Peer-to-peer
Introduction To Mobile Computing Mobile Computing is an umbrella term used to describe technologies that enable people to access network services anyplace, anytime, and anywhere. Ubiquitous computing and nomadic computing are synonymous with mobile computing. Information access via a mobile device is plagued by low available bandwidth, poor connection maintenence, poor security, and addressing problems. Unlike their wired counterparts, design of software for mobile devices must consider resource limitation, battery power and display size. Consequently, new hardware and software techniques must be developed. For example, applications need to be highly optimized for space, in order to fit in the limited memory on the mobile devices. For Internet enabled devices, the good old TCP/IP stack cannot be used; it takes too much space and is not optimized for minimal power consumption. Given the plethora of cellular technologies that have emerged in such a market, it becomes extremely difficult to provide support for inter-device communication. A new hardware technology solution, Bluetooth, has been proposed to overcome this barrier. Any device with a Bluetooth chip will be able to communicate seamlessly with any other device having a similar chip irrespective of the communication technologies they might be using. For the sake of explanation, an analogy can be drawn between the Java Virtual Machine and Blue tooth. In the recent past, cellular phone companies have shown an interesting growth pattern. The number of customers has been steadily increasing but the average airtime per user has slowed to a constant. To increase the user average connect time, many cellular providers have started providing data services on their networks which entices the user to use the mobile device for both voice and data communication. Typical data services include chat, e-mail, Internet browsing. An example of this type of service is SMS (Short Message Service). It is a data service in a GSM cellular network that allows the users to send a maximum of 160-character message at a time (similar to paging). Inherently, this service is not feasible for browsing, checking e-mail or chatting. GSM networks provide another service called GPRS (General Packet Radio Service) that allows information to be sent and received across the cellular network. There has also been a recent effort defining common standards for providing data services on hand-held devices. WAP (Wireless Application Protocol) and KVM (Kilobyte Virtual Machine) deserve a mention here. WAP is a protocol suite that comprises of protocols tailored for small devices. WAP has been developed by the WAP Forum [ ] and runs over an underlying bearer protocol like IP or SMS. In the WAP model, a service provider operates a WAP gateway to convert Internet content to a miniaturized subset of HTML that is displayed by a mini-browser on the mobile device. Companies like Nokia, Ericsson and Motorola have already developed WAP enabled phones. As of now, these phones are available and functional mostly in Europe. HTML, the de-facto Internet language, is not optimized for these devices. Handheld devices are characterized by small display sizes, limited input capabilities and limited bandwidth. The HTML document model consisting of headers, titles, paragraphs, etc, does not work well for a 10 row by 15 character wide screen. Keeping in mind the memory constraints of the mobile device, the browser should not be heavy (i.e. the markup language should not be too detailed). Alternative markup languages that have been proposed include HDML (Handheld Device Markup Language -- a prevalent standard), WML (Wireless Markup Language -- WAP brainchild) and Compact HTML . Details about these standards can be obtained from w3c site . Curious readers can also subscribe to the mailing list www-mobile@. The popular mini-browser in the market right now is UP.browser from []. The browser has been licensed to many cellular phone manufacturers like Motorola, Ericsson, Sony and Toshiba. Considertaion Of Data Link Layer The data link layer is the second layer in the OSI (open systems interconnection) seven-layer reference model. It responds to service requests from the network layer above it and issues service requests to the physical layer below it. The data link layer is responsible for encoding bits into packets prior to transmission and then decoding the packets back into bits at the destination. Bits are the most basic unit of information in computing and communications. Packets are the fundamental unit of information transport in all modern computer networks, and increasingly in other communications networks as well. The data link layer is also responsible for logical link control, media access control, hardware addressing, error detection and handling and defining physical layer standards. It provides reliable data transfer by transmitting packets with the necessary synchronization, error control and flow control. The data link layer is divided into two sublayers: the media access control (MAC) layer and the logical link control (LLC) layer. The former controls how computers on the network gain access to the data and obtain permission to transmit it; the latter controls packet synchronization, flow control and error checking. The data link layer is where most LAN (local area network) and wireless LAN technologies are defined. Among the most popular technologies and protocols generally associated with this layer are Ethernet, Token Ring, FDDI (fiber distributed data interface), ATM (asynchronous transfer mode), SLIP (serial line Internet protocol), PPP (point-to-point protocol), HDLC (high level data link control) and ADCCP (advanced data communication control procedures). The data link layer is often implemented in software as a driver for a network interface card (NIC). Because the data link and physical layers are so closely related, many types of hardware are also associated with the data link layer. For example, NICs typically implement a specific data link layer technology, so they are often called Ethernet cards, Token Ring cards, etc. There are also several types of network interconnection devices that are said to operate at the data link layer in whole or in part, because they make decisions about what to do with data they receive by looking at data link layer packets. These devices include most bridges and switches, although switches also encompass functions performed by the network layer. Data link layer processing is faster than network layer processing because less analysis of the packet is required. Channel Allocation In Mobile Computing Channel allocation deals with the allocation of channels to cells in a cellular network. Once the channels are allocated, cells may then allow users within the cell to communicate via the available channels. Channels in a wireless communication system typically consist of time slots, frequency bands and/or CDMA pseudo noise sequences, but in an abstract sense, they can represent any generic transmission resource. There are three major categories for assigning these channels to cells (or base-stations). They are Fixed Channel Allocation, Dynamic Channel Allocation and Hybrid Channel Allocation which is a combination of the first two methods. Fixed Channel AllocationFixed Channel Allocation (FCA) systems allocate specific channels to specific cells. This allocation is static and can not be changed. For efficient operation, FCA systems typically allocate channels in a manner that maximizes frequency reuse. Thus, in a FCA system, the distance between cells using the same channel is the minimum reuse distance for that system. The problem with FCA systems is quite simple and occurs whenever the offered traffic to a network of base stations is not uniform. Consider a case in which two adjacent cells are allocated N channels each. There clearly can be situations in which one cell has a need for N+k channels while the adjacent cell only requires N-m channels (for positive integers and m. In such a case, k users in the first cell would be blocked from making calls while m channels in the second cell would go unused. Clearly in this situation of non-uniform spatial offered traffic, the available channels are not being used efficiently. FCA has been implemented on a widespread level to date. Dynamic Channel AllocationDynamic Channel Allocation (DCA) attempts to alleviate the problem mentioned for FCA systems when offered traffic is non-uniform. In DCA systems, no set relationship exists between channels and cells. Instead, channels are part of a pool of resources. Whenever a channel is needed by a cell, the channel is allocated under the constraint that frequency reuse requirements can not be violated. There are two problems that typically occur with DCA based systems. First, DCA methods typically have a degree of randomness associated with them and this leads to the fact that frequency reuse is often not maximized unlike the case for FCA systems in which cells using the same channel are separated by the minimum reuse distance. Secondly, DCA methods often involve complex algorithms for deciding which available channel is most efficient. These algorithms can be very computationally intensive and may require large computing resources in order to be real-time. Hybrid Channel Alloction Scheme The third category of channel allocation methods includes all systems that are hybrids of fixed and dynamic channel allocation systems. Several methods have been presented that fall within this category and in addition, a great deal of comparison has been made with corresponding simulations and analyses [Cox, Elnoubi, Jiang, Katzela, Yue, Zhang]. We will present several of the more developed hybrid methods below. Channel Borrowing is one of the most straightforward hybrid allocation schemes. Here, channels are assigned to cells just as in fixed allocation schemes. If a cell needs a channel in excess of the channels previously assigned to it, that cell may borrow a channel from one of its neighboring cells given that a channel is available and use of this channel won't violate frequency reuse requirements. Note that since every channel has a predetermined relationship with a specific cell, channel borrowing (without the extensions mentioned below) is often categorized as a subclass of fixed allocation schemes. The major problem with channel borrowing is that when a cell borrows a channel from a neighboring cell, other nearby cells are prohibited from using the borrowed channel because of co-channel interference. This can lead to increased call blocking over time. To reduce this call blocking penalty, algorithms are necessary to ensure that the channels are borrowed from the most available neighboring cells; i.e., the neighboring cells with the most unassigned channels. Two extensions of the channel borrowing approach are Borrowing with Channel Ordering (BCO) and Borrowing with Directional Channel Locking (BDCL). Borrowing with Channel Locking was designed as an improvement over the simpler Channel Borrowing approach as described above [Elnoubi]. BCO systems have two distinctive characteristics [Elnoubi]: The ratio of fixed to dynamic channels varies with traffic load. Nominal channels are ordered such that the first nominal channel of a cell has the highest priority of being applied to a call within the cell. The last nominal channel is most likely to be borrowed by neighboring channels. Once a channel is borrowed, that channel is locked in the co-channel cells within the reuse distance of the cell in question. To be "locked" means that a channel can not be used or borrowed. Zhang and Yum [Zhang] presented the BDCL scheme as an improvement over the BCO method. From a frequency reuse standpoint, in a BCO system, a channel may be borrowed only if it is free in the neighboring cochannel cells. This criteria is often too strict. In Borrowing with Directional Channel Locking, borrowed channels are only locked in nearby cells that are affected by the borrowing. This differs from the BCO scheme in which a borrowed channel is locked in every cell within the reuse distance. The benefit of BDCL is that more channels are available in the presence of borrowing and subsequent call blocking is reduced. A disadvantage of BDCL is that the statement "borrowed channels are only locked in nearby cells that are affected by the borrowing" requires a clear understanding of the term "affected." This may require microscopic analysis of the area in which the cellular system will be located. Ideally, a system can be general enough that detailed analysis of specific propagation measurements is not necessary for implementation. ?Hybrid Channel Alloction Scheme Continue... A natural extension of channel borrowing is to set aside a portion of the channels in a system as dynamic channels with the remaining (nominal) channels being fixed to specified cells. If a cell requires an extra channel, instead of borrowing the channel from a neighboring cell, the channel is borrowed from the common "bank" of dynamic channels. An important consideration in hybrid systems of this type is the ratio of dynamic channels to fixed channels. Analysis by Cox and Reudlink [Cox - 1973] showed that given ten channels per cell, an optimum ratio was 8 fixed channels and 2 dynamic channels. In general, the optimum ratio depends upon the traffic load [Zhang]. In addition to BDCL, a second channel allocation method was presented by Yum and Zhang [Zhang]. Referred to as Locally Optimized Dynamic Assignment Strategy (LODA), this method is best described as a purely dynamic channel allocation procedure as opposed to a hybrid method. In this strategy there are no nominal channels; all channels are dynamic. When a given cell needs to accommodate a call, it chooses from among the bank of available channels according to some cost criteria. The channel with minimum cost is assigned. In a general sense, the cost is a measure of the future blocking probability in the vicinity of the cell given that the candidate channel is assigned. A more detailed description of the cost function will be addressed below. Dynamic Channel ReassignmentSimilar to the goals of dynamic channel assignment is the process of Dynamic Channel Reassignment (DCR). Whereas a DCA scheme allocates a channel to an initial call or handover, a DCR system switches a cell's channel (that is currently being used) to another channel which is closer to the optimum according to frequency reuse or other cost criteria. Thus, for example, a user communicating with channel n may be switched to channel m during the middle of her/his call if channel m is a more efficient use of the available bandwidth from a frequency reuse point of view. Philosophically, DCR is equivalent to DCA. Simulation and Comparison of Channel Allocation SchemesA great deal of work is available comparing various realizations of channel allocation schemes [Cox, Elnoubi, Jiang, Katzela, Yue, Zhang]. In comparing performance, typical system metrics include blocking probability of new calls and blocking probability of handover calls. These metrics are written as functions of offered traffic (where the traffic may be written in a variety of forms). It is generally assumed that a blocked new call is preferred over a blocked hand-off call. The idea being that with a blocked hand-off, users are forced to terminate communication in the middle of their session. If this blocking happens at a particularly inopportune time, the results could be disastrous (e.g., business partners cut off in the middle of a vital negotiation). In the case of a blocked new call, at least the business negotiation hasn't started and the involved parties aren't interrupted. Blocking probability is an important metric throughout the field of queueing theory and in the case of M/M/1 queues, the Erlang-B formula is often used for analysis of blocking probability. Because blocked calls can be very disconcerting, systems are typically designed to have blocking probabilities of no more than 1% or 2%. This is consistent with the assumption of small offered traffic loads. Hybrid Channel Alloction Scheme Continue... Cox and Reudink were the first researchers to present published comparisons of different channel allocation schemes. Their comparison was based on simulation of an outdoor vehicular wireless communication system [Cox - 1971, Cox - 1972, Jakes]. The simulation divided a region into a grid of square cells. The movement of vehicles had a two dimensional normal distribution with 0 mean and 30 mph standard deviation in each of the two orthogonal directions. Poisson arrivals were assumed for the rate of calls per vehicle and call durations were assume to have a truncated normal distribution (truncated on the left at zero) with a "mean" 90 seconds (true mean of 103.5 seconds). Cox and Reudink's study considered uniform and non-uniform distributions of spatial traffic. In the uniform case, all cells had approximately the same call arrival rate while in the non-uniform case, some cells had a significantly higher call arrival rate. With both the uniform and non-uniform spatial distributions, fixed channel allocation schemes were optimally matched so that the cells with the greatest numbers of calls had the greatest number of channels to deal with those calls. In both cases of uniform and non-uniform traffic, results showed that for low blocking probabilities, dynamic channel allocation schemes could handle more calls than fixed channel allocation schemes. More specifically, in the case of uniform traffic, the DCA approach outperformed the FCA approach when the blocking probability was lower than 10%. At a blocking probability of 1%, the DCA approach could handle over 10% more calls than the FCA approach. In the case of non-uniform traffic, the DCA approach outperformed FCA for blocking rates up to 60%. At a blocking rate of 1%, DCA could handle almost 70% more calls per cell than FCA. Cox and Reudink performed another comparison involving dynamic channel reassignment in [Cox - 1973]. In this hybrid procedure, the total number of available channels is broken into two groups: fixed and dynamic channels. When a cell requires a channel, it first searches for an available fixed channel that is preassigned to the cell. If none of the fixed channels are available, a dynamic channel is searched for from the common bank of dynamic channels. If this search is in vain, the call is blocked. When users who were assigned fixed channels end their calls, these freed fixed channels are then assigned to users in the same cell who are currently using dynamic channels. This frees the dynamic channel for future use and ensures that a large number of channels being used are the optimally-spaced, fixed channels. Results from Cox and Reudink's study of dynamic channel reassignment showed that channel use was increased by over 60% compared to fixed channel allocation for a blocking rate of 1%. This result corresponds to uniform offered traffic. Zhang and Yum compared four channel assignment Fixed Channel Assignment (FCA), Borrowing with Channel Ordering (BCO), Borrowing with Directional Channel Locking (BDCL) and Locally Optimized Dynamic Assignment (LODA). With respect to uniform offered traffic, their results showed that BDCL had the lowest blocking probability followed by BCO, LODA and FCA. With non-uniform offered traffic, the relative performance of the four methods was the same with the exception that in this case, LODA performed better than BCO. It makes sense that the ordering for BDCL, BCO and FCA was as found. Indeed, BDCL was specifically designed as an improvement over BCO and BCO was designed as an improvement over FCA [Zhang, Elnoubi]. The fact that the performance of LODA varies under uniform versus non-uniform traffic is rather interesting however. The reason behind this phenomenon is that LODA provides optimal channel allocation only in local regions. Given non-uniform traffic which consists of dense regions in certain local areas, LODA will accommodate these regions of high traffic offering. However, in a global sense, the LODA algorithm will not necessarily provide the optimal allocation. With uniform offered traffic, LODA does not have any regions with peak traffic to optimize; i.e., no local regions within which the benefits of LODA can be realized. Furthermore, with respect to the entire region, the optimization is generally not optimal in a global sense. The result is that with uniform traffic, LODA does not have any advantage to offer over BCO. From the previous discussion we see that one general result of all of the comparisons is that dynamic channel allocation outperforms fixed channel allocation for low blocking rates (below 10% in most cases). Blocking rates above 1% or 2% are generally not tolerated. This is generally an accepted guideline throughout the telecommunications industry and we will adhere to this design constraint as well. Common Principle Of Channel Allocation The large array of possible channel allocation systems can become cumbersome. However, all channel allocation methods operate under simple, common principles. Throughout this report we have touched on three points which an efficient channel allocation scheme should address: Channel allocation schemes must not violate minimum frequency reuse conditions. Channel allocation schemes should adapt to changing traffic conditions. Channel allocation schemes should approach (from above) the minimum frequency reuse constraints so as to efficiently utilize available transmission resources. As the first requirement suggests, all channel allocation schemes adhere to condition 1. From a frequency reuse standpoint, a fixed channel allocation system distributes frequency (or other transmission) resources to the cells in an optimum manner; i.e., common channels are separated by the minimum frequency reuse distance. Thus, a fixed channel allocation scheme perfectly satisfies condition 3 as well. However, a fixed allocation scheme does not satisfy condition 2. Philosophically, any dynamic channel allocation scheme will meet the requirements of all of the above three conditions to some degree. At the system architecture level dynamic channel allocation schemes may differ widely, but fundamentally, their only difference is in the degree to which they satisfy condition 3. Different DCA schemes attempt to satisfy condition 3 (in addition to conditions 1 and 2) by approaching the minimum frequency reuse constraint arbitrarily closely, and by doing so in as short a time period as possible. The above three conditions point to the fact that design of dynamic channel allocation schemes falls within the general class of optimization problems. Furthermore, since we can always assume that the available number of base stations is finite and the transmission resources will always be countable (due to FCC requirements if nothing else) then our problem can be reduced to the subclass of combinatorial optimization problems. As with all combinatorial optimization problems, there will exist a solution space and a cost function [Aarts & Korst]. A typical element of the solution space could be a particular layout of frequency channels among the base-stations. The cost function can be loosely characterized as the difference between the frequency reuse of an arbitrary solution and the frequency reuse of the optimized solution. The error associated with a non-optimized cost is realized as a future increased blocking probability or an otherwise unwarranted lack of channel availability. It is typically assumed that the solution to the wireless dynamic channel allocation problem is NP-complete [Yue, Cox - 1971]. The definition of np-completeness follows from the conjecture made in the late 1960's that there exists a class of combinatorial optimization problems of such inherent complexity that any algorithm, solving each instance of such a problem to optimality, requires a computational effort that grows superpolynomially with the size of the problem. In the case of dynamic channel allocation, the complexity is generally attributed to the required inclusion of cochannel interference in any analysis of dynamic channel allocation schemes [Yue]. The author is aware of one published article to date offering an analytical method (approximate) for calculating the performance of dynamic channel allocation [see Yue]. Recently, several approximation techniques have been proposed as methods for solving condition 3 of the dynamic channel allocation problem. In particular there has been interest in applying simulated annealing techniques [Duque-Anton] and neural network methods [Chan, Kunz, Funabiki] to dynamic channel allocation.eless Lan A wireless LAN or WLAN is a wireless local area network, which is the linking of two or more computers without using wires. WLAN utilizes pread-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.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. Google is even providing a free service to Mountain View, California and has entered a bid to do the same for San Francisco. New York City has also begun a pilot program to cover all five boroughs of the city with wireless Internet access.?left0History Of Wireless Lan In 1970 University of Hawaii, under the leadership of Norman Abramson, developed the world’s first computer communication network using low-cost ham-like radios, named ALOHAnet. The bi-directional star topology of the system included seven computers deployed over four islands to communicate with the central computer on the Oahu Island without using phone lines."In 1979, F.R. Gfeller and U. Bapst published a paper in the IEEE Proceedings reporting an experimental wireless local area network using diffused infrared communications. Shortly thereafter, in 1980, P. Ferrert reported on an experimental application of a single code spread spectrum radio for wireless terminal communications in the IEEE National Telecommunications Conference. In 1984, a comparison between Infrared and CDMA spread spectrum communications for wireless office information networks was published by Kaveh Pahlavan in IEEE Computer Networking Symposium which appeared later in the IEEE Communication Society Magazine. In May 1985, the efforts of Marcus led the FCC to announce experimental ISM bands for commercial application of spread spectrum technology. Later on, M. Kavehrad reported on an experimental wireless PBX system using code division multiple access. These efforts prompted significant industrial activities in the development of a new generation of wireless local area networks and it updated several old discussions in the portable and mobile radio industry.The first generation of wireless data modems was developed in the early 1980's by amateur radio operators. They added a voice band data communication modem, with data rates below 9600 bit/s, to an existing short distance radio system, typically in the two meter amateur band. The second generation of wireless modems was developed immediately after the FCC announcement in the experimental bands for non-military use of the spread spectrum technology. These modems provided data rates on the order of hundreds of kbit/s. The third generation of wireless modem [then] aimed at compatibility with the existing LANs with data rates on the order of Mbit/s. Several companies [developed] the third generation products with data rates above 1 Mbit/s and a couple of products [had] already been announced [by the time of the first IEEE Workshop on Wireless LANs].""The first of the IEEE Workshops on Wireless LAN was held in 1991. At that time early wireless LAN products had just appeared in the market and the IEEE 802.11 committee had just started its activities to develop a standard for wireless LANs. The focus of that first workshop was evaluation of the alternative technologies. [By 1996], the technology [was] relatively mature, a variety of applications [had] been identified and addressed and technologies that enable these applications [were] well understood. Chip sets aimed at wireless LAN implementations and applications, a key enabling technology for rapid market growth, [were] emerging in the market. Wireless LANs [were being] used in hospitals, stock exchanges, and other in building and campus settings for nomadic access, point-to-point LAN bridges, ad-hoc networking, and even larger applications through internetworking. The IEEE 802.11 standard and variants and alternatives, such as the wireless LAN interoperability forum and the European HIPERLAN specification [had] made rapid progress, and the unlicensed PCS [ Unlicensed Personal Communications Services ] and the proposed SUPERNet, later on renamed as U-NII, bands also presented new opportunities." On July 21, 1999, AirPort debuted at the Macworld Expo in New York City with Steve Jobs picking up an iBook supposedly to give the cameraman a better shot as he surfed the Web. Applause quickly built as people realized there were no wires. This was the first time Wireless LAN became publicly available at consumer pricing and easily available for home use. Before the release of the Airport, Wireless LAN was too expensive for consumer use and used exclusively in large corporate settings.Originally WLAN hardware was so expensive that it was only used as an alternative to cabled LAN in places where cabling was difficult or impossible. Early development included industry-specific solutions and proprietary protocols, but at the end of the 1990s these were replaced by standards, primarily the various versions of IEEE 802.11 (Wi-Fi). An alternative ATM-like 5 GHz standardized technology, HIPERLAN, has so far not succeeded in the market, and with the release of the faster 54 Mbit/s 802.11a (5 GHz) and 802.11g (2.4 GHz) standards, almost certainly never will.In November 2006, the Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO) won a legal battle in the US federal court of Texas against Buffalo Technology which found the US manufacturer had failed to pay royalties on a US WLAN patent CSIRO had filed in 1996. CSIRO are currently engaged in legal cases with computer companies including Microsoft, Intel, Dell, Hewlett-Packard and Netgear which argue that the patent is invalid and should negate any royalties paid to CSIRO for WLAN-based products.enifits Of Wireless Lan The popularity of wireless LANs is a testament primarily to their convenience, cost efficiency, and ease of integration with other networks and network components. The majority of computers sold to consumers today come pre-equipped with all necessary wireless LAN technology.The benefits of wireless LANs include:Convenience: The wireless nature of such networks allows users to access network resources from nearly any convenient location within their primary networking environment (home or office). With the increasing saturation of laptop-style computers, this is particularly relevant.Mobility: With the emergence of public wireless networks, users can access the internet even outside their normal work environment. Most chain coffee shops, for example, offer their customers a wireless connection to the internet at little or no cost.Productivity: Users connected to a wireless network can maintain a nearly constant affiliation with their desired network as they move from place to place. For a business, this implies that an employee can potentially be more productive as his or her work can be accomplished from any convenient location.Deployment: Initial setup of an infrastructure-based wireless network requires little more than a single access point. Wired networks, on the other hand, have the additional cost and complexity of actual physical cables being run to numerous locations (which can even be impossible for hard-to-reach locations within a building).Expandability: Wireless networks can serve a suddenly-increased number of clients with the existing equipment. In a wired network, additional clients would require additional wiring.Cost: Wireless networking hardware is at worst a modest increase from wired counterparts. This potentially increased cost is almost always more than outweighed by the savings in cost and labor associated to running physical cables.Disadvantage Of Wireless Lan Wireless LAN technology, while replete with the conveniences and advantages described above, has its share of downfalls. For a given networking situation, wireless LANs may not be desirable for a number of reasons. Most of these have to do with the inherent limitations of the technology.Security: Wireless LAN transceivers are designed to serve computers throughout a structure with uninterrupted service using radio frequencies. Because of space and cost, the antennas typically present on wireless networking cards in the end computers are generally relatively poor. In order to properly receive signals using such limited antennas throughout even a modest area, the wireless LAN transceiver utilizes a fairly considerable amount of power. What this means is that not only can the wireless packets be intercepted by a nearby adversary's poorly-equipped computer, but more importantly, a user willing to spend a small amount of money on a good quality antenna can pick up packets at a remarkable distance; perhaps hundreds of times the radius as the typical user. In fact, there are even computer users dedicated to locating and sometimes even cracking into wireless networks, known as wardrivers. On a wired network, any adversary would first have to overcome the physical limitation of tapping into the actual wires, but this is not an issue with wireless packets. To combat this consideration, wireless networks users usually choose to utilize various encryption technologies available such as Wi-Fi Protected Access (WPA). Some of the older encryption methods, such as WEP are known to have weaknesses that a dedicated adversary can compromise. (See main article: Wireless security.) Range: The typical range of a common 802.11g network with standard equipment is on the order of tens of meters. While sufficient for a typical home, it will be insufficient in a larger structure. To obtain additional range, repeaters or additional access points will have to be purchased. Costs for these items can add up quickly. Other technologies are in the development phase, however, which feature increased range, hoping to render this disadvantage irrelevant. (See WiMAX)Reliability: Like any radio frequency transmission, wireless networking signals are subject to a wide variety of interference, as well as complex propagation effects (such as multipath, or especially in this case Rician fading) that are beyond the control of the network administrator. In the case of typical networks, modulation is achieved by complicated forms of phase-shift keying (PSK) or quadrature amplitude modulation (QAM), making interference and propagation effects all the more disturbing. As a result, important network resources such as servers are rarely connected wirelessly.Speed: The speed on most wireless networks (typically 1-108 Mbit/s) is reasonably slow compared to the slowest common wired networks (100 Mbit/s up to several Gbit/s). There are also performance issues caused by TCP and its built-in congestion avoidance. For most users, however, this observation is irrelevant since the speed bottleneck is not in the wireless routing but rather in the outside network connectivity itself. For example, the maximum ADSL throughput (usually 8 Mbit/s or less) offered by telecommunications companies to general-purpose customers is already far slower than the slowest wireless network to which it is typically connected. That is to say, in most environments, a wireless network running at its slowest speed is still faster than the internet connection serving it in the first place. However, in specialized environments, the throughput of a wired network might be necessary. Newer standards such as 802.11n are addressing this limitation and will support peak throughputs in the range of 100-200 Mbit/s.Wireless LANs present a host of issues for network managers. Unauthorized access points, broadcasted SSIDs, unknown stations, and spoofed MAC addresses are just a few of the problems addressed in WLAN troubleshooting. rchitecture Of Wireless LanAll components that can connect into a wireless medium in a network are referred to as stations. All stations are equipped with wireless network interface cards (WNICs). Wireless stations fall into one of two categories: access points and clients.Access pointsAccess points (APs) are base stations for the wireless network. They transmit and receive radio frequencies for wireless enabled devices to communicate with.ClientsWireless clients can be mobile devices such as laptops, personal digital assistants, IP phones, or fixed devices such as desktops and workstations that are equipped with a wireless network interface.Basic service setThe basic service set (BSS) is a set of all stations that can communicate with each other. There are two types of BSS: independent BSS and infrastructure BSS. Every BSS has an identification (ID) called the BSSID, which is the MAC address of the access point servicing the BSS.Independent basic service setAn independent BSS is an ad-hoc network that contains no access points, which means they can not connect to any other basic service set.Infrastructure basic service setAn infrastructure BSS can communicate with other stations not in the same basic service set by communicating through access points.Extended service setAn extended service set (ESS) is a set of connected BSSes. Access points in an ESS are connected by a distribution system. Each ESS has an ID called the SSID which is a 32-byte (maximum) character string. For example, "linksys" is the default SSID for Linksys routers.Types Of Wireless Lan Peer-to-peerA 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.802.11 specs define the physical layer (PHY) and MAC (Media Access Control) layers. However, unlike most other IEEE specs, 802.11 includes three alternative PHY standards: diffuse infrared operating at 1 Mbit/s in; frequency-hopping spread spectrum operating at 1 Mbit/s or 2 Mbit/s; and direct-sequence spread spectrum operating at 1 Mbit/s or 2 Mbit/s. A single 802.11 MAC standard is based on CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). The 802.11 specification includes provisions designed to minimize collisions. Because two mobile units may both be in range of a common access point, but not in range of each other. The 802.11 has two basic modes of operation: Ad hoc mode enables peer-to-peer transmission between mobile units. Infrastructure mode in which mobile units communicate through an access point that serves as a bridge to a wired network infrastructure is the more common wireless LAN application the one being covered. Since wireless communication uses a more open medium for communication in comparison to wired LANs, the 802.11 designers also included a shared-key encryption mechanism, called wired equivalent privacy (WEP), or Wi-Fi Protected Access, (WPA, WPA2) to secure wireless computer networks.BridgeA 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.Bluetooth is an industrial specification for wireless personal area networks (PANs). Bluetooth provides a way to connect and exchange information between devices such as mobile phones, laptops, PCs, printers, digital cameras, and video game consoles over a secure, globally unlicensed short-srange radio frequency. The Bluetooth specifications are developed and licensed by the Bluetooth Special Interest Group.Uses??Bluetooth is a standard and communications protocol primarily designed for low power consumption, with a short range (power-class-dependent: 1 meter, 10 meters, 100 meters)[1] based on low-cost transceiver microchips in each device.Bluetooth enables these devices to communicate with each other when they are in range. The devices use a radio communications system, so they do not have to be in line of sight of each other, and can even be in other rooms, as long as the received transmission is powerful enough.ClassMaximum Permitted Power(mW/dBm)Range(approximate)Class 1100?mW (20?dBm)~100 metersClass 22.5?mW (4?dBm)~10 metersClass 31?mW (0?dBm)~1 meterIt has to be noted that in most cases the effective range of class 2 devices is extended if they connect to a class 1 transceiver, compared to pure class 2 network. This is accomplished by higher sensitivity and transmitter power of the Class 1 device. The higher transmitter power of Class 1 device allows higher power to be received by the Class 2 device. Furthermore, higher sensitivity of Class 1 device allows reception of much lower transmitted power of the Class 2 devices. Thus, allowing operation of Class 2 devices at much higher distances. Devices that use a power amplifier on the transmit, have improved receive sensitivity, and highly optimized antennas are available that routinely achieve ranges of 1km within the Bluetooth Class 1 standard.VersionData Rate?Version 1.21 Mbit/sVersion 2.0 + EDR3 Mbit/sWiMedia Alliance(proposed)53 - 480 Mbit/s?In order to use Bluetooth, a device must be compatible with certain Bluetooth profiles. These define the possible applications and uses of the technology.List of applicationsMore prevalent applications of Bluetooth include:Wireless control of and communication between a mobile phone and a hands-free headset or car kit. This was one of the earliest applications to become popular.Wireless networking between PCs in a confined space and where little bandwidth is required.Wireless communications with PC input and output devices, the most common being the mouse, keyboard and printer.Transfer of files between devices with OBEX.Transfer of contact details, calendar appointments, and reminders between devices with OBEX.Replacement of traditional wired serial communications in test equipment, GPS receivers, medical equipment, bar code scanners, and traffic control devices.For controls where infrared was traditionally used.Sending small advertisements from Bluetooth enabled advertising hoardings to other, discoverable, Bluetooth devices.Seventh-generation game consoles—Nintendo Wii, Sony PlayStation 3—use Bluetooth for their respective wireless controllers.Dial-up internet access on personal computer or PDA using a data-capable mobile phone as a modem.Receiving commercial advertisements ("spam") via a kiosk, e.g. at a movie theatre or lobbyBluetooth vs. Wi-Fi in networkingBluetooth and Wi-Fi have slightly different applications in today's offices, homes, and on the move: setting up networks, printing, or transferring presentations and files from PDAs to computers. Both are versions of unlicensed spread spectrum technology.Bluetooth differs from Wi-Fi in that the latter provides higher throughput and covers greater distances, but requires more expensive hardware and higher power consumption. They use the same frequency range, but employ different multiplexing schemes. While Bluetooth is a cable replacement for a variety of applications, Wi-Fi is a cable replacement only for local area network access. Bluetooth is often thought of as wireless USB, whereas Wi-Fi is wireless Ethernet, both operating at much lower bandwidth than the cable systems they are trying to replace. However, this analogy is not entirely accurate since any Bluetooth device can, in theory, host any other Bluetooth device—something that is not universal to USB devices, therefore it would resemble more a wireless FireWire.Bluetooth ??Bluetooth exists in a many products, such as phones, printers, modems and headsets. The technology is useful when transferring information between two or more devices that are near each other in low-bandwidth situations. Bluetooth is commonly used to transfer sound data with phones (i.e. with a Bluetooth headset) or byte data with hand-held computers (transferring files).Bluetooth simplifies the discovery and setup of services between devices. Bluetooth devices advertise all of the services they provide. This makes using services easier because there is no longer a need to setup network addresses or permissions as in many other networks.Wi-FiWi-Fi is more like traditional Ethernet networks, and requires configuration to set up shared resources, transmit files, and to set up audio links (for example, headsets and hands-free devices). It uses the same radio frequencies as Bluetooth, but with higher power output resulting in a stronger connection. Wi-Fi is sometimes called "wireless Ethernet." This description is accurate, it also provides an indication of its relative strengths and weaknesses. Wi-Fi requires more setup, but is better suited for operating full-scale networks because it enables a faster connection, better range from the base station, and better security than puter requirements????A personal computer must have a Bluetooth adapter in order to be able to communicate with other Bluetooth devices (such as mobile phones, mice and keyboards). While some desktop computers already contain an internal Bluetooth adapter, most require an external Bluetooth dongle. Most recent laptops come with a built-in Bluetooth adapter.Unlike its predecessor, IrDA, which requires a separate adapter for each device, Bluetooth allows multiple devices to communicate with a computer over a single adapter. ................
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