10CE100 Wireless LAN

1. INTRODUCTION

WIRELESS LOCAL AREA NETWORK

A wireless LAN (or WLAN, for wireless local area network, sometimes referred to as

LAWN, for local area wireless network) is one in which a mobile user can connect to a

local area network (LAN) through awireless (radio) connection. The IEEE 802.11 group

of standards specify the technologies for wireless LANs. 802.11 standards use

the Ethernet protocoland CSMA/CA (carrier sense multiple access with collision

avoidance) for path sharing and include an encryption method, the Wired Equivalent

Privacy algorithm .

High-bandwidth allocation for wireless will make possible a relatively low-cost wiring of

classrooms in the United States. A similar frequency allocation has been made in Europe.

Hospitals and businesses are also expected to install wireless LAN systems where

existing LANs are not already in place.

Using technology from the Symbionics Networks, Ltd., a wireless LAN adapter can be

made to fit on a Personal Computer Memory Card Industry Association (PCMCIA) card

for a laptop or notebook computer. 

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1.1Security Because wireless technology has roots in military applications, security has long

been a design criterion for wireless devices. Security provisions are typically built into

wireless LANs, making them more secure than most wired LANs. It is extremely difficult

for unintended receivers (eavesdroppers) to listen in on wireless LAN traffic. Complex

encryption techniques make it impossible for all but the most sophisticated to gain

unauthorized access to network traffic. In general, individual nodes must be security-

enabled before they are allowed to participate in network traffic.

1.2Cost A wireless LAN implementation includes both infrastructure costs for the wireless

access points and user costs for the wireless LAN adapters. Infrastructure costs depend

primarily on the number of access points deployed; access points range in price from

$800.00 to $2,000.00. The number of access points typically depends on the required

coverage region and/or the number and types of users to be serviced. The coverage area is

proportional to the square of the product range. 

Wireless LAN adapters are required for standard computer platforms, and range in

price from $200.00 to $700.00. The cost of installing and maintaining a wireless LAN is

generally lower than the cost of installing and maintaining a wired LAN for two reasons.

First, a WLAN eliminates the direct costs of cabling and the labor associated with

installing and repairing it. Second, because WLANs simplify moves, adds, and changes,

they reduce the indirect costs of user downtime and administrative overhead. 

1.3Scalability Wireless networks can be designed to be extremely simple or quite complex.

Wireless networks can support large numbers of nodes and/or large physical areas by

adding access points to boost or extend coverage. 

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1.4 Battery Life for Mobile Platforms

End-user wireless products are capable of being completely untethered, and run

off the battery power from their host notebook or hand-held computer. WLAN vendors

typically employ special design techniques to maximize the host computer’s energy usage

and battery life.

1.5 Safety The output power of wireless LAN systems is very low, much less than that of a

hand-held cellular phone. Since radio waves fade rapidly over distance, very little

exposure to RF energy is provided to those in the area of a wireless LAN system.

Wireless LANs must meet stringent government and industry regulations for safety. No

adverse health affects have ever been attributed to wireless LANs.

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2. HISTORY OF WIRELESS LAN

The first generation of wireless data modems was developed in the early 1980s

by amateur radio operators, who commonly referred to this as packet radio. They added a

voice band data communication modem, with data rates below 9600-bit/s, to an existing

short distance radio system, typically in the two meter amateur band. The second

generation of wireless modems was developed immediately after the FCC announcement

in the experimental bands for non-military use of the spread spectrum technology. These

modems provided data rates on the order of hundreds of kbit/s. The third generation of

wireless modem then aimed at compatibility with the existing LANs with data rates on

the order of Mbit/s. Several companies developed the third generation products with data

rates above 1 Mbit/s and a couple of products had already been announced by the time of

the first IEEE Workshop on Wireless LANs."

"The first of the IEEE Workshops on Wireless LAN was held in 1991. At that

time early wireless LAN products had just appeared in the market and the IEEE

802.11committee had just started its activities to develop a standard for wireless LANs.

The focus of that first workshop was evaluation of the alternative technologies. By 1996,

the technology was relatively mature, a variety of applications had been identified and

addressed and technologies that enable these applications were well understood. Chip sets

aimed at wireless LAN implementations and applications, a key enabling technology for

rapid market growth, were emerging in the market. Wireless LANs were being used in

hospitals, stock exchanges, and other in building and campus settings for nomadic access,

point-to-point LAN bridges, ad-hoc networking, and even larger applications through

internetworking. The IEEE 802.11 standard and variants and alternatives, such as the

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wireless LAN interoperability forum and the EuropeanHiperLAN specification had made

rapid progress, and the unlicensed PCS Unlicensed Personal Communications

Services and the proposed SUPERNet, later on renamed as U-NII, bands also presented

new opportunities."

WLAN hardware was initially so expensive that it was only used as an alternative

to cabled LAN in places where cabling was difficult or impossible. Early development

included industry-specific solutions and proprietary protocols, but at the end of the 1990s

these were replaced by standards, primarily the various versions of IEEE 802.11 (Wi-Fi).

An alternative ATM-like 5 GHz standardized technology, HiperLAN/2, has so far not

succeeded in the market, and with the release of the faster 54 Mbit/s 802.11a (5 GHz)

and 802.11g (2.4 GHz) standards, almost certainly never will.

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3. TYPES OF WLAN

3.1 Peer-to-peer

Peer-to-Peer or ad-hoc wireless LAN

An ad-hoc network is a network where stations communicate only peer to peer

(P2P). There is no base and no one gives permission to talk. This is accomplished using

the Independent Basic Service Set (IBSS).

A peer-to-peer (P2P) network allows wireless devices to directly communicate

with each other. Wireless devices within range of each other can discover and

communicate directly without involving central access points. This method is typically

used by two computers so that they can connect to each other to form a network.

3.2 Bridge

A bridge can be used to connect networks, typically of different types. A

wireless Ethernet bridge allows the connection of devices on a wired Ethernet network to

a wireless network. The bridge acts as the connection point to the Wireless LAN.

3.3 Wireless distribution system

A Wireless Distribution System is a system that enables the wireless

interconnection of access points in an IEEE 802.11 network. It allows a wireless network

to be expanded using multiple access points without the need for a wired backbone to link

them, as is traditionally required. The notable advantage of WDS over other solutions is

that it preserves the MAC addresses of client packets across links between access points.

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4. PHYSCAL LAYER

The PHY layer, which actually handles the transmission of data between nodes,

can use either direct sequence spread spectrum, frequency hopping spread spectrum, or

infrared (IR) pulse position modulation. IEEE 802.11 makes provisions for data rates of

either 1 Mbps or 2 Mbps, and calls for operation in the 2.4 - 2.4835 GHz frequency band

(in the case of spread-spectrum transmission), which is an unlicensed band for industrial,

scientific, and medical (ISM) applications, and 300 - 428,000 GHz for IR transmission.

4.1. Infrared (IR)

Infrared is generally considered to be more secure to eavesdropping, because IR

transmissions require absolute line-of-sight links (no transmission is possible outside any

simply connected space or around corners), as opposed to radio frequency transmissions,

which can penetrate walls and be intercepted by third parties unknowingly. Infrared

transmissions can be adversely Provide data rate between 1Mbs and 2Mbps at a

wavelength between 850nm and 950 nm. It is immune to electrical interface. However,

infrared transmissions can be adversely affected by sunlight, and the spread-spectrum

protocol of 802.11 does provide some rudimentary security for typical data transfers.

4.2. Spread Spectrum Radio

Spread-spectrum protocol of 802.11 does provide some rudimentary security for

typical data transfers. Its data rate is 2Mbps.I t is easy to generate and can travel long

distance. It can also penetrate through wall.

Two types of this technology

---Frequency Hopping Spread Spectrum (FHSS)

---Direct Sequence Spread Spectrum (DSSS)

4.3. Frequency Hopping Spread Spectrum (FHSS)

In a Frequency Hopping Spread Spectrum (FHSS) system, the data is modulated

on to the carrier in a manner identical to that employed for standard narrow band

communications. Most frequency hopping systems employ Gaussian Frequency Shift

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Keyed modulation, either two or four level. The carrier frequency is then changed

(hopped) to a new frequency in accordance with a pre-determined hopping sequence. If

the receiver frequency is then hopped in synchronism with the transmitter, data is

transferred in the same manner as if the transmitter and receiver were each tuned to a

single fixed frequency. If different transmitter-receiver pairs hop throughout the same

band of frequencies, but using different hopping sequences, then multiple users can share

the same frequency band on a non-interfering basis.

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5.WLAN Configurations

5.1 Independent WLANs The simplest WLAN configuration is an independent (or peer-to-peer) WLAN

that connects a set of PCs with wireless adapters. Any time two or more wireless adapters

are within range of each other, they can set up an independent network (Figure 3). These

on-demand networks typically require no administration or preconfiguration. 

 

Figure 3.Independent WLAN

Access points can extend the range of independent WLANs by acting as a repeater (see

Figure 4), effectively doubling the distance between wireless PCs. 

Figure 4. Extended-Range Independent WLAN Using Access Point as Repeater

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5.2Infrastructure WLANs

In infrastructure WLANs, multiple access points link the WLAN to the wired

network and allow users to efficiently share network resources. The access points not

only provide communication with the wired network but also mediate wireless network

traffic in the immediate neighborhood. Multiple access points can provide wireless

coverage for an entire building or campus.

 

Figure 5. Infrastructure WLAN

6. Advantages of WLANs

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WLANs have advantages and disadvantages when compared with wired LANs.

AWLAN will make it simple to add or move workstations and to install access points to

provide connectivity in areas where it is difficult to lay cable. Temporary or semi

permanent buildings that are in range of an access point can be wirelessly connected to a

LAN to give these buildings connectivity. Where computer labs are used in schools, the

computers (laptops) could be put on a mobile cart and wheeled from classroom to

classroom, provided they are in range of access points. Wired network points would be

needed for each of the access points. A WLAN has some specific advantages:

It is easier to add or move workstations.

It is easier to provide connectivity in areas where it is difficult to lay cable.

Installation is fast and easy, and it can eliminate the need to pull cable through

walls and ceilings.

Access to the network can be from anywhere within range of an access point.

Portable or semi permanent buildings can be connected using a WLAN.

Although the initial investment required for WLAN hardware can be similar to

the cost of wired LAN hardware, installation expenses can be significantly lower.

When a facility is located on more than one site (such as on two sides of a road), a

directional antenna can be used to avoid digging trenches under roads to connect

the sites.

In historic buildings where traditional cabling would compromise the façade,

a WLAN can avoid the need to drill holes in walls.

Long-term cost benefits can be found in dynamic environments requiring frequent

moves and changes.

7. DISADVANTAGES OF WLAN

WLANs also have some disadvantages:

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As the number of computers using the network increases, the data transfer rate to

each computer will decrease accordingly.

As standards change, it may be necessary to replace wireless cards and/or access

points.

Lower wireless bandwidth means some applications such as video streaming will

be more effective on a wired LAN.

Security is more difficult to guarantee and requires configuration.

Devices will only operate at a limited distance from an access point, with the

distance determined by the standard used and buildings and other obstacles

between the access point and the user.

A wired LAN is most likely to be required to provide a backbone to the WLAN;

a WLAN should be a supplement to a wired LAN and not a complete solution.

Long-term cost benefits are harder to achieve in static environments that require

few moves and changes..

8. WLAN limitations 

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The first limitation of a WLAN is often overlooked—WLANs transmit data via

radio waves. In the case of 802.11b and 802.11g, they use the 2.4GHz ISM (Industrial,

Scientific and Medical) band and 5GHz band respectively.

There are things which stop radio waves, like metal boxes—many industrial

buildings act like metal boxes. In such cases, you can use wireless transmission either

inside the building or outside it, but not both unless you install either a dual antenna

access point (with one antenna inside the building and one outside) or two access points.

Certain types of equipment, like X-Ray machines, are also metal boxes. Bank and/or

document vaults are, in effect, metal boxes too.

Metal boxes are not the only thing which can stop a radio wave, especially a low

power signal of the sort used by WLANs. Distance, ordinary walls, thick stone walls,

trees and vegetation, all interfere with the transmission of a microwave signal.

Naturally the people writing the standards for WLANs take all these situations

into account, but in practice one has to physically survey a proposed WLAN site with at

least a WLAN access point and one or more portable clients. Since the signal

characteristics can change from acceptable to unusable within a meter or two, such

surveys have to be done very carefully and in great detail.

The next problem when deploying a WLAN, especially in the unlicensed 2.4GHz

ISM band used by 802.11b and 802.11g, is competition for the spectrum. Since the

2.4GHz band is unlicensed, there are a lot of other devices which use the same band. Two

common examples of this are digital cordless telephones and devices using Bluetooth. If,

for example, a company chose to use Bluetooth telephone handsets, then it is very likely

that WLANs operating in the 2.4GHz band would be unreliable. The company would

have to restrict itself to a WLAN in the 5GHz band—i.e. 802.11a—which would be more

expensive than the 2.4GHz option. A 5GHz WLAN may also incur a license fee

depending on local regulations, and would almost certainly require more access points as

higher frequencies do not propagate as well as lower ones in a physically cluttered

environment. Changing from 2.4GHz units to 5GHz would require a complete resurvey of

the WLAN site, which would further add to the cost of the upgrade.

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Another significant consideration for would-be WLAN builders is that the 802.11

standards family provide shared bandwidth; access points are bridges not switches. For

this reason all the limitations of shared bandwidth in copper Ethernet environments apply

to WLANs, but unfortunately the most common solution—segmentation—is much harder

to apply.

In a wired network you segment the system by simply breaking the network in

half and adding a bridge or a switch, but you can't break a wireless link in half. You can

add more access points to a given area, but not without limit as they are all using the same

set of channels. The 802.11 Task Groups are working to ensure that WLAN channels are

used as efficiently as possible, but the fact remains that copper-based LANs are always

going to permit greater client density than WLANs.

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9. APPLICATIONS OF WIRELESS LAN

Wireless LANs frequently augment rather than replace wired LAN networks-often

providing the final few meters of connectivity between a backbone network and the

mobile user. The following list describes some of the many applications made possible

through the power and flexibility of wireless LANs:

Doctors and nurses in hospitals are more productive because hand-held or

notebook computers with wireless LAN capability deliver patient information

instantly.

Consulting or accounting audit engagement teams or small workgroups

increase productivity with quick network setup.

Network managers in dynamic environments minimize the overhead of moves,

adds, and changes with wireless LANs, thereby reducing the cost of LAN

ownership.

Training sites at corporations and students at universities use wireless

connectivity to facilitate access to information, information exchanges, and

learning.

Network managers installing networked computers in older buildings find

that wireless LANs are a cost-effective network infrastructure solution.

Retail store owners use wireless networks to simply frequent network

reconfiguration.

Trade show and branch office workers minimize setup requirements by

installing preconfigured wireless LANs needing no local MIS support.

Warehouse workers use wireless LANs to exchange information with central

databases and increase their productivity.

Network managers implement wireless LANs to provide backup for mission-

critical applications running on wired networks.

Senior executives in conference rooms make quicker decisions because they

have real-time information at their fingertips.

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10. REFERENCES

1. http:// searchmobilecomputing.techtarget.com/definition/wireless-LAN

2. http://en.wikipedia.org/wiki/Wireless_LAN

3. http://www.pulsewan.com/data101/wireless_lan_basics.htm

4. http://www.cwnp.com/learning_center/index_applications.html

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