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

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eliminate jitter. Devices called jitter buffers, in endpoints and VoIP servers, can mini-

mize the effect of jitter, too. But, like PLC measures, they do so by increasing latency.

Echo

When you hear the words you’ve just spoken repeated back to you a split second

later on the telephone, you’re experiencing echo. Chances are, if the echo occurs less

than 150 ms from the time you actually said the words, then you won’t notice it. But

when the echo occurs above this threshold, it can be particularly annoying. Echo is

an unfortunate by-product of the gateway electronics that bridge soft-based PBX sys-

tems to analog or TDM links. It is caused by three conditions, and it’s at its worst

when they exist together:

• Interfacing between TDM and VoIP endpoints or analog and VoIP endpoints.

The more interface points in the network, the bigger pain echo is likely to be.

• Long round-trip latency between caller and receiver. The higher the latency, the

more annoying echo is likely to be.

• Interfacing of a call path between two-wire analog and TDM or four-wire analog

devices. In this case, echo is caused by an inability of the TDM or four-wire cir-

cuit to cancel the local side-tone signal on the two-wire device (side-tone is cov-

ered in the sidebar).

Hybrids and ...