Specifications

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Cisco 1751 Router Software Configuration Guide
OL-1070-01
Another handling delay is the time it takes to generate a voice packet. In VoIP, the DSP generates a
frame every 10 milliseconds. Two of these frames are then placed within one voice packet; the packet
delay is therefore 20 milliseconds.
Another source of handling delay is the time it takes to move the packet to the output queue. Cisco IOS
software expedites the process of determining packet destination and getting the packet to the output
queue. The actual delay at the output queue is another source of handling delay and should be kept under
10 milliseconds whenever possible by using whatever queuing methods are optimal for your network.
Output queue delays are a QoS issue in VoIP and are discussed in the “Configure IP Networks for
Real-Time Voice Traffic” section on page 2-2.
In Voice over Frame Relay, you need to make sure that voice traffic is not crowded out by data traffic.
Strategies on how to manage Voice-over-Frame-Relay voice traffic are discussed in the “Configure
Frame Relay for VoIP” section on page 2-24.
Jitter
Jitter is another factor that affects delay. Jitter occurs when there is a variation between when a voice
packet is expected to be received and when it actually is received, causing a discontinuity in the
real-time voice stream. Voice devices such as the Cisco 3600 router, Cisco MC3810, and the Cisco 1751
router compensate for jitter by setting up a playout buffer to playback voice in a smooth fashion. 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.
End-to-End Delay
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 end points 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
Echo is hearing your own voice in the telephone receiver while you are talking. When timed properly,
echo is reassuring to the speaker; 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 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 limited by design by the
total amount of time they will wait for the reflected speech to be received, which is known as an echo
trail. The echo trail is normally 32 milliseconds.
G.729 CS-ACELP 8 15
G.729a
CS-ACELP
815
Table 2 CODEC-Induced Delays
CODEC Bit Rate (kbps) Compression Delay (ms)