VoIP White Paper
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Revised: February 8, 2007
Page 26
15 Glossary
AEC – Acoustic Echo Canceller. An echo canceller that cancels the effects of audible room echo, such as in a
desktop speakerphone. See “Echo Canceller.”
Echo Canceller – An algorithmic device, usually implemented in software in a digital signal processor chip to
remove the echo of the speaker’s voice from a full-duplex telephone connection. Echo cancellers are more
sophisticated and differ from echo suppressors in that they eliminate echo without having to impose half-duplex
communications on the link (like echo suppressors do).
Full Duplex – Allowing communication (including the movement of data or voice) in both directions of the circuit
at the same time – simultaneous two-way communication. A half-duplex connection allows communication in
both directions, but in only one direction at a time.
Jitter – The amount of variability in latency as a function of time. In VoIP systems, jitter describes the
irregularity of packet arrivals over the course of time. As network jitter increases, deeper receive buffers are
required to smooth the effects (and not lose packets). Increased jitter typically results in increased overall end-
to-end latency.
Latency – A specific type of delay in time of transmission or response to events. More formally, is the amount
of time elapsed between two fixed reference points in a system that transmits information or takes action
based on certain events. Latency is sometimes thought of as the time delay between cause and effect. In
VoIP, there are several sources of latency, including network latency, coder latency, packetization latency,
buffering latency, and more.
LEC – Line Echo Canceller. An echo canceller that cancels the effects of electrical circuit echo in phone lines.
See “Echo Canceller.”
NAT – Network Address Translation. Generically, NAT refers to devices that translate the IP addresses of
packets as they transit from one subnet to another. NAT is normally associated with firewall devices and is
used to hide private, non-routable IP addresses of a LAN from the public Internet. Using NAT has security
advantages and also works to minimize the number of public IP addresses required at a single site. Minimizing
the use of public IP addresses is important because the pool of available addresses is a scarce resource.
Side-Tone – Feedback of speaker’s voice from the microphone to the ear piece to make speech sound as
natural as possible. Without side-tone, phone handsets sound like they are broken – even though speech is
still being transmitted to the remote end.
Author:
Jeffrey Szczepanski
Chief Technical Officer
InSciTek Microsystems, Inc.
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