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Appendix B
Synopsis of Basic VoIP Concepts
End-to-End Delay
Echo
Signaling
OL-2031-02
Playout control is handled through RTP encapsulation, either by selecting
adaptive or non-adaptive playout-delay mode. In either mode, the default value
for nominal delay is sufficient.
Figuring out the end-to-end delay is not difficult if you know the end-to-end
signal paths/data paths, the codec, and the payload size of the packets. Adding the
delays from the endpoints to the codecs at both ends, the encoder delay (which is
5 milliseconds for the G.711 and G.726 codecs and 10 milliseconds for the G.729
codec), the packet delay, and the fixed portion of the network delay yields the
end-to-end delay for the connection.
Echo is hearing your own voice in the telephone receiver while you are talking.
When timed properly, echo is reassuring to the speaker. But if the echo exceeds
approximately 25 milliseconds, it can be distracting and cause breaks in the
conversation.
In a traditional telephony network, echo is normally caused by a mismatch in
impedance from the four-wire network switch conversion to the two-wire local
loop and is controlled by echo cancellers. In voice-packet-based networks, echo
cancellers are built into the low bit-rate codecs and are operated on each DSP.
Echo cancellers are, by design, limited by the total amount of time they will wait
for the reflected speech to be received. This amount of time is called an echo trail.
The echo trail is normally 32 milliseconds. VoIP has configurable echo trails of
8, 16, 24, and 32 milliseconds.
Although there are various types of signaling used in telecommunications today,
this document describes only those with direct applicability to Cisco voice
implementations. One signaling type involves access signaling, which determines
Catalyst 4224 Access Gateway Switch Software Configuration Guide
A Voice Primer
B-7
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