The Wlan

A wireless LAN or WLAN is a wireless local area network, which is the linking of two or more computers without using wires. It is the same as LAN, but has a wireless interface. WLAN utilizes spread-spectrum technology based on radio waves to enable communication between devices in a limited area, also known as the basic service set. This gives users the mobility to move around within a broad coverage area and still be connected to the network.
Wireless Local Area networks (WLANs) have been employed to add mobility features to office and campus networks since the late 1980s. This article presents a discussion of the current state of WLAN technology and some of the products available.
Physically, there are two ways to implement wireless LANs: infrared and radio. Since radio is currently the most popular choice, we will restrict this discussion to radio wireless LANs.
WLANs are a distinct category of products and technology that must be differentiated from Metropolitan Area Networks (MANs) and Wide Area Networks (WANs). Examples of wireless MANs are Ricochet, Ardis, and RAM Mobile Data which provide city-wide coverage for low bit-rate data services. The most visible wireless WAN system is the cellular telephone system which can be used for data services just as telephone land lines can be used with a modem for data services. However, the bandwidth limitations when using switched cellular technology are severe, and data connections generally are not tolerant of the extended drop-outs that can occur.
Conversely, WLANs are generally accepted to be 1Mbps links or above (although a few drop into the 100's of Kbps), short range (100's of meters) technologies which do not need to support vehicular mobility (high speed handoffs) or wide area coverage. What they do provide is the wireless equivalent of a LAN for file sharing, remote database access, file server access, internet access, e-mail, and all the other applications which operate over LANs, only the user is no longer tethered to an RJ45 wall jack. Note also that there are a few applications of WLAN technology which operate over great distances (1-20 miles) but they are not generally regarded as WLANs, but as wireless bridges or point-to-point data links.
This technology is becoming more and more popular, especially with the rapid emergence of small portable devices such as PDAs (personal digital assistants).

The notebook is connected to the wireless access point using a PCMCIA wireless card.



54 MBit WLAN PCI Card (802.11g)
WLAN, (Wireless Local Area Network), is a high-speed wireless technology for accessing the Internet or corporate intranet. Two or more WLAN-enabled devices such as Nokia 9500 Communicators or WLAN-enabled laptops can also make an independent connection between each other.
The Wi-Fi Alliance has developed specifications for three common WLAN implementations - 802.11b, 802.11g, and 802.11a. Most WLAN networks today are based on these three standards. As a result, WiFi and WLAN are almost interchangeable
terms.
HISTORY
WLAN products were initially developed to support vertical market applications, i.e. sold as components of a specific solution to a specific problem where user and station mobility were required. You may be familiar with one or more of the following WLAN vertical applications:
•    Hertz rental car portable check-in terminals,
•    Wal-Mart inventory and pricing terminals, and
•    Hospital portable patient information terminals.
In each of these applications, the WLAN was deployed to support the specific application. The WLAN was not a part of the general purpose LAN infrastructure. More recently, particularly because of recent steps towards WLAN standardization, WLANs are being deployed to support general purpose LANs.
IEEE 802.11 - A standard is born
Until the summer of 1997, there were no standards for wireless LAN products. Each vendor defined the protocols and signaling for their own products, and these proprietary products did not interoperate with one another. In 1990, the Institute of Electrical and Electronics Engineers (IEEE) formed a working group, identified as 802.11, to standardize wireless LAN signaling and protocols.
The standard developed by working group 802.11 was accepted by the IEEE board during the summer of 1997 and became IEEE standard 802.11-1997. The new standard defines three different physical implementations (signaling techniques and modulations), a Media Access Control function, and a Management function. The three physical implementations are:

1.    Direct sequence spread spectrum radio (DSSS) in the 2.4 GHz band,
2.    Frequency hopping spread spectrum radio (FHSS) in the 2.4 GHz band, and
3.    Infrared light (IR).
All of the implementations support data rates of 1 Mb/s and, optionally, 2 Mb/s. The 802.11 working group is currently considering additions to the standard that will provide higher data rates (5.5 and 11 Mb/s) in the 2.4 GHz band and additions that will allow wireless LANs to operate in a 5 GHz band. Acceptance of the standard for higher data rates in the 2.4 GHz band is expected in October of 1998.
Need For WLANs
One characteristic of the IT industry is that it tends to over hype 'new' technology while ignoring the applicable lessons taught by older technologies—many of which were over hyped in their day. Wireless LAN (WLAN) technology, specifically the 802.11 family of standards, is a current case in point.
Yes, it can be very useful and when deployed properly, very convenient. A wireless network can be used to solve all sorts of tricky sites and situations, but it has its limitations. These limitations are often the same ones that older technology, such as shared Ethernet, had. Of course, limitations of new technology often get lost in the hype.
WLAN Limitations
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 stonewalls, 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.
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.
Versions of WLAN:
802.11b: The most common type of WLAN today, operating in the 2.4GHz band. Top transfer speed is 11 Mbps.
802.11g: Also
operates at 2.4GHz, but transports data over improved and faster protocols. Top transfer speed is 54 Mbps.
802.11a: Operates at 5GHz. Uses the same protocols as 802.11g. Top transfer speed is 54 Mbps.
Working of WLANs
Wireless LAN cards are PC cards - interface cards complying with the PCMCIA standard - with a transmitter and receiver inside.   PCMCIA cards require a PCMCIA slot, something now standard on laptops, but rarely found on a desktop machine. However, there is technology available to provide a PC card interface on a desktop platform using a standard PCI slot.
The technology supports Windows 9x, ME, NT4 and 2000 as well as LINUX and also works on the MAC. Once the adapter is installed it can be used with any make of wireless LAN PC card (or in fact with ANY PCMCIA card, including flash, SRAM, compact flash and modem cards.) and you can even hot-swap cards without restarting the PC.
Architecture of a Wireless LAN

Wireless Local Area Network Architecture using an Infrastructure BSS
Stations
All components that can connect into a wireless medium in an a network are referred to as stations. All stations are equipped with wireless network interface cards (WNICs). Stations fall into one of two categories: Wireless Clients and Access Points.
Access Points (AP’s)
Access Points are base stations for the wireless network. They transmit and receive radio frequencies for wireless enabled devices to communicate with.
Wireless Clients
Wireless clients can be mobile devices such as laptops,
personal digital assistants (PDAs), IP phones or fixed devices such as desktops and workstations that are equipped with a wireless network interface card.
Basic Service Set
The Basic Service Set (BSS) is a set of all stations that can communicate with each other. There are two types of BSS: Independent BSS and Infrastructure BSS. Every BSS has an id called the BSSID, it is the MAC address of the access point servicing the BSS.
Independent Basic Service Set
Independent BSS are an ad-hoc network that contain no Access Points. Since they do not use Access Points they can not connect to any other basic service sets.

Infrastructure Basic Service Set
An Infrastructure BSS can communicate with other stations not in the same basic service set by communicating to each other through Access Points.
Extended Service Set
An Extended Service Set (ESS) is a set of connected BSS. Access Points in an extended service set are connected by a distribution system. Each ESS has an ID called the SSID which is a 32 byte (maximum) character string. Example: linksys (the default SSID for Linksys routers).
Distribution System
A distribution system connects Access Points in an extended service set. A distribution system is usually a wired LAN but can be a wireless LAN.
Wireless LAN Applications
Wireless LANs are useful for a wide range of applications. There are some applications, however, that are more effective and efficient than others. Let's take a look at several "killer applications," which are convincing
situations that prompt the use of WLANs.
Sharing Internet Access
The most compelling reason to install a WLAN is to share a single high-speed Internet connection. This ability can benefit almost anybody, from the enterprise businessman to the college student to the person surfing the Web at home. The use of a WLAN to share a high-speed Internet connection allows the user to stay mobile and save money, because there are no wires to buy or install.
One example of a WLAN for this purpose is within a small office or home setting. Every member of a family or small business can easily share a single high-speed connection through the use of a cable/DSL modem, router, access point (or wireless router), and radio-equipped end user devices. This is very convenient and saves money because everybody can simultaneously have access to a single connection and roam anywhere in the house or office.
A WLAN increases the flexibility of the network because you can add new workstations at any time without having to run cable. The relocation of workstations, along with any printers or servers, is also very easy. WLANs also provide a high level of convenience in a larger enterprise environment because guests and corporate visitors with wireless devices can quickly connect to the network with very little configuration.
Transmitting Voice over WLANs
The use of a WLAN to transmit voice is a great solution when people need to constantly be in contact with each other. WLAN phones, which work just like cell
phones when they are in the coverage of the WLAN, are very useful in places where workers are moving around. Some examples of WLAN phone solutions include the following:
•    Retail stores where employees can communicate with each other to locate certain clothes for a customer and/or check inventory.
•    Buildings where security guards can quickly call for onsite help, the police or an ambulance.
•    Schools that need constant communications with teachers and custodians.
•    Hotels where staff members are spread out on different floors and need to respond quickly to requests.
The most attractive aspects of WLAN phones are that they carry no monthly fee for use and there is no major installation associated with them (assuming a WLAN is already in place). While the phones themselves are a little expensive at an average cost of about $475, they are still only about half the per phone cost of an equivalent wired system because they do not require any installation or wires. Conversely, a single wired phone usually runs about $900, which includes the phone and installation of wires.
Managing Manufacturing and Inventory
Many businesses benefit from using WLANs to manage their manufacturing processes. This lowers operating costs. Because the connections between the manufacturing equipment and main control systems are wireless, the company can reconfigure the assembly process at any time from anywhere, saving time and money.
A WLAN can also track and update inventory in real-time, enabling efficiency
and accuracy to increase dramatically. In a retail environment, as soon as a clerk purchases or stocks a product, a wireless management solution can update the inventory. In the manufacturing environment, WLANs can keep the raw materials and finished product statistics up-to-date. Employees equipped with wireless-enabled bar code scanners can check or change product prices and/or check the number in stock.
The improved accuracy provided by using a WLAN to manage inventory creates a chain reaction of benefits. Because the clerks enter the information directly into the main computer via handheld scanners, there is no paperwork to deal with. This significantly reduces human error when entering data, which leads to very accurate financial records. This is important to manufacturing companies because accurate financial records ensure correct taxes are paid and fines (and possible law suits) are kept to a minimum.
WLAN products offer a fast, reliable, cost-effective solution for wireless client access to the network by the following applications and environments:
    Remote access to corporate network information
    E-mail, file transfer and terminal emulation
    Difficult-to-wire environments
    Historic or older buildings
    Buildings with asbestos insulation
    Open areas where wiring is difficult to employ
    Frequently changing environments
    Retailers, manufacturers or other organizations that frequently rearrange the workplace or relocate
    Temporary LANs for special projects or
peak time usage
    Trade shows, exhibitions and construction sites that employ temporary networks. Retailers, airline and shipping companies that need additional workstations for a peak period and Auditors that require workgroups at customer sites.
    Access to database for mobile workers
    Medical, Technical and Retail specialists that require roaming access to a database or other network resources.
    SOHO (Small Office and Home Office) users
    Perfect for users that need a small, easy-to-install network that deploys rapidly.
    Inter-building connection
Wireless building-to-building networks are quickly and easily installed, require no monthly lease fees, and provide the flexibility to reconfigure easily.

Security Features

The concern with wireless networking is that the range could extend far beyond the walls and parking lots of your building or office locations. There is no way to limit the propagation. Therefore, anyone wishing to attack your network and/or steal information can easily do so by "war driving". War driving is the act of locating and possibly exploiting connections to Wireless Local Area Networks (WLANs). War driving requires the use of a computer device, an antenna, and software tools that can detect and exploit wireless access points.
WLANs may be considered similar to Broadband connections, that is, when active they expose networks to continuous probing. Once discovered, unprotected networks make for easy targets. PCs residing on WLANs may be compromised
and can expose proprietary data and be used as back doors to more secure network locations. Without appropriate safeguards, distributed denial-of-service attacks, worms and viruses can traverse these wireless access connections right into your home or business network (LAN) and hosts (Servers, PCs, ...).
A wireless access point (WAP) is a public access, just like connecting your LAN (wired or not) to the Internet, but with one major difference: only your neighborhood can access it. Is this worse or better than Internet? Even with a broadband connection, when connected to the Internet, you can change regularly form IP address and nobody knows it.


802.11x is a standard of defining port-based authentication and key distribution for wired and wireless networks. It's based on a protocol called EAP (Extensible Authentication Protocol) which facilitates the authentication process between the authenticator (access point) and the suppliant (NIC of the user computer).
Once the server approves access an additional authentication process like LEAP (Lightweight Extensible Authentication Protocol) verifies the user based on a digital certificate and then dynamically generates WEP keys. 802.11x enhances WEP by changing the shared keys as often as the network manager desires but the drawback is that it is still based on the 40-bit encryption level of WEP.
Wireless VPN - VPNs have provided security for many wired LANs and may eventually provide the same protection for WLANs as well. A VPN secures a connection
by acting as a boundary between the enterprise LAN and the Internet. Through integration with firewall software VPNs can offer authentication, privacy, access control, and traffic shaping capabilities to control bandwidth consumption. Companies can now implement a VPN through a VPN gateway to enhance security over WEP, but do not allow for much customization. New software products are being developed that involve unique policy-based controls to meet the different needs of each company.
Advantages of WLAN
Flexibility and wirelessness
Wireless networks have many advantages compared to wired networks. One of the advantages of wireless network is its flexibility. The radio waves go through the walls (or wooden floors, typically max 2) and the wireless computer can be anywhere within range of an access point (up to 300 feet for 802.11b with few obstructions). There is no need to install new network cables when a workstation is moved or deal with a tangle of cables to give everyone network access in a conference room.
Easy to use
The wireless networks are superficially very easy to set-up and use. Just plug-in a base station and equip your laptops with wireless LAN cards (or purchase new Laptops with built in WLAN capability). This is seen as a big advantage, however ignoring the need to configure the Wireless LAN with the same care as a wired network is one of the main reasons that a company's biggest security 'hole' is often its Wireless LAN. It is also a myth that wireless networks do
not need the same kind of control and maintenance that wired networks do. In fact, the need to enable IPSEC (or change WEP keys on a daily basis) and restrict access to specific computers (usually to those with specified MAC address) often means that a wireless network requires more attention (per user).
Planning
Wireless networks have some advantages related to planning. Both types of network must be carefully planned (especially with respect to capacity) and of course the Wireless base-stations still have to be plugged into the wired LAN at some point. The physical components of wired networks (cables, hubs, switches, etc.) have to be planned carefully while wireless networks are much easier in that sense. It is a mistake not to plan a wireless network for actual usage patterns - many offices quickly find that their carefully sited base-stations may give good general coverage but one becomes totally overloaded when workers move from the desks to concentrate in one area (for example, to attend a meeting).
Setting of equipment
A Wireless LAN can be almost invisible. Equipment can be placed in roof cavities or behind screens. A Wireless LAN is especially suited to sites where a wired network could be very awkward (or expensive) to implement, such as large open spaces (including temporary sites and open plan offices), museums (and other old buildings) and 'one-off' usage (such as an exhibition or fair)
Robustness
Wireless networks can be robust. If one base station goes down, users may be able
to physically move their PCs to be in range of another. It is also possible to site base stations with overlapping ranges so that users can remain connected if one base station goes down, although this is more expensive and requires careful planning. However wireless networks can suffer from radio-interference from other devices (like microwave ovens), and performance can collapse when too many users attempt to use the same access point.
Application transparency
One thing related to easy use of wireless networks is called application transparency. This means that applications which function in wired networks are made to function also in wireless networks. But there are longer delays and a narrower bandwidth which can cause problems when using the same applications in wireless networks.
Prices
The price of wireless products were once quite high for home users. PCI cards for wireless network cost 100 euros in 2000. At the end of 2004 the same card cost only 30 euros. Prices are not high anymore and wireless LAN is a common choice for home networks.
The other Benefits of WLAN are:
1.    CAMP: Convenience, Affordability, Mobility, Productivity
2.    Deployment advantages: Installation flexibility, speed and scalability
3.    Regions without or with limited wired infrastructure can easily establish wireless communication
4.    Wireless networks have a better chance of surviving disasters
5.    802.11 wireless LANs, WiMAX and 3G+ cellular networks promise high bandwidths, global mobility, quality of
service and seamless integration with one another.
6.    High speed access without cables
7.    Complements mobile networks when high speed access is required and the computer or mobile device is moving
Wi-Fi Alliance standardisation has established steady interoperability between WLAN devices from different vendors

Disadvantages of WLAN
Safety and security
Security is a major concern. The fact that someone can sit in the car park (or building next door), pick-up and store all your wireless traffic, and crack the most common implementation of Wireless encryption (WEP) in less than a week (and then read all the stored traffic) has to be a concern to any business operation (see Wardriving). Changing the wireless LAN encryption key on a daily or weekly basis, and then 'rolling it out' (by some means other than 'plain text' emails) is an extra overhead that many Companies never manage to achieve. IPSec is often used by larger businesses that have the infrastructure to support it. Wi-Fi Protected Access (WPA) is now available in most access points and provides adequate security for most purposes.
The transmission frequencies and powers used are such that wireless networks are safe to use in most environments, however (especially in the USA) many may decide that the potential risk (or liability) is just not worth the reward. Use (for example) in hospitals may be impossible to obtain liability insurance against. Most airlines also prohibit the use of any type of wireless system during flight.
Limitations
related to radio waves
Equipment has to be located to take into account the limitations of wireless operation. Users need 'line of sight' to a base station. Typical 802.11b base stations have a range of approx 300 feet (further in an open plan office). However the more walls (or floors) that are in the 'line of sight' the more the transmissions will be degraded (and the slower the network will run). Too many base-stations will result in overlapping areas of coverage and cross-interference - the base stations will adjust their frequency bands in an attempt to avoid clashing, however in 802.11b and 802.11g there are only 3 complete bands that do not overlap at least in some frequencies.
Often the unauthorized user in the car park will get better reception (as the radio waves go straight through the glass windows) than the company workers in the meeting rooms (where foil insulated partition walls block radio transmission).
Data Transfer speed
Data transfer speeds will typically not be as good as in a wired network. All users of the same base station have to share the bandwidth (typically 11Mb/s or 54Mb/s) whilst those wired to a hub or switch typically get almost the full bandwidth (typically 10 Mb/s or 100Mb/s) as long as transfer is not bottlenecked by the uplink of the hub/switch. Also, wireless data rates degrade rapidly with signal strength/interference.
Data transfer protocols
The use of wireless adds some overhead to each 'packet' of information exchanged. The TCP/IP control
protocol requires many small packets to be exchanged (to establish a link) and even after the link is established data is transferred in packets. The overhead added by wireless may cause problems with 'latency' that could lead to other equipment in the network assuming a packet has been 'dropped' and thus requesting a retransmit. This can result in a much slower connection than expected as bandwidth becomes overloaded by retransmissions.

Standards and limitations
There are also limitations concerning the wireless networks. All the wireless products have to support international standards. The international institutions introduce practices and also limit the frequencies used in wireless equipment. Publishing and implementation of regulations of this kind takes time. For this reason some companies are producing products to which they have applied patents. This means that the products of two separate companies may not work together at all. In this situation the products should be ordered only from one company if those products are used in a same wireless network. The Wi-Fi Alliance was formed to test and certify compliance with standards and promote interoperability, but standards are still being developed and vendors include their own proprietary features so interoperability is often difficult to achieve.
Global operations
There is also a disadvantage related to global operation. In theory, wireless equipment that conforms to the common standard has to function all over the world. A device
which is bought in Finland has to function also in the USA. However changing the Wireless adaptor settings to operate at the radio frequencies permitted in another country may well require re-installing the drivers (or re-running the Wireless installation software) which may be beyond the capabilities of many users.

WLAN Vs BLUETOOTH
IEEE 802.11b & 802.11a    Bluetooth
Time Table Standard in 1998, Products in 2000    Standard in 2000, products in 2001 and 2002
Frequency Band and bandwidth    IEEE 802.11b -   2.4 GHz
IEEE 802.11a - 5GHHz
IEEE 802.11g - 2.4 2.4 GHz
Speed    11 Mbps- 54 Mbps   (Effective speed - half of rated speed)    1-2 Mbps (Effective speed - less than 50% rated speed)
Modulation Technique    Spread Spectrum
OFDM
Distance Coverage    Up to 300 feet - 802.11b
Up to 60 ft - 802.11a    Up to 30 feet now   - efforts to increase coverage and speed
Number of access points required    every 200 feet - 802.11b
Every   50 feet - 802.11a Every 30 feet - 25 to 30 times number of Bluetooth access points;
Maturity    More matured products    Less matured but progressing fast
Market Penetration    Quite widespread Just starting in 2002
Interference with other devices 2.4 GHz band is polluted - significant interference here 2.4 GHz band is polluted - significant interference here
Interoperability Current problems   expected to be resolved in future Problems now but expect resolution soon
Cost    Much more expensive than Bluetooth Cost incremental in PDAs and phones