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APPENDIX A

Brief Technical Description of the IEEE 802.11 Standard

802.11 is the standard for Wireless Local Area Networks (WLAN's) 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. The YDI

WL2400 models comply with standard for DSSS for use with BPSK modulation at a 2 Mbps data rate.

The Media Access Control (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. This technique 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 back off interval) to wait before

"listening" again to verify a clear channel on which to transmit. If the channel is still busy, another back off

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 radio data link

characteristics [i.e. the Physical Level (PHY)]. This access method is attractive because it provides

spectral efficiency as well as asynchronous data transfer. Frequency Division Multiple Access (FDMA) and

Code Division Multiple Access (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 Physical Level (PHY) under 802.11 includes diffused infrared (DFIR), Direct Sequence Spread

Spectrum (DSSS), and Frequency Hopping 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.