Installation guide

Latency Network Requirements and Preparation

ShoreTel 14.2 Planning and Installation Guide 47

* In network traces, G.722 often appears as “G722/8000” even though it should be “G722/16000”. This

is due to an historical error in notation. ShoreTel follows the Internet Engineering Task Force

recommendation of maintaining the G722/8000 notation.

Latency

Latency is the amount of time it takes for one person’s voice to be sampled, digitized (or encoded),

packetized, sent over the IP network, de-packetized, and replayed to another person. The ITU G.114

Recommendation recommends not more than 150 ms one-way delay, from “mouth-to-ear.” If the

latency is too high, it interferes with the natural flow of the conversation, causing the two parties to

confuse the latency for pauses in speech. The resulting conversation is reminiscent of international

calls over satellite facilities. The maximum latency for ShoreTel phones on an ideal intranet is 130 ms.

Networks can inject additional latency if not properly designed. Potential trouble spots to be aware of

are routers and wide-area-network interfaces. Care should be taken to ensure that all network

congestion points are designed to provide sufficient capacity to ensure low latency throughput.

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 ShoreTel voice switches and 400-Series IP phones 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.

Table 12: Supported Codecs

Codec Sample Rate Data Rate

L16/1600 (Linear

Broadband)

16 KHz 256 Kbps

L16/8000 (Linear) 8 KHz 128 Kbps

G.711 µ-law (PCMU) 8 KHz 64 Kbps

G.711 A-law (PCMA) 8 KHz 64 Kbps

G.722 16 KHz* 32 Kbps

G.729a 8 KHz 8 Kbps

iLBC 8 KHz 13.33 Kbps

AAC_LC 32 KHz

BV-32 16KHz 32 Kbps

BV-16 8KHz 16 Kbps

DV-14 8KHz

T.38