Tech Info - VoIP CODECs

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

CODEC Performance

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:

• IP packet encapsulation = 20 bytes
• UDP packet encapsulation = 8 bytes
• RTP packet encapsulation = 12 bytes
• LAN encasulation = 18 bytes VLAN = 22 bytes

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

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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