. DiffServ) should be mapped on the underlying ATM QoS classes. L2 Tunneling protocol (L2TP). The L2TP is typically used over IP as opposed to L2TP over ATM. A given L2TP tunnel can be associated with a certain traffic engineering specification, but the constituent flows in that L2TP tunnel do not receive differ- entiated service. So, the flows aggregated in a single L2TP tunnel receive the same aggregate QoS treatment. IP QoS. This is a network layer QoS provisioning, which includes standardized approaches, such as DiffServ (from the IETF), as well as customized approaches (e.g. chosen scheduling schemes and their combinations). In this case the main QoS control is done at BRAS as the gateway router which interconnects the IP core network of the telecom operator and the DSL access network. Regarding the DSL access network, three models for QoS support can be distin- guished, which include the following (they may be used separately or jointly, in the same access network): Partitioning of bandwidth. In such a case rate limits are established on broadband network gateways, based on business or application characteristics of the gateways that control each partition. Distributed precedence and scheduling. This marks services (based on layer 2 precedence) and lowest traffic classes will be dropped in the case of congestion appearance. This model provides weighted fairness among traffic classes of the same precedence, but it cannot provide fairness between users within the same class. Priority in this case is used to enhance the option with bandwidth partitioning (illustrated in Figure 4.4). However, if traffic bursts of higher-priority traffic above its partitioned rate are allowed, then it may discard the bandwidth portion for other traffic. So, the burst traffic will be delivered only in cases when there is excess bandwidth that is not being utilized by other traffic 4 QoS for Fixed Ultra-Broadband ADSL2+ N video (IPTV) streams: Total bandwidth DL: 24 Mbit/s Total bandwidth UL: 3 Mbit/s Min. bandwidth N X 3 Mbit/s E