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WaveLAN: Making waves

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CIOL Bureau
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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.

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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

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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.

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