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Tech Stuff - VoIP CODECs

This section shows some comparisons between three standard CODECs used in VoIP

Note: This table was created when the VoIP world was young. Many newer standards are missing but it still proves the point that you need to consider not just the head-line compression ratio but the sample duration since the normal block overhead can loom exceedingly large on short duration codecs

CODEC Performance

Standard Packet Size Sample
Duration
Data Rate
Bits
PPS
Simplex
G.711 120 15ms 64K 66
G.729A 10 10ms 8K 100
G.723.1 24 30ms 5.3/6.4K 33

Notes:

  1. Figures above are for simplex traffic - normal duplex voice traffic will double these figures.

  2. PPS = packets per second.

IP Overhead and Data Rates

This table compares the same three CODECs adding the following IP overheads:

For a grand total of 40 bytes for non-LAN traffic and either 58 or 62 bytes for a LAN which gives the following real-world VoIP duplex data rates:

Adjusted Data Rates

Standard PPS
Duplex
Raw Data
Duplex
IP Data
Duplex
IP + LAN
Duplex
IP + VLAN
Duplex
G.711 132 128K bits/s 174K bit/s 193K bit/s 198K bit/s
G.729A 200 16K bits/s 80K bit/s 108K bit/s 115K bit/s
G.723.1 66 10.6 or
12.8K bits/s
33K bit/s 43K bit/s 45K bit/s

Notes:

  1. PPS = packets per second.

  2. All figures assume a full duplex conversation.

  3. RFC 2508 defines a method for compressing the RTP overhead from 12 bytes to 4 (including the checksum) which would drop the total data rate for G.723.1 from 33K to 26K a substantial saving.

  4. Unless very slow lines are being used, data rates for all CODECs are reasonable. Transaction rates however depending on the CODEC being used can be frighteningly high and would cripple most modest 8 or 16 bit embedded CPUs.



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