Evolving UC communications requires increasingly complex quality-of-service strategies. Have we reached the point where QoS is too complex for normal use? Do cloud services over the Internet mean that QoS is irrelevant?
The differentiated services (DiffServ) model of QoS prioritizes different types of traffic so that time-critical applications like voice, video, and interactive business applications receive priority over traffic flows that aren't as time critical. In the DiffServ model, a Differentiated Services Code Point (DSCP) value is set within the IP packet header. While up to 64 different per-hop behaviors can be specified with DSCP, most QoS designs use only six to 12 of them. (Note: An Integrated Services QoS model is also available, but it hasn't gained widespread acceptance.) IETF standards documents describe each of the per-hop behaviors, one of them being a priority queue that's typically reserved for time-critical protocols like voice. Some QoS designs may also prioritize critical device communications, such as from bedside monitors in healthcare or process control alerts in manufacturing plants.
The DSCP version of QoS consists of three basic functions: classification, marking, and queueing. Classification, which is typically performed at the ingress edge of the network, identifies different classes of traffic. Once a packet has been classified, its headers are marked by adding a QoS tag. Performing classification and marking at the network's edge allows successive network equipment in the forwarding path to make forwarding decisions efficiently -- meaning, they don't have to repeat the classification function. Packets are then queued according to the applied classification and marking, with higher-priority packets selected for transmission over congested links before lower-priority packets.
It's important to note that QoS only applies during congestion, when more packets need to be sent over a link than the link can handle. The network equipment only needs to make a queueing decision when multiple packets need to be sent on a link. Ideally, the highest-priority packets get selected to go first, followed by lower-priority packets. The 64 traffic classes defined by DSCP provides a lot of flexibility.
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