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Wireless Networking

PC - Radio Interface

INTRODUCTION TO WIRELESS COMMUNICATION
 
As standards emerge, data rates rise, and laptops seek connectivity, Wireless LANs are finding a market niche as extensions to wired networks. Still the world’s wireless lan market remains a small one. The technology is new, so components are expensive and data rates are low. A network interface card(NIC) needed to hook up a PC to a wired Ethernet LAN with a peak data rate of 10 Mbps cost less than $100. On the other hand, the card needed to interface the same PC to a wireless radio LAN costs $500 with peak data rates of just 1 to 2 Mbps. The popularity of wireless data links is also damped by the absense of standards for radio LANs. Swapping wires for radio waves means more than buying new adapters. It involves with dealing with interference, ensuring that all areas are covered, tracking a mobile users’ whereabouts, and managing battery power, among other things.

Three physical media are defined in the current wireless standard.
 

 Whether short range IR or long range radio, all wireless data technologies assume one of the two basic configurations : Infrastructure or AdHoc . In the infrastructure networks - by far the most common type of radio lans - computers outfitted with wireless transceivers communicate with each other and a wired lan through access points scattered throughout a building. In an Adhoc wireless network, computers fitted with the appropriate adapters communicate directly without installed access points. Such arrangements may be semipermanent or temporary.

Linking With Light - Infrared

There are two basic types of IR links.

LOS links are highly focussed narrow beams that send signals directly over the shortest path from the sender to the receiver. These LOS systems transmit IR beams from one computer to the next in an adhoc setup using optical nodes attached to standard NICs in each machine. Communicating nodes may be as far as 25m apart, and like all IR devices, can be used for single room applications. Diffuse IR floods an entire room. The light is beamed in a roughly spherical pattern that bounces off various room surfaces to reach its destination. It is similar to radio LANs.

Spread Spectrum Techniques

The Federal Communications Commission (FCC) has opened up three frequency bands for licenced commercial use.

To prevent interfernce within these so called Industrial, Scientific and Medical (ISM) bands, the FCC required commercial equipment to operate at low power levels of 1 watt or less and to employ Spread Spectrum (SS) transmision. SS messsages resist interference better than narrow band signals.

 In DSSS, RF enegry is spread across a wide frequency band ( thus lowering its power density ). By replacing each data bit by multiple sub-bits, called CHIPS, that occupy the same time interval. The length of the chip sequence is known as the processing gain or spreading ratio. The upper limit on processing gain is determined mainly by the available RF B/W.

FHSS spreads the radio energy over a wide band by moving between frequencies in time. A transmitter jumps from one narrow band frequency to another at a specific rate and in accordance with a code sequence, sending several data bits at each frequency.Interference is minimised by limiting the time spent at each frequency, thus lowering the chance that two transmitters will try to use the same frequency at the same time. Besides limiting interference from outside sources, SS technology can help prevent two separate networks installed in the same building from getting into each others way. Frequency Hopping may be employed with each network using its own hopping sequence.
 
Standards for Wireless LANs

The first set of standards for wireless radio LANs( WLANs ) and diffuse IR is called IEEE 802.11. It hopes that the new radio standard for the 2.4 Ghz ISM band will provide inter-operability among different manufacturers’ WLAN products, giving consumers greater choice in what they are buying. Standards in addition promise to lower prices through increased vendor competetion and reliance on mass produced, tandard chipsets in place of expensive , proprietary technologies.

802.11 covers
 

Another standard developed by European Telecommunication Standards Institute (ETSI) for second generation wireless technology is HIPERLAN I - a high speed packet standard promising data rates upto 24 Mbps. A new emerging standard is the HIPERLAN II, which offers much higher data rates for video and multimedia applications.