IEEE 802.11

802.11 is the standard for Wireless Local Area Networks (WLANs) developed by the Institute of Electrical and Electronics Engineers (IEEE). It can be compared to the 802.3 standard for ethernet wired LANs. The goal of this standard is to tailor a model of operation in order to resolve compatibility issues between manufacturers of WLAN equipment manufacturers. Thus far, the IEEE 802.11 standards committee is revising a version of a Media Access Control - Physical Level (MAC-PHY) level.

The MAC under 802.11 is composed of several functional blocks. These include mechanisms to provide contention and contention-free access control on a variety of physical layers. The functions within the MAC are independent of data rates or physical characteristics.

The fundamental access method of the 802.11 MAC is known as Carrier Sense Multiple Access with collision avoidance, or CSMA/CA. CSMA/CA works by a "listen before talk scheme". This means that a station wishing to transmit must first sense the radio channel to determine if another station is transmitting. If the medium is not busy, the transmission may proceed. The CSMA/CA scheme implements a minimum time gap between frames from a given user. Once a frame has been sent from a given transmitting station, that station must wait until the time gap is up to try to transmit again. Once the time has passed, the station selects a random amount of time (called a backoff interval) to wait before "listening" again to verify a clear channel on which to transmit. If the channel is still busy, another backoff interval is selected that is less than the first. This process is repeated until the waiting time approaches zero and the station is allowed to transmit. This type of multiple access ensures judicious channel sharing while avoiding collisions.

This scheme allows automatic medium sharing between several devices with compatible PHYs. This access method is attractive because it provides spectral efficiency as well as asynchronous data transfer. FDMA and CDMA schemes would not be adequate because they require bandwidth used by the modulation scheme. Strict TDMA would not work well because it requires synchronization. Thus CSMA/CA, which may be thought of as a version of TDMA, is better suited to this application.

The PHY under 802.11 includes diffused infra-red (DFIR), direct sequence spread spectrum (DSSS), and frequency hopped spread spectrum (FHSS). Both spread spectrum techniques are used in the 2.4 GHz band because of wide availability in many countries and lower hardware costs in comparison to the higher microwave frequencies.

The IEEE standard supports DSSS for use with BPSK modulation at a 1 Mbps data rate, or QPSK modulation at a 2 Mbps data rate. The general band plan consists of five overlapping 26 MHz sub-bands centered at 2.412, 2.427, 2.442, 2.457, and 2.470 GHz. This scheme is used in an attempt to combat interference and selective fading.

FHSS is supported under 802.11 with GFSK modulation and two hopping patterns with data rates of 1 Mbps and 2 Mbps. Under this scheme, the band is divided into 79 sub-bands with 1 MHz bandwidth each. Each sub-band is subject to a minimum rate of 2.5 hops/s using any of three possible hop patterns (22 hops in a given pattern). The minimum hop rate ensures that each packet sent could be transmitted in a single hop so that destroyed information could be recovered in another hop. This allows an effective frequency diversity that provides excellent transmission characteristics.

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