13
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.
