Quality of Service


Earlier we described how residential VoIP is becoming popular. These calls are being sent over the Internet, not over a private corporate wide-area network (WAN). You saw in Chapter 6, "Quality of Service Design," how important QoS is to voice calls, to ensure that the delay and variation in delay (jitter) remain within acceptable levels. However, the Internet is known as a best-effort service: in general it has no QoS support. Thus, the quality of these voice calls over the Internet is not guaranteed. For residential and small-business needs, the quality might be acceptable, especially when weighed against the cost savings. For organizations that require enterprise-class voice-enabled network services and quality, though, Cisco does not recommend using best-effort WAN connections such as the Internet. For these networks, dedicated WAN connections with QoS support should be used for voice traffic between sites.[2]

Note

It is interesting to consider the quality of service provided by the best-effort Internet. For example, when we send an e-mail to someone, it usually arrives almost immediately. While we have probably all experienced a few instances of e-mails never arriving at their destination, in general, we think of the Internet as providing good service.


Voice traffic is sensitive to delays, jitter, and extensive packet loss. Recall from Chapter 6 that the guidelines for ensuring acceptable voice quality are as follows:

  • The one-way delay should be no more than 150 milliseconds (ms).

  • The jitter should be no more than 30 ms.

  • No more than 1 percent of the packets should be lost.

Note

While 150 ms is the standard for acceptable voice delay, tests have shown that a negligible quality difference exists with a 200-ms delay.


Chapter 6 also identifies the tools that are available to implement the QoS required by voice traffic, including classification and marking, policing and shaping, congestion avoidance, congestion management, and link-specific tools.




Campus Network Design Fundamentals
Campus Network Design Fundamentals
ISBN: 1587052229
EAN: 2147483647
Year: 2005
Pages: 156

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