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Two main features of QoS are:
System resources are allocated by assigning different weights to different classes of service. A weight is the relative portion of the available resources allocated to a class. Allocating resources according to weights ensures full utilization of the line even if a specific class is not using all of its resources. In such a case, the remaining resources are divided among the remaining classes in accordance with their relative weights.
For some types of traffic, such as voice and video, it is necessary to minimize the latency (delay) of packets. Latency is controlled by defining special LLQ (low-latency queuing) classes. These classes are handled in a strict priority manner. LLQ packets are handled immediately upon arrival, and before packets that do not belong to LLQ classes.
QoS supports multiple LLQ classes. In some cases, it may be necessary to define more than one Low Latency class, for example when different types of traffic have a different sensitivity to delays. In such cases, a class with the higher sensitivity to delay receives a higher priority than a class with the lower sensitivity.
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Note - When LLQ classes are used, it is assumed that the expected traffic will not exceed a relatively small amount of the available resources. Although QoS does not allow LLQ traffic to starve non-LLQ traffic, too much LLQ traffic reduces overall network quality of service and prevents efficient management of weighted resources. |
RED (Random Early Drop) is a congestion avoidance mechanism for detecting and preventing congestions. It takes advantage of TCP's congestion control mechanism by randomly dropping packets during periods of congestion. This causes TCP senders to slow down their transmission, thus preventing high congestion.
QoS implements WRED (Weighted RED) in which packets are dropped according to their priority. WRED mostly affects traffic which is of low priority and which exceeds its weight.
