10-01-2010, 11:35 PM
seminar report and ppt presentation on WLAN
10-01-2010, 11:35 PM
seminar report and ppt presentation on WLAN
23-09-2010, 10:31 AM
hi,
please go through the following thread to get more information on 'WLAN' http://project-seminars.com/attachment.php?aid=1427
06-11-2012, 02:07 PM
Wireless LAN – IEEE 802.11
![]() INTRODUCTION Local Area Networks have evolved over the past 20 or so years to become a crucial ingredient in the success of businesses, large and small. From the smallest office to the largest multinational corporation shared access to information resources is an indispensable part of modern business processes. Local Area Networks (LAN) have been traditionally connected with wired infrastructure and a multi-billion dollar industry has grown up to supply customers needs for wired networking products. Companies like Cisco, 3Com, Bay Networks and Cabletron have developed a vast range of products to implement and manage Local Area Networks of all sizes and to interconnect them throughout the enterprise. Over the past ten or so years an alternative to wired LAN structures has evolved in the form of the Wireless LAN (WLAN). In a manner analogous to the growth of the wired LAN, initial application and market success of the WLAN was in specialized, vertical markets. Thus applications that highly valued the mobile, untethered connectivity were the early targets of the WLAN industry. These first generation products, which operated in the unlicensed 902-928 MHz ISM (Industrial Scientific and Medical) band had limited range and throughput, but proved useful in many factory floor and warehouse applications. These systems took advantage of emerging semiconductor processes developed for cellular telephone applications to enable inexpensive WLAN products. Unfortunately these same inexpensive components also enabled a wide variety of other 900 MHz products like cordless telephones. Consequently, the band quickly became crowded with a variety of unlicensed products. Building upon technology originally developed for military applications, spread spectrum techniques were employed to minimize sensitivity to interference. This approach allowed the design and manufacture of 900 MHz WLAN products having nominal data rates of 500 kilobits per second. Ultimately, the growing popularity of the band for a large range of unlicensed products, aggravated by the limited bandwidth caused users of WLAN to look to a different frequency band for growth in performance. AD-HOC Network In the ad-hoc network, computers are brought together to form a network "on the fly." As shown in Figure 1, there is no structure to the network; there are no fixed points; and usually every node is able to communicate with every other node. A smallest Wireless LAN may consist of computers each equipped with wireless n/w interface card. This mode of operation is possible when stations are able to communicate directly and the network does not have an AP. This type of network is often formed when a station is not able to locate an AP and starts communicating with the peer stations directly. It can share printer, but cannot share resource of wired LAN unless one of the computer act as bridge to the wired LAN using special s/w (bridge) PROTOCOL ARCHITECTURE To provide a basis for the further discussions of the technology and standards issues related to WLAN, a brief review of network structures is in order. The first concept to keep in mind is that networks represent an interactive collection of oftenpowerful computers. The complexities of the interactions among these members of the network are many. To provide a common framework for describing and understanding, the International Standards Organization approved a standard called ISO-7498 that defines a seven-layered model to describe the interconnection processes between various members of a network. This model, which is officially known at the Open System Interconnect model, is the basis for most discussions of network function. The seven layers are shown in Figure 1. WLAN products, in common with other networking products, typically work at the two bottoms most layers of the 7-layered model. The Physical Layer (usually referred to as simply PHY) is the actual physical method by which data is passed from one member of the network to another. For a WLAN its description includes such things as frequency of operation, data rate, modulation method, etc. In addition to the PHY, the lower half of the Data Link layer, usually known as the Media Access Control (or MAC) layer is defined by the WLAN product. The MAC layer is conventionally defined as the protocol by which data is transferred between network members. In Figure 1, the shaded areas represent the PHY and MAC layers. 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. 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 [5], and the spread-spectrum protocol of 802.11 does provide some rudimentary security for typical data transfers. 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 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. The operation of a pair of frequency hopping transmitter-receiver pairs is shown schematically in Figure 2. The obvious question arises: why not just assign a fixed frequency to each user and share the bandwidth in that manner? The answer lies in how a FHSS responds to interferors. Direct Sequence Spread Spectrum (DSSS) The second type of spread spectrum is known as Direct Sequence Spread Spectrum (DSSS). In this system, the data stream is multiplied by a pseudo-random spreading code to artificially increase the bandwidth over which the data is transmitted. This is shown in Figure 3. The resulting data stream is then modulated onto the carrier using either Differential Binary Phase Shift Keying or Differential Quadrature Phase Shift Keying. By spreading the data bandwidth over a much wider frequency band, the power spectral density of the signal is reduced by the ratio of the data bandwidth to the total spread bandwidth. In a DSSS receiver the incoming spread spectrum data is fed to a correlator where it is correlated with a copy of the pseudo-random spreading code used at the transmitter. Since noise and interference are by definition de-correlated from the desired signal, the desired signal is then extracted from a noisy channel. While the block diagram of a DSSS WLAN product is somewhat simpler than a FHSS product, there are some very subtle difficulties that come into play in the presence of strong interfering |
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