 control Setup, teardown or change a DOCSIS service flow signaling Data and control flows between UE and CM CM - Cable modem CMTS - Cable modem termination system IMS - IP multimedia subsystem PCRF - Policy and charging rules function UE - User equipment Figure 4.5 PacketCable QoS architecture. 4.3 QoS for Fixed Ultra-Broadband Access packets. Although ATM lost the "battle" with the Internet technologies in the 1990s, certain QoS solutions in ATM are well defined and they are still being used on a link level for QoS provisioning in broadband access links. The downstream GTC frame is broadcast to every ONT with the aim of ONT using the synchronization information carried in the physical control block downstream (PBCd) field. For the purposes of synchronization between the OLT and ONTs, the downstream frames are transmitted continuously even when there is no user data to carry in them. Meanwhile, the upstream traffic contains multiple transmission bursts coming from multiple ONTs. Each of the upstream burst frames contains physical layer overhead upstream (PLOu), which is in fact bandwidth allocation interval that contains the dynamic bandwidth report upstream (DBRu) field as well as allocation identifiers (Alloc-IDs). When the upstream traffic from ONTs arrives at the OLT, it is scheduled based on the class of service (CoS) dependent on the type of transmission containers (T-CONTs) specified in the Alloc-ID. The T-CONT frames are used for establishing a virtual connection between ONTs and the OLT. In GPON there are five defined types of T-CONT frames (Table 4.5) which one may find equal to five QoS classes in the upstream direction. Single T-CONT may have one or multiple GEM ports. Then, a GPON interface on an ONT may contain one or multiple T-CONTs. Because different ONTs may be deployed on different distances of the same OLT to which they are connected, the OLT also has a ranging function to avoid overlapping of frames coming from different ONTs in the uplink direction. The ranging is based on calculating a spec