WaveLAN: Making waves

The buzzword these days seems to be wireless; anything wired seems to be on
its way out. Likewise, LAN, or to be precise, wired Local Area Network is also
becoming passe with the coming of age of WLAN or wireless LAN. The standard for
WLAN was proposed by IEEE and called IEEE802.11, 802.11a, 802.11b of which
802.11b is by far the most popular. This specification is a WLAN standard that
defines a set of specifications for the two layers of WLAN, the physical layer (PHY)
and Medium Access Control (MAC) layer.

The PHY layer handles the transmission of data between nodes while the MAC
layer is a set of protocols responsible for maintaining order in the use of the
shared medium. It serves as the interface between the PHY layer and the host
device. Any product that is compliant with the IEEE802.11b specification gets
the Wi-Fi certification, which is the seal of approval given by WECA
(Wireless Ethernet Compatibility Alliance). Wi-Fi here stands for Wireless
Fidelity just like hi-fi stands for high fidelity in audio systems. The Wi-Fi
certification ensures product interoperability between vendors. It can cover
distances up to 1,000 feet in open areas and about 250 to 400 feet in closed
areas.

Operation

WLAN uses radio waves to transmit and receive information, thus replacing
physical media like wires and fiber optic cables. The data being transmitted is
super imposed on the radio carrier, which is the process of modulation. The
radio receiver extracts data by tuning in on that radio frequency while
rejecting others. To do this WLAN requires two components, the access point and
the adapter. An access point actually connects the wired and wireless LAN. It is
a transceiver that connects to the wired network from a fixed location using
standard Ethernet cable. Users access the WLAN through wireless LAN adapters
equipped with transceiving capabilities.

IEEE802.11b layers

PHY layer

The IEEE standard specifies 3 different kinds of modulation in the PHY layer
which are Direct Sequence Spread Spectrum (DSSS), Frequency Sequence Spread
Spectrum (FSSS) or IR pulse position modulation. Spread spectrum is used to
overcome the problems created by noise, resulting in the degradation of any
signal sent through that noise cloud. It uses multiple frequencies in the band
to increase the immunity to noise. Spread Spectrum implies that data is sent in
small pieces over a number of discrete frequencies available for use at any time
in the specified range.

The two ways in which spread spectrum is implemented are DSSS and FSSS. DSSS
devices communicate by splitting each byte of data into several parts and
sending them concurrently on different frequencies. Hence, DSSS uses a lot of
available bandwidth, about 22MHz. FSSS devices on the other hand send a short
burst of data and then shift frequencies (known as hopping), followed by another
short burst. Both DSSS and FSSS operate in the unlicensed ISM band of 2.4-2.4835
GHz frequency band.

It is all very well to talk of wireless LAN, but one of the most important
aspects that haunt wireless communication is the speed of data transfer. Well,
in this case data rates as high as 11Mbps are achievable and 22Mbps are in the
offing.

Initially, the IEEE802.11 systems employed either FHSS or DSSS and data rates
of 1 to 2Mbps could be achieved. But, as the demand for greater data rates grew,
the IEEE 802.11b spec was released. This supports 11Mbps and focuses on DSSS
using complementary code keying (CCK) modulation scheme. One thing to be noted
here is that the two technologies, DSSS and FSSS, are not interoperable. Also,
data rates of 11Mbps at all times cannot be assured. In case of disturbances,
data rates fall to 5.5Mbps, 2Mbps or 1Mbps.