 HiperMAN standard), and OFDMA (2,048-subcarrier OFDM). The WiMax standard can also adjust the trans- mit power by incorporating adaptive burst profiles. In addition, the standard provides for forward error correction (FEC) coding. Therefore, WiMax can accommodate a wide variety of radio conditions. WiMax provides for quality of service (QoS) through the use of a request/grant protocol. The base station controls access to the inbound channel. In order to trans- mit, users must first send requests on a contention-based access channel. The base station allocates the exclusive right to use the inbound traffic channel, using a system of transmission grants. Because only one station can be given permission to send at a time, there are no inbound collisions. WiMax supports four main types of QoS: Unsolicited Grant Service-Real-Time-In this isochronous service for real-time voice and video, stations are allocated inbound transmission capacity on a scheduled basis. Real-Time Polling Service-In this service for real-time voice and video, the base station polls each user device in turn. Variable Bit Rate-Non-Real-Time-This data service provides capacity guarantees and variable delay. Variable Bit Rate-Best Effort-This residential data service offers IP-like best effort. WMANs Because WiMax is intended for public networks, encryption is a key compo- nent. The initial specification identifies the 168-bit Triple Data Encryption Stan- dard (3DES) as mandatory. Future plans call for including Advanced Encryption Standard (AES) on an optional basis. The WiMax standard incorporates Dynamic Frequency Selection, which means the radio automatically searches for an unused channel. WiMax can take advantage of multiple duplexing modes, including Time Division Duplex (TDD) dynamic asymmetry, allowing the uplink/downlink bandwidth to be allocated according to current traffic conditions. WiMax standards also define an optional mesh configuration. 802.16 Revisions IEEE 802.16 first issued standards for the PHY (physical) and MAC (Media Acc