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Реферат: Wireless connectivity

Реферат: Wireless connectivity

Wireless Connectivity





Wireless Application Design



Data reduction

Mobile WWW Browsers

Active documents

Dynamic URLs

Data over Cellular links

Radio-based wireless connectivity

IEEE 802.11 protocol

Interworking Units for wireless connectivity

Internet Mobile Host Protocol

IP Tunneling


Traditional networking technologies offer tremendous capabilities from an office or home via the Web. But, limitations to networking through the use of wired-based systems exist because you cannot utilize these network services unless you are physically connected to the system. As mobile computing becomes more prevalent, systems and applications must deal with scarcity of resources such as bandwidth. Mobile devices and wireless workstations should handle some of the work that has been traditionally carried-out by the network through techniques such as document partitioning. Dynamic documents can also be used to cache and prefetch documents while the network connection is not being utilized fully.

Meanwhile, the need for higher speed wireless connections is growing with multimedia rich contents on the World Wide Web (WWW). The IEEE 802.11 protocol and the Medium Access Control part of the protocol (DFWMAC) will allow wireless networks to operate at high data rates (1 to 20 Mbps). Furthermore, the 802.11 only effects the bottom two layers of the OSI's seven layered architecture; hence, through an access point (Router), wireless packets are routed to the Web.



Wireless LANs will provide the first layer of connectivity between mobile users and the global information infrastructure. Wireless devices such as Personal Digital Assistants (PDAs) and Notebooks will be an extension of the Web. The user should not know nor care whether the information travels over a wire or a radio frequency. Depending on the power of the transmitters and the sensitivity of the receivers, wireless devices may become the first truly universal form of virtual LAN. By mixing the wireless Networks with other wireless communication technologies such as cellular and satellite, the user can have full connectivity at all times and more importantly everywhere on the globe.

Wireless connectivity to the web can also be achieved through the use of existing cellular telephone links. Using Spread Spectrum Technologies (SST) such as time-division multiple access (TDMA), code-division multiple access (CDMA) and extended time-division multiple access (ETDMA) has allowed the cellular links to carry more information and as a result better suited for data transmission. Although the overhead in cellular data transmission is somewhat high, but data reduction techniques, and caching is used to reduce network latency.

With the Introduction of PDA, people began to see the natural progression of Wireless technology into these devices. However, the current state of these devices has obvious limitations. Computational power, storage, communication bandwidth, display size and power consumption are just a few of these limitations. Nevertheless, presently such devices are running Web browser, mail clients and etc.Presently there is a variety of pen based computer systems like palmtops, notebooks and different versions of what John Sculley, Apple's vice president in 1992 introduced as a PDA.

Personal communication is the primary motivation for wireless connectivity, but in addition, wireless users need access to on-line information in real time. There are three reasons why users need to be connected to the Web. First, it is often difficult, if not impossible, to determine the data of interest ahead of time and download it to the hand-held device. Second, even then, space limitations may prevent caching of all data. Finally, some data changes dynamically with time such as weather forecast, or stock market activities. (Watson, 1994)

The current application environment is ill-suited for the wireless Web, the wired web squanders bandwidth through unusable information on the client's side. In the wired world these inefficiencies amount to only milliseconds, but as the bandwidth is reduced over wireless links, milliseconds can add up to seconds and perhaps time-outs by the underlying protocols such as TCP/IP. As a result various groups have proposed new HyperText Mark-up Language (HTML) or new protocols such as HTTP+. But these avenues of solutions are rigid and the need for standardization is greater than a temporary increase in throughput for a particular scheme.



Similar to any transmission system, a wireless system needs a transmitter, a receiver and a transmission medium. In a wireless system, the transmission medium is air rather than the cables used by conventional wired systems. The use of air as a transmission medium utilizes two major spectra: infrared and radio frequency.

The key difference between the use of infrared and radio frequency is the support of roaming. Infrared is a line-of-sight technology. There has to be a direct line of sight or at least a surface to bounce the waves from the transmitter to the receiver. On the other hand, radio frequency systems can penetrate through objects such as walls and doors in most office buildings; hence their popularity in present wireless systems. FCC rules allow only small sections of the electromagnetic spectrum (figure below) to be used for wireless data networks; thus techniques are needed to avoid interference from other devices that share the space or perhaps multiple stations using the same frequency.

A technique developed by the military in the 1970s to help secure transmissions offers a way around this problem. This technique is called Spread Spectrum Technology (SST). It involves spreading transmissions across a range of frequencies, rather than transmitting on one frequency all the time.

One approach known as Frequency-Hopping Spread Spectrum (FHSS) involves dividing a range of the radio spectrum into individual channels, each on a specific frequency. A transmitter can hop from one channel to the next and if the receiver is aware of the hopping pattern of the transmitter, it can follow the pattern and receive the information. The second method of spread spectrum is Direct Sequence Spread Spectrum (DSSS). The source data to be transmitted is first exclusive ORed with a pseudorandom binary sequence. The bits making up the sequence are random but the same sequence is made much larger than the source data rate. When this data is modulated and transmitted it occupies a wider frequency band than the original source data bandwidth. This would make the signal appear as noise to any other devices using the same frequency spectrum. All the members of this wireless system know the binary sequence being used .(Halsall, 1996) Thus, all receivers first search for the known preamble sequence, once it has been recognized, the receivers start to interpret the bit stream.

FCC rules for DSSS transmission requires 10 or more redundant data bits to be added to each signal. This limits the maximum throughput of DSSS transmitters to approximately 2 Mbps when using the 902-MHz band, and approximately 8 Mbps in the 2.4-GHz band.

Wireless Application Design

Designing a web application for a wireless node is different from designing a web application for a workstation. Bandwidth is a precious resource in the wireless domain and it must be utilized in the most efficient fashion. Research focuses on streamlining applications to make the best use of the available bandwidth. These options include using dynamic documents which use the resources of the mobile node itself to generate parts of a document or partitioning the application between a client and the server.

Dynamic documents can address the variable resources requirement of mobile computers accessing the Web. Dynamic documents are programs executed by programs such as Web browsers in order to generate the actual information displayed to the user. Execution of a dynamic document cause the client to perform any number of actions in order to generate a final presentation to the user. Dynamic documents are flexible enough to address many mobile computing resource constraints. Documents can be customized at the client depending on available resources. (Kaashoek, 1994)

Application partitioning can also be used over a wireless link for more effective use of the wireless link. Much like a client/sever system, applications and their functionality can be divided into different parts. the boundaries between how much of the application should be run on the client side vs. the server side can be determined dynamically and based upon the availability of the bandwidth. The data and their functions are packaged into hyperobjects. The purpose of hyperobjects is to expose a certain level of application structure and semantics to the system in a uniform and manageable way. The system will use this hyperobject structure, along with observations of access patterns to make informed decisions. (Watson, 1995)

Partitioning documents are combined with several other well-known techniques to increase the effectiveness of wireless clients such as browsers.

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