QoS control Mechanism of Network Elements

This section explains the mechanism to allow QoS control at each network element.

Packet Classifer

This is used to classify packets based on some predefined rules. As determined by the IETF DiffServ Working Group, packet classifier can be of 2 types: Multi-field (MF) classfier and Behaviour Aggregate classfier (BA). MF classifier is used at the edges of the network and BA is used at the core network to ensure scalability.

Multi-Field Classi®er This classifies based on the com-bination of one or more header (like source/destinationaddress, DS field, protocol ID) information and input interface information.

Behaviour Aggregate Clasi®er It classifies packet based only on their DS value.

Traffic Conditioner

Traffic Conditioner measures the input traffic and assures that the packet behaviour follows the predefined profiles. The traffic conditioner consists of a combination of Meter, Marker, Shaper and Dropper. In the DiffServ network, packets are conditioned at the edges only.


The scheduler controls the packet trans-mission sequence from queues of individual classes to pro-vide traffic of each class with the QoS level appropriate to PHB of the respective class. The scheduling algorithms are grouped into
two major categories: Priority Based Scheduling and Bandwidth-Sharing Scheduling. The DiffServ uses the scheduling at both edge and core of the network.

Queue Management

Queue control can accurately con-trol packets (including packet discarding) during congestion. As the Queue Control uses a packet priority discarding mech-anism, it can differentiate traffic and provide different loss and delay characteristics to the packets belonging to different service classes of the same queue. The common queue control systems include the Random Early Detection (RED).When the queue length reaches the preset threshold, the RED randomly discards packets and this
discard rate is decided by the function of queue length. Packets are discarded in proportion to the flow before the tail packet discarding occurs due to queue overflow.