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Quality of Service for Packetized Voice 619
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Transmission delay over the serial link
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Queuing delay and serialization delay over each device in the network path
Loss
As stated earlier in this chapter, packet loss is acceptable and expected in data networks.
Packet loss in voice networks has a deleterious impact on voice quality and should be kept to
a minimum.
Echo
Echo is the result of speech signals from one direction reflecting into the opposite direction.
Some echo is a good thing. A little echo should always be present because it helps the end user
hear his or her own voice. Too much echo is undesirable. Talker echo occurs when the speech
signal travels toward the destination and is reflected into the return path at a point near the
destination. A talker hears the reflected signal as an echo. If the echo signal is reflected, the
listener hears the echoed signal.
Delay
Queuing delay is a function that occurs during the transmission that increases delay. Queuing
delay is also called transmission delay. Simply stated, queuing delay is the amount of time
required to put data on the wire. It is sometimes called serialization delay for a single packet.
Router Design Concerns
Three areas of concern should be addressed at the router: prioritization, slow-speed links, and
traffic shaping.
Simply stated, prioritization is making sure the voice packets maintain top priority in the packet
food chain. Voice packets belong in the top sirloin section of the supermarket. Large data frames
that take a long time to clock out can create delays. To a voice packet, a delay is the kiss
of death.
Prioritization involves two steps: classification and queuing.
IP Precedence
The IP Precedence bits are the three high-order bits in the Type of Service (ToS) field of the IP
header. Table 16-4 shows the values for each setting of IP Precedence. Cisco recommends that
voice traffic be set at IP Precedence bit value 5.
87200333.book Page 619 Wednesday, August 22, 2001 1:41 PM