dium (a single cable in this case), the uplink and downlink directions are treated differently regarding QoS at the end node of the cable access segment, which is the CMTS (cable modem termination system). This is in fact a consequence of the fact that uplink RF channels are contentious and shared-access mediums, based on a topology with many CMs connected to a single 4.3 QoS for Fixed Ultra-Broadband Access CMTS. On the other side, the downlink RF channels behave similarly to a traditional IP router where IP packets arrive and are queued and then forwarded to one (for unicast) or multiple (for multicast traffic, such as TV) destinations. So, applied QoS mechanisms are different for service flows in uplink (i.e. upstream) and downlink (i.e. downstream) directions. There are five service flow scheduling mechanisms for the uplink and one scheduling mechanism for the downlink. Upstream service flows may be defined with one of five service flow scheduling types: Best-effort. This is a standard contention-based resource management which is based on a FCFS (First Come First Served) principle, although it is being coordinated by the CMTS scheduler. For this scheduling type it is possible to set a limit on the maximum bitrate for a given service flow. Non-real-time polling. This is a reservation-based resource management approach in which the CMTS polls CMs on a fixed time interval basis to determine whether ser- vice flow data has been queued for transmission, and, if so, to provide a transmission grant for the given service flow. Real-time polling. This is similar to non-real-time-polling, with the main difference in the duration of the fixed polling interval, which in this case is usually very short (e.g. shorter than 500 ms). Both polling scheduling types are more suitable for traffic which has variable bit rates and is more tolerant to delay and throughput. Unsolicited grant. This is reservation-based resource management based on a fixed-size grant that is allocated to a given service flow at almost fixed time inter