WiMax has two main topologies ~V namely Point to Point for backhaul and Point to Multi Point Base station for Subscriber station.

In each of these situations, multiple input multiple output antennas are used. The protocol structure of IEEE 802.16 ~V Broadband wireless MAN standard is shown below:

Figure 1: Protocol Structure of IEEE 802.16

The above picture shows four layers ~V Convergence, MAC, Transmission and Physical. These layers map to two of the lowest layers ~V physical and data link layers of the OSI mo del.

WiMax provides many user applications and interfaces like Ethernet, TDM, ATM, IP, and VLAN.

The IEEE 802.16 standard is versatile enough to accommodate time division multiplexing (TDM) or frequency division duplexing (FDD) deployments and also allows for both full an d half-duplex terminals.

802.16 supports three physical layers. The mandatory physical mode is 256-point FFT OFDM (Orthogonal Frequency Division Multiplexing). The other modes are Single carrier (SC) and 2048 OFDMA (Orthogonal Frequency Division Multiplexing Access) modes. The corresponding European standard - the ETSI Hiperman standard defines a single PHY mode identical to the 256 OFDM modes in the 802.16d standard.

The MAC was developed for a point-to-multipoint wireless access environment and can accommodate protocols like ATM, Ethernet and IP (Internet Protocol). The MAC frame structure dynamic uplink and downlink profiles of terminals as per the link conditions. This is to ensure a trade-off of capacity and real-time robustness.

The MAC uses a protocol data unit of variable length, which increases the standards efficiency. Multiple MAC protocol data unit can be sent as a single physical stream to save overload. Also, multiple Service data units (SDU) can be sent together to save on MAC header overhead. By fragmenting, you can send large volumes of data (SDUs) across frame boundaries and can guarantee a QoS (Quality of Service) of competing services. The MAC uses a self-correcting bandwidth request scheme to avoid overhead and acknowledgement delays.

This also allows better QoS handling than the traditional acknowledged schemes. The terminals have a variety of options to request for bandwidth depending on the QoS and other parameters. The signal requirement can be polled or a request can be piggybacked.

The 802.16 MAC protocol performs mainly two tasks ~V Periodic and Aperiodic activities. Fast activities (periodic) like scheduling, packing, fragmentation and ARQ are hard-pr essed for time and have hard tight deadlines. They must be performed within a single frame.

The slow activities, on the other hand, typically execute as per pre-fixed timers, but are not associated with any timers. They also do not have specific time frame or deadline.