hold. Requests to change DIUC are done with DBPC-REQ or RNG-REQ messages. METROPOLITAN AREA NETWORKS Burst Profile Z Overlap Burst Profile Y Overlap Burst Profile Figure 13.17 Burst profiles. DBPC stands for downlink burst profile change RNG stands for ranging REQ stands for request The DL-MAP message defines usage of downlink and contains carrier-specific data. The DL-MAP message is the first message in every frame and its decoding is time-critical, typically done in hardware. Entries denote instants when the burst profile changes. Uplink transmissions are invited transmissions and are in contention slots, which include bandwidth requests. Transmissions are in initial ranging slots and are ranging requests (RNG-REQs). The contention is resolved using truncated exponential back- off. The bursts are defined by UIUCs (uplink interval usage codes). Transmissions are allocated by the uplink-MAP message. All transmissions have synchronization pream- ble. Ideally, all data from a single SS (subscriber station) is concatenated into a single PHY burst. The uplink channel descriptor is sent regularly. It defines uplink profiles. All uplink burst profiles are acquired. Burst profiles can be changed on the fly. The uplink channel descriptor establishes association between UIUC and actual PHY parameters. The uplink MAP message defines usage of the uplink and contains the "grants." Grants are addressed to the subscriber station. The time is given in mini-slots, the unit of uplink bandwidth allocation. There are 2m physical slots. In the 10- to 66-GHz band PHY, the physical slot is 4 symbols long. The time is expressed as the arrival time at the BS (base station). There are four classes of uplink service, which is a characteristic of the service flow. 1. Unsolicited grant services (UGS). This class is for constant bit-rate (CBR) and CBR-like service flows (SFs) such as T1/E1. 2. Real-time polling services (rtPS) For rt-VBR-like SFs such as MPEG video 3. Non-real-time polling services (nrtPS) 13.6 IEEE 802.16 STANDARD For nrt S