Different Queuing for Video/Voice
Queuing is an important design and performance issue that must also be examined
when discussing IP telephony. Queuing has traditionally been a Layer 3
function for WAN connections, but when discussing a converged network, specifically
that dealing with voice or video traffic, attention must also be given to the
LAN. Layer 2 traffic can be classified by type of service using the 802.1Q protocol.
It is recommended that when using this protocol you separate voice and
video traffic from regular data traffic and place this traffic in a higher-priority
queue. 802.1Q specifies seven classes of service (COS), 0 being lowest priority
and 7 being of the highest priority. It is recommended that COS 4–7 be used for
voice and video, and that 0–3 be used for normal data operations. An important
note to make regarding Layer 2 queuing is that once the packet encounters a
router, the Layer 2 information is lost—in other words, 802.1Q is only a LAN
solution. For traffic crossing WAN links, Layer 3 queuing must be incorporated.