Packet prioritization has become more important because many types of data traffic need to
share a data path through the network, often congesting WAN links. If the WAN link is not
congested, you don’t need to implement traffic prioritization, although it might be appropriate
to add more bandwidth in certain situations.
Prioritization of traffic will be required on your network if you have, for example, a mixture
of file transfer, transaction-based, and desktop video conferencing. Prioritization is most
effective on WAN links where the combination of bursty traffic and relatively lower data rates
can cause temporary congestion. This is typically necessary only on WAN links slower than
T-1/E-1. However, prioritization can also be used across OC (optical carrier)-12 and OC-48
links, because at times, tests can be run to saturate these links, but you might still want voice
and video to have a priority.