his, it grants bit rate capacity to the subscriber station (SS) while maintaining QoS and fairness. The SS scheduler maintains QoS among its connections and is responsible to share the bandwidth among the connection, again while maintaining QoS and fairness. The algorithms in the BS and SS can be very different; while the subscriber station may use the bandwidth in a way unforeseen by the base station. During SS initialization, there is a scan for a downlink channel and synchronization is established with the BS. The SS must then obtain the transmit parameters from the uplink channel descriptor (UCD) message. Ranging is then performed followed by negotiation of basic capabilities. The subscriber station (SS) is then authorized and there is a key exchange carried out. Registrations are then performed and IP connectivity is established; time of day is exchanged. Operational parameters are then transferred and connections are set up. Ranging is an important element in uplink and downlink communication. For uplink transmissions, times are measured at the base station. At startup, the SS sends a RNG- REQ (ranging-request) in a ranging window. The BS now measures arrival time and signal power, then calculates required (time) advance and power adjustment. The BS sends adjustment in RNG-RSP (range response). The SS adjusts its advance and its power and then sends a new RNG-REQ message. This loop continues until power and timing are acceptable. Registration is a form of capability negotiation. The Subscriber Station (SS) sends a list of capabilities and parts of the configuration file to the Base Station (BS) in the Type (6) BR msb (8) BR Isb (8) CID msb (8) CID Isb (8) HCS (8) Figure 13.18 Typical bandwidth request message. 13.6 IEEE 802.16 STANDARD REG-REQ (registration request) message. The Base Station replies with a REG-RSP (registration respond) message telling which capabilities are supported/allowed. The SS acknowledges the REG-RSP with a REG-ACK message. The IP connectivity and configuration file downloads as fol