Switching to VoIP

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Chapter 9: Quality of Service

point, bandwidth availability can be compromised while still allowing data through;

the transmission just slows down. Some applications tolerate congestion and slow

throughput better than others. The more tolerance an application has, the higher its

error budget is said to be.

Slowness of transmission—latency—is the enemy of Voice over IP, and one of the

key contributors to failure with the technology. Aside from careful network design

and bandwidth provisioning, which are factors in building any IP network, there is

an elegant solution to the latency problem, one that allows local and end-to-end

guarantees of bandwidth and prioritization of real-time traffic over less sensitive traf-

fic. This chapter covers that solution, which comes in the form of QoS protocols and

standards: 802.1p, 802.1q VLAN, DiffServ, RSVP, and MPLS.

Call-quality scoring

Historically, the quality of phone calls’ audio has been measured using the mean

opinion score (MOS) from a group of listeners. These listeners hear sound samples

from calls of varying quality, recorded during different sets of network conditions. A

sound sample from each set of conditions is played for the opinion group, and each

rates the sample’s quality on a scale of 1 to 5, with 5 being the best quality. ...