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WLAN A wireless LAN, or WLAN WLAN, is a local area network that does not have wired Ethernet connections. A WLAN can be either an extension to a current wired network or an alternative to it. Use of a WLAN adds flexibility to networking. A WLAN allows users to move around while keeping their computer connected, without having to depend on Ethernet cables. A wireless LAN, or WLAN WLANs try to provide all the features of wired LANs, but without the wires. The only differences that is noticeable to the end user Isthe speed (ranging from 1 to 54Mbps, with some manufacturers currently offering proprietary 108Mbps solutions) And security (the wireless access point is shared among everybody nearby, so security issues exist with WLANs that don't exist for wired networks). A wireless LAN, or WLAN Areas covered A small office A large campus, with neighborhood and city-wide ranges planned for the future. Commonly, WLANs employ access points that provide access within a radius of 65 to 300 feet. WLAN types The private home or small business WLAN: This consists of one or two access points covering around a 100- to 200-foot radius. WLAN types The enterprise class WLAN: This type has a larger number of individual access points covering a wider area. The access points themselves have features not needed for a home or small office, like better security, authentication, remote management, and tools to help integrate with existing networks. Each access point has a larger coverage area than home or small office products, and all are designed to work together to cover a much larger area. WLAN Types The Wireless Metropolitan Area Network (WMAN): A WMAN covers an area from multiple city blocks up to a city's boundaries. The most common type of WMAN is a collection of individual enterprise class wireless networks that collectively allow users to access all of them. In most places, WMANs usually consist of wireless networks belonging to several businesses or Internet service providers. The WMAN is also the point where you start seeing different technologies and standards. Again, the most common WMAN is basically a group of individual access points and WLANs. While you will see the term Metropolitan Area Network (MAN), don't confuse it with a Wireless Metropolitan Area Network (WMAN). Aside from the fact that MANs tend to be wired networks, they usually exist to provide connectivity to local ISPs, or to business and enterprise class LANs. In contrast, a WMAN exists to provide connectivity directly to an end user. In other words, a MAN normally acts as a backbone, while a WMAN acts as the "last mile" connection directly to a user's computer. WLAN Types Wireless WAN (Wide Area Network): Although a WAN by definition is the exact opposite of a LAN, Wireless WANs (WWANs) deserve brief mention here. Most WANs exist to connect LANs that are not in the same geographical area, and until recently this was also the case for WWANs. But recently, cellular phone companies like AT&T have begun offering WWAN technology that the end user can access directly. Those WWANs use cellular data technology to cover extremely wide areas. While they are considerably slower than wireless LAN speeds (most advertise between 50 to 144Kbps; compare this to dial-up speeds, which are around 56Kbps), they're still better than the lowest end of DSL speeds (128Kbps), plus they allow far greater mobility than standard 802.11a, b, or g wireless. Since they rely on coverage by the major cellular network providers, coverage areas for wireless Internet access tend to be more or less the same as they are for cell phones. There are many different standards competing at this level (GSM, CDMA, GPRS, 3G, to name a few). Most of them are mobile data standards that previously were used only on cell phones. How does it work? WLAN is a wireless local area network, which is the linking of two or more computers without using wires. WLAN utilizes spread-spectrum or OFDM modulation 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. Prospects For the home user, wireless has become popular due to ease of installation, and location freedom with the gaining popularity of laptops. Public businesses such as coffee shops or malls have begun to offer wireless access to their customers; some are even provided as a free service. Large wireless network projects are being put up in many major cities. WLAN The WLAN consists of nodes and access points. Node A computer with an antenna/ network adapter A peripheral with an antenna/ network adapter Access points Transmitters/receivers between the nodes WLAN Data transfer Data transfer is implemented by FHSS DSSS Infrared Frequency-hopping spread spectrum (FHSS) FHSS is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. A spread-spectrum transmission offers three main advantages over a fixed- frequency transmission: Spread-spectrum signals are highly resistant to narrowband interference. The process of re-collecting a spread signal spreads out the interfering signal, causing it to recede into the background. Spread-spectrum signals are difficult to intercept. A frequency-hop spread- spectrum signal simply sounds like an increase in the background noise to a narrowband receiver. Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spread- spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently. Direct-sequence spread spectrum (DSSS) In telecommunications, direct-sequence spread spectrum (DSSS) is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 'spread spectrum' comes from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency. Direct-sequence spread spectrum (DSSS) It phase-modulates a sine wave pseudorandomly with a continuous string of pseudonoise (PN) code symbols called "chips", each of which has a much shorter duration than an information bit. Each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate. It uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal. Transmission method Direct-sequence spread-spectrum transmissions multiply the data being transmitted by a "noise" signal. This noise signal is a pseudorandom sequence of 1 and −1 values, at a frequency much higher than that of the original signal, thereby spreading the energy of the original signal into a much wider band. The resulting signal resembles white noise, like an audio recording of "static". However, this noise-like signal can be used to exactly reconstruct the original data at the receiving end, by multiplying it by the same pseudorandom sequence (because 1 × 1 = 1, and −1 × −1 = 1). This process, known as "de-spreading", mathematically constitutes a correlation of the transmitted PN sequence with the receiver's assumed sequence. Transmission method For de-spreading to work correctly, the transmit and receive sequences must be synchronized. This requires the receiver to synchronize its sequence with the transmitter's sequence via some sort of timing search process. However, this apparent drawback can be a significant benefit: if the sequences of multiple transmitters are synchronized with each other, the relative synchronizations the receiver must make between them can be used to determine relative timing, which, in turn, can be used to calculate the receiver's position if the transmitters' positions are known. This is the basis for many satellite navigation systems. Transmission Method The resulting effect of enhancing signal to noise ratio on the channel is called processing gain. This effect can be made larger by employing a longer PN sequence and more chips per bit, but physical devices used to generate the PN sequence impose practical limits on attainable processing gain. If an undesired transmitter transmits on the same channel but with a different PN sequence (or no sequence at all), the de-spreading process results in no processing gain for that signal. This effect is the basis for the code division multiple access (CDMA) property of DSSS, which allows multiple transmitters to share the same channel within the limits of the cross-correlation properties of their PN sequences. As this description suggests, a plot of the transmitted waveform has a roughly bell- shaped envelope centered on the carrier frequency, just like a normal AM transmission, except that the added noise causes the distribution to be much wider than that of an AM transmission. In contrast, frequency-hopping spread spectrum pseudo-randomly re-tunes the carrier, instead of adding pseudo-random noise to the data, which results in a uniform frequency distribution whose width is determined by the output range of the pseudo- random number generator. Infrared Technology Uses high frequencies for transmission just below visible light. Cannot penetrate opaque objects. Either directed or diffused technology is used. In directed it is the line of sight where the emitter directs the IR to the detector. In diffused the Emitter transmits the IR signal it bounces off the ceiling and the detectors are capable of detecting the reflected signal. WLANs typically use this IR. WLAN modes of operation It has two basic modes of operation: Adhoc (distributed control) Infrastructure LAN (Centralized control) Adhoc Mode Ad hoc mode enables peer-to-peer transmission between mobile units. A peer-to-peer (P2P) 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. Infrastructure Mode Infrastructure mode in which mobile units communicate through an access point that serves as a bridge to a wired network infrastructure 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 bridge This is a hardware component used to connect two or more network segments which are physically separated. Consumers have been presented with wireless bridges operating in different frequencies and licensing models. Wireless bridges usually work only in pairs or more, and can be used in two types of implementations. They are the point-to-point link, or the point to multipoint link. In point to point link, there are a pair of bridges which are used to connect two network segments, typically in two separate buildings. In a point to multipoint scenario, one bridge is installed as the "root bridge", and multiple non-root bridges connect to this root bridge. With this arrangement, if one non-root network segment wants to pass data to the other non-root segment, it passes it through the root bridge.
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