robust to packet loss and bit errors. In the octet aligned format the payload header, table of contents entries and the payload frames are individually octet aligned to make implementations efficient, but in the bandwidth efficient format only the full payload is octet aligned. If the option to transmit a robust sorted payload is signalled the full payload SHALL finally be ordered in descending bit error sensitivity order to be prepared for unequal error protection or unequal error detection schemes.
Robustness against packet loss can be accomplished by using the possibility to retransmit previously transmitted frames together with the current frame or frames. This is done by using a sliding window to group the speech frames to send in each payload, see figure 1. A packet containing redundant frames will not look different from a packet with only new frames. The receiver may receive multiple copies or versions (encoded with different modes) of a frame for a certain timestamp if no packet losses are experienced. If multiple versions of a speech frame is received, it is RECOMMENDED that the mode with the highest rate is used by the speech decoder.
references:
https://www.ietf.org/proceedings/51/I-D/draft-ietf-avt-rtp-amr-10.txt
The payload format consists of one payload header with an optional interleaving extension, a table of contents, optionally one CRC per payload frame and zero or more payload frames.
The payload format is either bandwidth efficient or octet aligned, the mode of operation to use has to be signalled at session establishment. Only the octet aligned format has the possibility to use the robust sorting, interleaving and CRC to make it robust to packet loss and bit errors. In the octet aligned format the payload header, table of contents entries and the payload frames are individually octet aligned to make implementations efficient, but in the bandwidth efficient format only the full payload is octet aligned. If the option to transmit a robust sorted payload is signalled the full payload SHALL finally be ordered in descending bit error sensitivity order to be prepared for unequal error protection or unequal error detection schemes.
Robustness against packet loss can be accomplished by using the possibility to retransmit previously transmitted frames together with the current frame or frames. This is done by using a sliding window to group the speech frames to send in each payload, see figure 1. A packet containing redundant frames will not look different from a packet with only new frames. The receiver may receive multiple copies or versions (encoded with different modes) of a frame for a certain timestamp if no packet losses are experienced. If multiple versions of a speech frame is received, it is RECOMMENDED that the mode with the highest rate is used by the speech decoder.
references:
https://www.ietf.org/proceedings/51/I-D/draft-ietf-avt-rtp-amr-10.txt
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