cols 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. В В
Operation
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.
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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...
