WLAN (Wireless Local Area Networks )

Introduction:

A Wireless Local Area Network (WLAN) links two or more devices using some wireless distribution method (typically spread-spectrum or OFDM radio), and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network.
Networks, especially LAN, go wireless with the advent of new technologies, offering the dynamic option of mobile access—a virtual reality that will add a new dimension to efficiency and functionality related to the work place.

Basic:

Acomputer network, as we all know, is the linking of two or more computers within a well-defined area. The most common networks that are used today are wide area networks (WANs), metropolitan area networks (MANs) and local area networks (LANs), of which the latter serve to provide solutions of the most general interest. Conventionally, LANs have always been wired, i.e. connected by coaxial cables, optical fibres etc. But suppose you are in your car en route to the airport or are lounging about at home, waiting for that all important E-mail from your client/employee/boss—it is then that you wish that you were somehow connected to your office network and database. All this is now a reality with the advent of new technologies, which have seen networks, especially LANs, go wireless. Wireless LANs come in the hierarchy of wireless information networks, as seen in Table I. Another related concept, akin to wireless networks, is mobile computing. But the two are not identical, as portable (or mobile) computers are often wired. For example, if a traveller plugs a portable computer (with a modem in this case) into the telephone jack of a hotel, then the service provided is mobile computing. On the other hand, some wireless computers are not portable at all. So why are networks (LANs for that matter) needed that are wireless? The answer to this question is manifold: (i) One plans for LAN wiring just as one would plan for telephone and electric power wiring while setting up a new office today. This proves to be not only a complex and cumbersome task but also very expensive. A wireless system would do away with all these problems. (ii) Buildings of historical value, and those with marble and decorative interiors etc, all pose serious problems for wiring installation. (iii) Underground wiring, though an apparent solution to the above problem, suffers from constraints like expensive installation, and difficult relocation and maintenance. (iv) A wireless network brings the processing and database capabilities of a large computer (such as a mainframe) directly to any location. Two kinds of wireless LAN (WLAN) technologies are available as of today—radio & infrared. Radio WLANs use radio waves and IR LANs use infrared light to communicate.

Types of wireless LANs:

• Peer-to-peer:

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.
If a signal strength meter is used in this situation, it may not read the strength accurately and can be misleading, because it registers the strength of the strongest signal, which may be the closest computer.

• 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.

• Wireless distribution system:

A Wireless Distribution System is a system that enables the wireless interconnection of access points in an IEEE  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.
An access point can be either a main, relay or remote base station. A main base station is typically connected to the wired Ethernet. A relay base station relays data between remote base stations, wireless clients or other relay stations to either a main or another relay base station. A remote base station accepts connections from wireless clients and passes them to relay or main stations. Connections between "clients" are made using MAC addresses rather than by specifying IP assignments.
All base stations in a Wireless Distribution System must be configured to use the same radio channel, and share WEP keys or WPA keys if they are used. They can be configured to different service set identifiers. WDS also requires that every base station be configured to forward to others in the system.
WDS may also be referred to as repeater mode because it appears to bridge and accept wireless clients at the same time (unlike traditional bridging). It should be noted, however, that throughput in this method is halved for all clients connected wirelessly.
When it is difficult to connect all of the access points in a network by wires, it is also possible to put up access points as repeaters.

• Roaming:

Internal Roaming (1): The Mobile Station (MS) moves from one access point (AP) to another AP.
A Mobile Station roaming from one access point to another often interrupts the flow of data between the Mobile Station and an application connected to the network.
External Roaming (2): The MS(client) moves into a WLAN of another Wireless Internet Service Provider (WISP) and takes their services (Hotspot)

Categories:

• Infrared LANs:

The technology used is similar to that used in consumer products such as TV set remote controls, wireless keyboards, printers etc. The transmitter uses simple, inexpensive IR LEDs and the receivers use photodiodes. The advantages of using IR technology are: (i) Simple design of transmitter and receivers. (ii) Low cost (a direct implication of the above). (iii) IR does not come under FCC (or the WPC, in India’s case) regulations. (iv) Mutual interference is relatively low, because of low power levels involved. (v) Since IR does not penetrate walls, data is confined within the four walls of a building (which implies data security). The disadvantages are: (i) IR rays are susceptible to interference from sunlight and ambient light, though this is not a serious problem and can be thwarted by ingenious use of filters. (ii) The range is limited with respect to radio LANs, as seen in Table I. There are two types of IR LANs: Direct beam IR (DBIR) LANs. These use what are referred to as line-of- sight links. This involves the trans- mission of highly focussed narrow IR beams that connect one terminal to another. Consequently, the receiver and transmitter must be properly aligned. This gives a higher signal- to-noise ratio, longer ranges and higher data rates (up to 10 megabits per second). Because of the need for aligning the directions of transmitter and receiver, these are best suited for fixed terminals and especially for large file transfers between mainframes. Diffused IR (DFIR) LANs. DFIR LANs provide ease of installation as they are more mobile. These function by the transmission of signals in all directions and IR energy flooding an entire room. The transmission-reception is made more efficient by using a passive reflector or an active repeater, as seen in Fig. 2.Because of the multiple paths involved, data rates are limited to about 1 Mbps. The standards for DFIR LANs are given as IEEE 802.11. New generation laptops have an in-built IR transceiver chip (as small as 1 cm2) which enables communication between the portable laptop and a fixed terminal, printer or any peripheral. The main advantage of using infrared is the reduced cost (they are much cheaper than radio LANs), and this is at the cost of limited ranges and, of course, interrupted transmissions when obstacles are present (since infrared does not penetrate solid matter).

• Radio LANs:

These are the alternate solutions to IR technologies. These have their own pros and cons and use 902-928MHz, 2.4-2.4835GHz and 5.725-5.85GHz (ISM bands) frequencies. These bands are most susceptible to interference from radar, microwave ovens, medical equipment etc, and so the radiated power must be limited to below 1W. Radio frequency (RF) LANs are of two types: (i) spread-spectrum LANs and (ii) microwave LANs. Spread-spectrum LANs. In spread-spectrum transmission, the signal is made to occupy a fairly broad band of frequencies, but with very low average power density. Spread-spectrum messages resist interference better than narrow band signals. Because they take up so much bandwidth, most interfering transmissions affect just a small part of the message, instead of wiping it out completely. Two types of techniques are used in concordance with this technology: (i) direct sequence (DS) transmission and (ii) frequency hopping (FH) transmission

• Frequency hopping transmission:

A very important advantage of using spread-spectrum LANs is that no licensing is required, as the radia- ted power is kept very low. As a consequence, they are the most popular variety of radio LANs and are used where the ranges required are significantly higher (compared to infrared LANs), though setting up a radio LAN is in itself an expensive business. Microwave LANs. These LANs operate at radio frequencies in the microwave range. The most common frequencies are the ones between 18-19 GHz. They offer higher data rates than IR LANs but have to be licensed and coordinated within geographic areas to prevent interference between systems. Microwave LANs are best suited for areas like an open office where there are very few obstructions like concrete walls and floors.

Applications:

As is obvious, the applications for WLAN systems exist only in cases where a wired solution is not feasible, either because of exorbitant installation costs, or because just a temporary network is required. Another very common situation is one where a wired network is complemented with a wireless set-up (wireless access is possible to an existing wired network). For example, at the Carnegie Mellon University in the US, an experimental wireless LAN set-up enables access to the university’s optical fibre network from any part of the campus. All that the students have to do is point their laptops (with IR transceiver chips) to a wireless ‘access point’ (a network hub) to hook onto the optical fibre network. Other applications typically include offices which need frequent relocation, e.g. political campaign offices, large industrial fairs, conference centres, sites of natural calamities (where the existing telephone system has broken down and cannot support data communication). Another exciting possibility is that of using a cellular telephone with a traditional analogue modem to create, in effect, a wireless connection. This service, called CDPD (cellular digital packet data), is becoming available in many cities.

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