Switching to VoIP

This is the Title of the Book, eMatter Edition

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

Latency, Packet Loss, and Jitter

|

193

Latency, Packet Loss, and Jitter

Latency (also called lag), being the chief cause of poor perceived call quality, is

caused primarily by slow network links. End-to-end latency, in the case of VoIP, is

the time it takes from the instant the caller utters something until the time the

receiver hears that utterance. Research has established that round-trip latency less

than 150 ms is not immediately noticeable, but latency higher than 150 ms is dis-

couraged, and latency higher than 300 ms is considered unacceptable. (Cisco Sys-

tems states that round-trip latency higher than just 150 ms is unacceptable.)

Latency has the following effects on telephony applications:

• Can slow down the human conversation

• Can result in caller and receiver unintentionally interrupting each other

• Can exacerbate another Quality-of-Service problem: echo

• Can cause synchronization delays in conference-calling applications

The best ways to beat latency are to use low-packet-interval codecs and maintain fast

network links, because QoS protocols alone cannot directly improve latency’s

impact. That is, they can’t speed up your network. One negative effect of latency in

telephony systems is echo. It’s discussed in more detail in the following section,

“Echo.”

ITU-T recommendation ...