VoIP Quality of Service

Perhaps the biggest issue with VoIP is ensuring that the Quality of Service
(QoS) is comparable to the QoS achieved in traditional circuit-switched telephony.
As we have seen, IP and UDP provide no quality guarantees whatsoever.
Although RTP and RTCP provide QoS-related information (such as
jitter, number of lost packets, and so on), they do not provide any assurance
of quality. In order to ensure that VoIP is not a low-quality service, specific
solutions must be implemented in the network.
One way to help ensure that VoIP offers high quality is to ensure that
more than enough bandwidth is available—both in terms of throughput
on transmission facilities and in terms of processing power within routers.
By overprovisioning the network, one can reduce the likelihood of congestion
and thereby improve quality. This, however, is an expensive option
that leaves much of the network capacity unused much of the time. Moreover,
it does not guarantee quality. Thus, one needs technical solutions
within the network.
The following sections provide a brief overview of some QoS techniques.
For more detailed explanations, the reader is referred to the applicable
IETF specifications.