Wireless Connectivity
Content
Abstract
Introduction
Operation
Wireless Application Design
Caching
Prefetching
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
Abstract
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.
Introduction
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.
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. ............
