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G. Marconi invented the wireless telegraph in 1896. In 1901, he sent telegraphic signals across the Atlantic Ocean from Cornwall to St. Johns Newfoundland; a distance of 1800 miles. His invention allowed two parties to communicate by sending each other alphanumeric characters encoded in an analog signal. Over the last century, advances in wireless technologies have led to the radio, the television, the mobile telephone, and communication satellites. All types of information can now be sent to almost every corner of the world. Recently, a good attention has been focused on wireless networking.
Early wireless LAN products, introduced in the late 1980s, were marketed as substitutes for traditional wired LANs. Wireless networking is allowing businesses to develop WANs, MANs, and LANs without cabling. A wireless LAN saves the cost of the installation of LAN cabling and eases the task of relocation and other modifications to network structure. The IEEE has developed 802.11 as a standard for wireless LANs. The Bluetooth industry consortium is also working to provide a seamless wireless networking technology. The impact of wireless communications has been and will continue to be profound. Very few inventions have been able to "shrink" the world in such a manner. The standards that define how wireless communication devices interact are quickly converging and soon will allow the creation of a global wireless network that will deliver a wide variety of services.
1.2 Over view
As the name suggests, a wireless LAN is one that makes use of a wireless transmission medium. Until recently, wireless LANs were little used. The reasons for this included high prices, data rates, occupational safety concerns, and licensing requirements. As these problems have been now addressed, the popularity of wireless LANs has grown rapidly.
1.3 Wireless LAN Applications Areas:
There are four application areas for wireless LANs:
Cross- building interconnect,
Nomadic access and
Ad hoc networks.
The motivation for wireless LANs was overtaken by events. First, as awareness of the need for LANs became greater, architects designed new buildings to include extensive pre wiring for data applications. Second, with advances in data transmission technology, there is an increasing reliance on twisted pair cabling for LANs and in particular, Category3 and Category 5 unshielded twisted pair.
However, in some environments, there is a role for the wireless LAN as an alternative to a wired LAN. Examples include buildings with large open areas. In most of these cases, an organization will also have a wired LAN to support servers and some stationary workstations. Thus, this application area is referred to as LAN extension.
There is a backbone wired LAN, such as Ethernet, that supports servers, workstations, and one or more bridges or routers to link with other networks. In addition, there is a Control Module (CM) that acts as an interface to a wireless LAN. The control module includes either bridge or router functionality to link the wireless LAN to the backbone. It includes some sort of access control logic, such as a polling or token-passing scheme, to regulate the access from the end systems.
Cross- Building Interconnect
Another use of wireless LAN technology is to connect LANs in nearby buildings, be they wired or wireless LANs. In this case, a point-to-point wireless link is used between two buildings. The devices so connected are typically bridges or routers. This single point-to-point link is not a LAN per se, but it is usual to include this application under the heading of wireless LAN.
Nomadic access provides a wireless link between a LAN hub and mobile data terminal equipped with an antenna, such as a laptop computer or notepad computer. Nomadic access is also useful in an extended environment such as a campus or a business operating out of a cluster of buildings.
Ad Hoc Networking
An ad hoc network is a peer-to-peer network (no centralized server) set up temporarily to meet some immediate need. For example, a group of employees, each with a laptop or palmtop computer may convene in a conference room for a business or classroom meeting. The employees link their computers in a temporary network just for the duration of the meeting.
There are differences between a wireless LAN that supports LAN extension and nomadic access requirements and an ad hoc wireless LAN. In the former case, the wireless LAN forms a stationary infrastructure consisting of one or more cells with a control module for each cell. Within a cell, there may be a number of stationary end systems. Nomadic stations can move from one cell to another. In contrast, there is no infrastructure for an ad hoc network. Rather, a peer collection of stations within range of each other may dynamically configure themselves into a temporary network.
Fig 1.1 Wireless LAN Configurations
1.4 Wireless LAN Requirements
A wireless LAN must meet the same sort of requirements typical of any LAN, including high capacity, ability to cover short distances, full connectivity among attached stations, and broadcast capability. In addition, there are a number of requirements specific to the wireless LAN environment. The following are among the most important requirements for wireless LANs.
Throughput: The medium access control protocol should make as efficient use as possible of the wireless medium to maximize capacity.
Number of nodes: Wireless LANs may need to support hundreds of nodes across multiple cells.
Connection to backbone LAN: In most cases, interconnection with stations on a wired backbone LAN is required. For infrastructure wireless LANs, this is easily accomplished through the use of control modules that connect to both types of LANs. There may also need to be accommodation for mobile users and ad hoc wireless networks.
Service area: A typical coverage area for a wireless LAN has a diameter of 100 to 300m.
Battery power consumption: Mobile workers use battery-powered workstations that need to have a long battery life when used with wireless adapters.
This suggests that a MAC protocol that requires mobile nodes to monitor access points constantly or engage in frequent handshakes with a base station is inappropriate. Typical wireless LAN implementations have features to reduce power consumption while not using the network, such as a sleep mode.
Transmission robustness and security: Unless properly designed, a wireless LAN may be interference prone and easily eavesdropped. The design of a wireless LAN must permit reliable transmission even in a noisy environment and should provide some level of security from eavesdropping.
Collocated network operation: As Wireless LANs become more popular, it is quite likely for two or more wireless LANs to operate in the same area or in some area where interference between the LANs is possible. Such interferee may thwart the normal operation of a MAC algorithm and may allow unauthorized access to a particular LAN.
License-free operation: Users would prefer to buy and operate wireless LAN products without having to secure a license for the frequency band used by the LAN.
Handoff/roaming: The MAC protocol used in the wireless LAN should enable mobile stations to move from one cell to another.
Dynamic configuration: The MAC addressing and network management aspects of the LAN should permit dynamic and automated addition, deletion, and relocation of end systems without disruption to other users.
1.5 The Trouble with Wireless
Wireless is convenient and often less expensive to deploy than fixed services, but wireless is not perfect. There are limitations, political and technical difficulties that may ultimately prevent wireless technologies from reaching the other side with full potential. Two limiting issues are incompatible standards and device limitations.
Device limitations also restrict the free flow of data. The small LCD on a mobile telephone is inadequate for displaying more than a few lines of text. In additions, most mobile wireless devices cannot access the vast majority of WWW sites on the Internet. The browsers use a special language, wireless markup language (WML), instead of the de facto standard HTML.
Most likely, no one wireless device will be able to meet every need. The potential of wireless can be met but not with a single product. Wireless will succeed because it will be integrated into a variety of devices that can meet a variety of needs.
1.6 Fading in the Mobile Environment
Perhaps the most challenging technical problem being faced by communication systems engineers is fading in a mobile environment. The term fading refers to the time variation of received signal power caused by changes in the transmission medium or path(s). In a fixed environment, fading is affected by changes in atmospheric conditions, such as rainfall. But in a mobile environment, where one of the two antennae is moving relative to the other, the relative location of various obstacles changes over time, creating complex transmission effects.
1.6.1 Types of Fading
Fading effects in a mobile environment can be classified as either fast or slow. Referring to Fig 1.2, as the mobile unit moves down a street in an urban environment, rapid variations in signal strength occur over distances of about one-half a wavelength. The rapidly changing waveform is an example of the spatial variation of received signal amplitude. The changes of amplitude can be as much as 20 or 30 dB over a short distance. This type of rapidly changing fading phenomenon, known as fat fading, affects not only mobile devices in automobiles, but even a mobile phone user walking down an urban street.
As the mobile user covers distances well in excess of a wavelength, the urban environment changes, as the user passes buildings of different heights, vacant lots, intersections, and so forth. Over these longer distances, there is a change in the average received power level about which the rapid fluctuations occur. This is referred to as slow fading.
Fig 1.2 Mobile unit signal reflections
Fading channel models are often used to model the effects of electromagnetic transmission of information over the air in cellular networks and broadcast communication. Fading channel models are also used in underwater acoustic communications to model the distortion caused by the water. Mathematically, fading is usually modeled as a time-varying random change in the amplitude and phase of the transmitted signal.
1.6.2 Slow vs. Fast Fading
The terms slow and fast fading refer to the rate at which the magnitude and phase change imposed by the channel on the signal changes. The coherence time is a measure of the minimum time required for the magnitude change of the channel to become decorrelated from its previous value.
Slow fading arises when the coherence time of the channel is large relative to the delay constraint of the channel. In this regime, the amplitude and phase change imposed by the channel can be considered roughly constant over the period of use. Slow fading can be caused by events such as shadowing, where a large obstruction such as a hill or large building obstructs the main signal path between the transmitter and the receiver. The amplitude change caused by shadowing is often modeled using a log-normal distribution with a standard deviation according to the Log Distance Path Loss Model.
Fast Fading occurs when the coherence time of the channel is small relative to the delay constraint of the channel. In this regime, the amplitude and phase change imposed by the channel varies considerably over the period of use.
In a fast-fading channel, the transmitter may take advantage of the variations in the channel conditions using time diversity to help increase robustness of the communication to a temporary deep fade. Although a deep fade may temporarily erase some of the information transmitted, use of an error-correcting code coupled with successfully transmitted bits during other time instances (interleaving) can allow for the erased bits to be recovered. In a slow-fading channel, it is not possible to use time diversity because the transmitter sees only a single realization of the channel within its delay constraint. A deep fade therefore lasts the entire duration of transmission and cannot be mitigated using coding.
Flat vs. Frequency-selective Fading
As the carrier frequency of a signal is varied, the magnitude of the change in amplitude will vary. The coherence bandwidth measures the minimum separation in frequency after which two signals will experience uncorrelated fading.
In flat fading, the coherence bandwidth of the channel is larger than the bandwidth of the signal. Therefore, all frequency components of the signal will experience the same magnitude of fading.
In frequency-selective fading, the coherence bandwidth of the channel is smaller than the bandwidth of the signal. Different frequency components of the signal therefore experience decorrelated fading.
In a frequency-selective fading channel, since different frequency components of the signal are affected independently, it is highly unlikely that all parts of the signal will be simultaneously affected by a deep fade. Certain modulation schemes such as OFDM and CDMA are well-suited to employ frequency diversity to provide robustness to fading. OFDM divides the wideband signal into many slowly modulated narrowband subcarriers, each exposed to flat fading rather than frequency selective fading. This can be combated by means of error coding, simple equalization or adaptive bit loading. Inter-symbol interference is avoided by introducing a guard interval between the symbols. CDMA uses the Rake receiver to deal with each echo separately.
Frequency-selective fading channels are also dispersive, in that the signal energy associated with each symbol is spread out in time. This causes transmitted symbols that are adjacent in time to interfere with each other. Equalizers are often deployed in such channels to compensate for the effects of the inter symbol interference.
Fading effects can also be classified as flat or selective. Flat fading, or nonselective fading, is that type of fading in which all frequency components of the received signal fluctuate in the same proportions simultaneously. Selective fading affects unequally the different spectral components of a radio signal. The term selective fading is usually significant only relative to the bandwidth of the overall communications channel. If attenuation occurs over a portion of the bandwidth of the signal, the fading is considered to be selective; nonselective fading implies that the signal bandwidth of interest is narrower than, and completely covered by, the spectrum affected by the fading.