Hz single carrier air interface. WirelessMAN OFDM Orthogonal frequency division multiplexing, with TDMA controlled multiple user access. This interface is mandatory for unlicensed bands. WirelessMAN OFDMA An OFDM interface with user access controlled by allocating to individual users a subset of the full set of available carrier frequencies. of these systems comes down, they will play an important role in improving wireless network performance, particularly in the increasingly crowded unlicensed frequency bands. MAC Layer In fulfilling the needs of metropolitan area networking, the 802.16 MAC layer must provide flexible and efficient access for a wide range of different service types. The main shortcoming of the contention based Table 12-5: 802.16a Key Parameters Parameter 802.16a standard RF band 2-11 GHz Modulation Single carrier, OFDM Data rate Peak data rates to 70 Mbps Multiple access OFDMA, TDMA Channelisation Flexible channel widths, from 1.75 to 20 MHz Duplex method TDD and FDD, as well as half-duplex via TDMA Network topology Point-to-multipoint and mesh topologies Bandwidth allocation Grant per subscriber station (GPSS) or grant per connection (GPC) - see Section 12.1.3 "MAC Layer" Chapter Twelve media access of 802.11 networks, based on carrier sensing (CSMA-CA), is that, prior to the 802.11e enhancements, no particular service quality level could be guaranteed. A subscriber requiring a latency-sensitive service such as VoIP may be subject to the exposed station or hidden station problems (Section "Radio Transmission as a Network Medium, p. 75") with the consequent deterioration of service quality. Connection-Oriented Versus Connectionless One key to the effectiveness of the 802.16 MAC is its connection- orientation. Every service is mapped to a connection and referenced using a 16-bit connection ID (CID). This includes services which are inherently connectionless such as UDP services (for example RIP, SNMP or DHCP messaging). Each connection can then be associated with specific parameters such as; ban