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Speex Actual Bandwidth Calculation
  
Oct 31, 2007
We have finally almost finished Speex coding on AR1688 DSP. This open source voice codec turned out to be much more complicated than we expected. There are 8 different bitrate with 8k sampling, from 2.15kpbs to 24.6kpbs. I am writing this article to answer a question of how much bandwidth this codec actually use during a call.
Let us calculate based on 802.3 ethernet, using SIP protocol:
Ethernet CRC: 4 bytes
MAC header: 14 bytes
IP header: 20 bytes
UDP header: 8 bytes
RTP header: 12 bytes
Speex data: x bytes, x depends on bitrate used and number of frames packed in an ethernet packet ("Frames per TX" option in AR1688)
Total: (58 + x) bytes
When different frames are included in an ethernet frame, the actually used bandwidth are:
Bitrate (kbps) 2.15 3.95 5.95 8 11 15 18.2 24.6
20ms frame (byte) 6 10 15 20 28 38 46 62
1 frame in ethernet packet (kbps) 25.6 27.2 29.2 31.2 34.4 38.4 41.6 48
2 frames in ethernet packet (kbps) 14 15.6 17.6 19.6 22.8 26.8 30 36.4
3 frames in ethernet packet (kbps) 10.1 11.7 13.7 15.7 18.9 22.9 26.1 32.5
4 frames in ethernet packet (kbps) 8.2 9.8 11.8 13.8 17 21 24.2 30.6

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