Saturday, November 14, 2009

Medium Access Control | WiMAX Operation

Medium access control is the process used by communication devices to gain access to a shared communications medium or channel. The methods for controlling access to WiMAX systems may be assigned (“non-contention based”) or random (“contention based”).

When the WiMAX system uses contention free access control the subscriber station must wait for polling messages before responding. If contention based access control is used (e.g. best effort service), the subscriber device must compete for access to send its packets. The WiMAX system can mix contention free and contention based access on the same radio channel.

Contention free access is provided by defining time periods that specific devices will use when communicating with the system. Because all the devices listening to the WiMAX radio channel can hear these messages, devices will not transmit during the assigned time periods.

Contention based access is provided through the use of contention slots and the collision sense multiple access (CSMA) process. The WiMAX channel descriptors define specific time periods (“contention slots”) that contention based WiMAX devices must use when accessing the WiMAX system. Contention slots are dedicated time intervals (time slots) on a communication channel that can be used to allow devices to randomly request service from a system.

When contention based WiMAX subscriber stations access the system, they first obtain the contention time slot interval and the system access parameters (e.g. initial access transmit power level). After the contention slot time period has started, the subscriber station begins to transmit an access message at a low RF power level. If the subscriber station hears a positive response to its access request message, it can transmit its package. If the subscriber device does not hear a response (e.g. another device has transmitted at the same time), it must stop transmitting and wait a random amount of time before attempting to access the system again. Each time the device attempts to access the system and fails, it must wait a longer amount of time before attempting to access the system again. This prevents the possibility of many collisions between devices that are attempting to access the system at approximately the same time.

Figure 1 illustrates how the WiMAX system can mix contention free and contention based access control on a WiMAX radio channel. This diagram shows that the downlink channel contains downlink and uplink descriptor messages that define when subscriber stations are allowed to transmit. For unicast polled devices (contention free), they are assigned specific time periods to transmit from a polling message. For multicast polled, broadcast polled or best effort devices (contention based), they compete during the contention time slot periods.

Figure 1: WiMax Access Control
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