Jitter - Cisco WS C4224V 8FXS - 200Mbps Ethernet Switch Software Configuration Manual

A Voice Primer

Jitter

Catalyst 4224 Access Gateway Switch Software Configuration Guide
B-6
Handling delay (sometimes called serialization delay) is caused by the devices
that handle voice information. Handling delays significantly degrade voice
quality in a packet network.
Delays caused by codecs are considered handling delays.
delay introduced by different codecs.
Table B-2 codec-Induced Delays
codec Bit Rate (kbps)
G.711 PCM 64
G.729 8
CS-ACELP
G.729a 8
CS-ACELP
Another handling delay is the time it takes to generate a voice packet. In VoIP,
the DSP generates a frame every 10 milliseconds. Two of these frames are then
placed within one voice packet, so the packet delay is 20 milliseconds.
Another source of handling delay is the time it takes to move the packet to the
output queue. Cisco IOS software expedites the process of determining packet
destination and getting the packet to the output queue. The actual delay at the
output queue is another source of handling delay and should be kept under
10 milliseconds whenever possible by using queuing methods that are optimal for
your network.
In Voice over Frame Relay, you need to make sure that voice traffic is not
crowded out by data traffic.
Jitter is another factor that affects delay. Jitter is the variation between the time a
voice packet is expected to be received and when it actually is received, causing
discontinuity in the real-time voice stream. Voice devices such as the Cisco 3600,
Cisco MC3810, and the Catalyst 4224 compensate for jitter by setting up a
playout buffer to play back voice in a smooth fashion.
Appendix B Synopsis of Basic VoIP Concepts
Framing size (ms) Compression Delay (ms)
0.125 5
10 15
10 15
Table B-2 shows the
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