Tuesday, November 17, 2009

Radio Link Control (RLC) | WiMAX Operation

Radio link control protocol is a layer 2 (link layer) that is used to coordinate the overall flow of data packets across the radio link. RLC uses error detection and data retransmission to increase the reliability of the radio link while reducing the error rate. WiMAX radio link control functions include power level control, periodic ranging, burst profile changes and bandwidth requests.

Power control is the process of adjusting the power level in a wireless system where the base station receiver monitors the received signal strength of mobile radios. Control messages are transmitted from the base station to the mobile telephone commanding it to raise and lower its transmitter power level as necessary to maintain a good radio communications link.

Ranging may need to be performed after the subscriber station has been inactive for a while. A timer (the T4 timer) that is continuously reset as the subscriber station communicates with the system helps determine this. If the subscriber station (SS) has not communicated with the system in awhile, the timer will not be reset and it will expire. If the timer expires, the SS must again perform ranging with the system.

The base station is responsible for assigning burst profiles. However, the subscriber station may request changes to the burst profile. This may occur as a result of an increase in the bit error rate of the received signal due to fading or interference. The subscriber station may request a change in burst profile that is more robust or offers a higher data transmission rate. The base station may grant the request, negotiate parameters or reject the request.

During a WiMAX communication session, changes in bandwidth may be requested. The subscriber station may send bandwidth request messages to the base station to increase or decrease its allocated bandwidth. Bandwidth request messages may be sent as independent messages or they may be piggybacked with other messages.

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