Information Technology Reference
In-Depth Information
Packet Loss
Packet loss is another item that affects voice and video quality. It causes voice and video
clipping and skips. It is caused by several factors: congested links, improper QoS configu-
ration, bad packet buffer management, and routing issues. Packet loss is also caused by
packets received outside of the dejitter buffer range, which are packets that are discarded.
Cisco VoIP uses 20-ms samples of voice payload per VoIP packet. Codec algorithms can
then correct up to 30 ms of lost voice. For the codec correction to be effective, only 1
packet can be lost during any given time. When this occurs, the DSP interpolates the con-
versation with what it thinks the audio should be.
Echo Cancellation
In phone calls, sometimes speech is echoed back to the speaker. This is usually caused by
an impedance mismatch. Echo cancellation involves first recognizing the originally trans-
mitted signal that reappears, with some delay, in the transmitted or received signal. Once
the echo is recognized, it can be removed by subtracting it from the transmitted or re-
ceived signal. ITU-T defines that echo delays more than 15 ms should be suppressed with
echo cancellers. Echo delays up to 15 ms do not need to be suppressed.
QoS and Bandwidth Mechanisms for VoIP and Video Networks
Cisco provides different QoS tools that you should use on edge and backbone routers to
support VoIP networks. First, the CCDA should understand the different categories of
QoS mechanisms:
Key
To p i c
Classification:
Process that identifies the class or group a packet belongs to. Matches
are based on protocol, input port, IP precedence, DSCP, or 802.1P class of service
(CoS). Classification is accomplished using class maps, access lists, and route maps.
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Marking:
Process of marking packets with differentiated service codepoint (DSCP)
values for QoS.
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Congestion avoidance:
Mechanism that seeks to avoid congestion by preemptively
dropping packets to signal traffic flows to slow sending rates. Examples are Weighted
Random Early Discard (WRED) and Distributed WRED (DWRED).
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Traffic conditioners:
These are of two types: traffic shaper and policer. The shaper
delays excessive traffic by using a buffer or queuing mechanism and shape the flow
of traffic. Traffic policing drops traffic or reclassifies excessive traffic to a lower pri-
ority. Frame Relay Traffic Shaping and Committed Access Rate (CAR) are examples.
Figure 14-25 shows the differences.
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