Network Requirements and Preparation
Planning and Installation Guide
9 – 7
When the codec is G.729a, the encoding process takes an additional 10 msecs and the
decoding process can take an additional 10 msecs.
See Table 9-4 for specific information about latency on the ShoreTel 6.1 system.
Jitter for Voice Switches
Jitter is the variation of latency across the network and the variation in packet
processing inside the switches. To compensate for jitter, the ShoreGear voice switches
continuously measure the jitter in the system and dynamically change the size of the
receive jitter buffers to optimize voice quality.
If the jitter buffer is too small, there can be packet loss from buffer underflows. This
occurs when the jitter buffer runs out of valid voice samples. If the jitter buffer is too
large, there will be unnecessary latency. Both conditions have a negative impact on
voice quality.
The jitter buffer starts at the minimum size of 0 msecs as packets from the network are
placed into the switchboard queue for immediate processing. When jitter is detected on
the network, the jitter buffer dynamically increases in increments of 5 msecs to
compensate for increased jitter and decreases in size in reaction to less jitter. The
maximum value of the jitter buffer is set by ShoreWare Director and ranges from 20 to
300 msecs, with a default of 50 msecs.
As the jitter increases on the network and the jitter buffer needs to be increased to
guarantee timely audio play, the latency of the audio also increases. The system
attempts both to maintain a minimum jitter buffer size that provides good-quality voice
without dropping packets and to provide minimum latency.
For IP phones that are configured into the ShoreTel system, the jitter buffer is not
configurable. The minimum jitter buffer is 10 msecs, and the maximum is 80 msecs.
NOTE
Maximum values greater than 100 should rarely be necessary. If needed, this
could indicate a problem in your network that should be addressed in another
way.
Table 9-4 Latency
Configuration Overhead Encoding Frame Size -5 Jitter Buffer
a
Decoding Total (+/– 5 msec)
b
Switch
17
0
0
Varies
0
17
LAN
17
5
5
Varies
5
32 + Jitter Buffer
WAN
17
5
15
Varies
5
42 + Jitter Buffer
G.729a
(LAN and WAN)
17
15
15
Varies
15
62 + Jitter Buffer
a. The jitter buffer varies, depending on network conditions. See below for more information.
b. If a call comes in on a trunk through either T1/E1 or analog loop-start, the total latency is increased
by the delay in the PSTN. You must add this latency to the total latency. Latency for the PSTN varies;
however, it is probably a minimum of 10 msecs (for local), and it could be as high as hundreds of
msecs (for long international calls).
