Wednesday, August 22, 2012

WiMAX Media Access Control (MAC)

The IEEE 802.16 MAC is significantly different from IEEE 802.11b Wi-Fi MAC. In Wi-Fi, the MAC uses contention access — all subscribers wishing to pass data through an AP compete for the AP’s attention on a random basis. This can cause distant nodes from the AP to be repeatedly interrupted by less sensitive, closer nodes, greatly reducing their throughput. This makes services, such as VoIP or IPTV which depend on a determined level of QoS, difficult to maintain for large numbers of users.

The MAC layer of 802.16 is designed to serve sparsely distributed stations with high data rates. Subscriber stations are not required to listen to one another because this listening might be difficult to achieve in the WiMAX environment. The 802.16 MAC is a scheduling MAC where the subscriber only has to compete once (for initial entry into the network). After that it is allocated a time slot by the base station. The time slot can enlarge and constrict, but it remains assigned to the subscriber, meaning that other subscribers are not supposed to use it but take their turn. This scheduling algorithm is stable under overload and oversubscription. It is also more bandwidth-efficient. The scheduling algorithm allows the BS to control QoS by balancing the assignment among the needs of subscribers.

Duplexing , a station’s concurrent transmission and reception, is possible through TDD and FDD. In TDD, a station transmits then receives (or vice versa) but not at the same time. This option helps reduce subscriber station costs, because the radio is less complex. In FDD, a station transmits and receives simultaneously on different channels.

The 802.16 MAC protocol is connection-oriented and performs link adaptation and ARQ functions to maintain target BER while maximizing the data throughput. It supports different transport technologies such as IPv4, IPv6, Ethernet, and ATM. This lets service providers use WiMAX independently of the transport technology they support.

The recent WiMAX standard, which adds full mesh networking capabilities, enables WiMAX nodes to simultaneously operate in “ subscriber ” and “ base station ” mode. This blurs the initial distinction and allows for widespread adoption of WiMAX-based mesh networks and promises widespread WiMAX adoption. Mobile WiMAX with OFDMA and scheduled MAC allows wireless mesh networks to be much more robust and reliable.

Source of Information : Elsevier Wireless Networking Complete

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