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Voice Payload

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  • #21882 Reply
    Dave02
    Guest

    VoIP with Codec 729a: I understand the 8kbps sampling rate, but how do you come to have a 20 byte payload? IP/UDP/RTP header I understand, but with the codec, how do they determine they have a 20 or 40 byte payload with a 729a codec?

    #21883 Reply
    rincon
    Guest

    look at rfc deocuments

    #21884 Reply
    ksabc
    Guest

    Easy.

    All Codec use 8K bit/s sampling rate(PCM), but for different Codec, it generate different voice data size in different duration according to the Codec’s algorithm, for example :

    G.723.1(5.3K) – generate 20 byte in 30ms, so the rate is (1000ms/30ms)*20byte*8bit=5280bit/s=5.3k

    G.723.1(6.4k) – generate 24 byte in 30ms, so rate is (1000ms/30ms)*24byte*8bit=6336bit/s=6.4k

    G.729A(8K) – generate 10 byte in 10ms, so rate is (1000ms/10ms)*10byte*8bit=8000bit/s=8k

    You should have more clear understanding now, it’s hard to find answers from rfc documents for this.

    #21885 Reply
    ksabc
    Guest

    Some supplement.

    why sometimes you will see the G.729A have 20byte or 40byte pure data in one RTP package? because in this case the Voice Packetizer output the RTP data after two/four 10ms, so in fact it have 2/4 frames/package, it will save bandwidth when compare to 1 frame/package because now 2/4 frames share the same IP/UDP/RTP header;

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