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WLAN: no more strings attached
A WIRELESS LAN is an extension of a wired LAN. WLAN supports network communication over short distances. Its components convert data packets into radio waves or infrared (IR) pulses and send them over to other wireless networks.
A Wireless LAN is built by attaching a device called the access point (AP) to the edge of the wired network that serves as a gateway to the wired LAN. Clients then communicate with the AP using a wireless network adapter similar to the traditional Ethernet adapter.
Standards
Most WLANs today are based on the IEEE 802.11 and 802.11b standards. These standards permit data transmissions at 1 to 2 Mbps or 5 to 11Mbps, respectively, and specify a common architecture, transmission methods, and other aspects of wireless data transfer to improve interoperability amongst products.
The IEEE has defined the 802.11 standard for wireless LANs. The standard defines three alternative physical interfaces, two radio frequency (direct sequence spread spectrum and frequency hopping spread spectrum) and one infrared (IR). The spread spectrum RF products provide 1 to 2 Mbps data rates and a range between 50 to 1000 feet depending on the environment (type of building construction, interference sources, etc).
Another infrared standard, IrDA, is mainly concerned with communication between two IR-equipped devices, whereas 802.11 deals with communication between multiple IR-equipped devices. There are few devices that implement the 802.11 IR standard. Many companies produce 802.11 RF products. The standard was finalized in 1997 after seven years of work. Some vendors began shipping standard compliant products in 1998, many have been shipping pre-standard products for years.
Wireless LANs have a range of technologies to choose from when designing a wireless LAN solution. Each technology comes with its own set of advantages and limitations.
Narrowband
A narrowband radio system transmits and receives user information on a specific radio frequency. Narrowband radio keeps the radio signal frequency as narrow as possible just to pass the information. Undesirable crosstalk between communications channels is avoided by carefully coordinating different users on different channel frequencies.
A private telephone line is much like a radio frequency. When each home in a neighbourhood has its own private telephone line, people in one home cannot listen to calls made to other homes. In a radio system, privacy and non-interference are accomplished by the use of separate radio frequencies. The radio receiver filters out all radio signals except the ones on its designated frequency.
From a customer standpoint, one drawback of narrowband technology is that the end-user must obtain an FCC license for each site where it is employed.
Spread spectrum
Most wireless LAN systems use spread-spectrum technology, a wideband radio frequency technique developed by the military for use in reliable, secure, mission-critical communications systems. Spread-spectrum is designed to trade off bandwidth efficiency for reliability, integrity, and security. In other words, more bandwidth is consumed than in the case of narrowband transmission, but the tradeoff produces a signal that is, in effect, louder and thus easier to detect, provided that the receiver knows the parameters of the spread-spectrum signal being broadcast. If a receiver is not tuned to the right frequency, a spread-spectrum signal looks like background noise. There are two types of spread spectrum radio: frequency hopping and direct sequence.
Frequency-hopping spread spectrum technology: FHSS uses a narrowband carrier that changes frequency in a pattern known to both transmitter and receiver. Properly synchronized, the net effect is to maintain a single logical channel. To an unintended receiver, FHSS appears to be short-duration impulse noise.
Direct-sequence spread spectrum technology: DSSS generates a redundant bit pattern for each bit to be transmitted. This bit pattern is called a chip (or chipping code). The longer the chip, the greater the probability that the original data can be recovered (and, of course, the more bandwidth required). Even if one or more bits in the chip are damaged during transmission, statistical techniques embedded in the radio can recover the original data without the need for retransmission. To an unintended receiver, DSSS appears as a low-power wideband noise and is rejected by most narrowband receivers.
Infrared
A third technology, little used in commercial wireless LANs, is infrared. Infrared (IR) systems use very high frequencies, just below visible light in the electromagnetic spectrum, to carry data. Like light, IR cannot penetrate opaque objects; it is either directed (line-of-sight) or diffuse technology. Inexpensive directed systems provide very limited range (3 ft) and typically are used for personal area networks but occasionally are used in specific wireless LAN applications. High performance directed IR is impractical for mobile users and is therefore used only to implement fixed sub-networks. Diffuse (or reflective) IR wireless LAN systems do not require line-of-sight, but cells are limited to individual rooms.
How WLANs work
Wireless LANs use electromagnetic airwaves (radio or infrared) to communicate information from one point to another without relying on any physical connection. Radio waves are often referred to as radio carriers because they simply perform the function of delivering energy to a remote receiver.
The data being transmitted is superimposed on the radio carrier so that it can be accurately extracted at the receiving end. This is generally referred to as modulation of the carrier by the information being transmitted. Once data is superimposed (modulated) onto the radio carrier, the radio signal occupies more than a single frequency, since the frequency or bit rate of the modulating information adds to the carrier.
Multiple radio carriers can exist in the same space at the same time without interfering with each other if the radio waves are transmitted on different radio frequencies. To extract data, a radio receiver tunes in one radio frequency while rejecting all other frequencies.
In a typical wireless LAN configuration, a transmitter/receiver (transceiver) device, called an access point, connects to the wired network from a fixed location using standard cabling. At a minimum, the access point receives, buffers, and transmits data between the wireless LAN and the wired network infrastructure. A single access point can support a small group of users and can function within a range of less than one hundred to several hundred feet. The access point (or the antenna attached to the access point) is usually mounted high but may be mounted essentially anywhere that is practical as long as the desired radio coverage is obtained.
End-users access the wireless LAN through wireless-LAN adapters, which are implemented as PC cards in notebook or palmtop computers, as cards in desktop computers, or integrated within hand-held computers. wireless LAN adapters provide an interface between the client network operating system (NOS) and the airwaves via an antenna. The nature of the wireless connection is transparent to the NOS.
Security issues
Security has always been lower in WLANS compared to the regular networks. Work is still being done to improve the level of security in all transmissions. Breakthrough technologies like WEP have promised to raise the level of security on wireless networks to rival that of a traditional wired network.
The writer
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