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WiMAX Media Access Control (MAC)
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Bog'liq garg200721.15.2 WiMAX Media Access Control (MAC)
The IEEE 802.16 MAC is signifi cantly different from that of IEEE 802.11b
Wi-Fi MAC. In Wi-Fi, the MAC uses contention access — all subscribers wishing
to pass data through an access point compete for the access point’s (AP’s) atten-
tion 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,
diffi cult 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 diffi cult 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 effi cient. The scheduling algorithm allows the base station
to control QoS by balancing the assignment among the needs of subscribers.
Duplexing, a station’s concurrent transmission and reception, is possible
through time division duplex (TDD) and frequency division duplex (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 adapta-
tion and ARQ functions to maintain target bit error rate while maximizing the
data throughput. It supports different transport technologies such as IPv4, IPv6,
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