oes not provide a bandwidth guarantee but packets are given a higher priority to be transmitted over the network than the packets from the BE PHB. In congestion situations the user of the Assured Forwarding service should encounter less bandwidth decrease than BE PHB users. The AF PHB group provides four independently forwarded AF classes. Each of these classes has three levels of dropping precedence: low, medium, and high. Therefore, 12 instances of AF with recommended DSCPs can exist in a DiffServ domain. Their defined DSCP codes are given following: Low drop: 001010 010010 011010 100010 Medium drop: 001100 010100 011100 100100 High drop: 001110 010110 011110 100110 In each DiffServ node, each AF class is allocated a certain amount of forwarding resources (buffer space and bandwidth). DiffServ Router A DiffServ router consists of five components shown in Figure 3.12 [Man03]. On arrival, packet is firstly classified by the classifier according to the bilateral service level agreement (SLA). Afterwards, the classifier forwards the packet to the traffic conditioner. The traffic conditioner may include a meter, a marker, a shaper, and a dropper. If accepted, the packet is enqueued into a corresponding buffer and then transmitted according to a specific scheduler policy. 3 UMTS Terrestrial Radio Access Network (UTRAN) Traffic Conditioner Meter Packets Shaper/ Classifier Marker Dropper Figure 3.12: IP DiffServ Router Building Blocks Mapping Function: Mapping is the function of translating the QoS of one system to the QoS parameters understood by another system. It is a necessary function for a UMTS packet entering the DiffServ-aware network, or when a packet coming from an external network enters the UMTS network. This function is usually integrated with the classifier into one functioning block. In the IP-based lub domain (at the Edge Router), it is responsible for translating the QoS of the UMTS system to the corresponding IP DiffServ PHB, and vice versa. Classifier: the packets are classified according to DSCP in 