Wireless Application Design
Mobile WWW Browsers
Data over Cellular links
Radio-based wireless connectivity
IEEE 802.11 protocol
Interworking Units for wireless connectivity
Internet Mobile Host Protocol
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.
Applications specify the caching attributes of an object or a number of objects. The default is to optimistically replicate objects on the mobile device. Explicit synchronization can be used to make the cache consistent with the wired network if the wireless link is up. (Watson 1995)
As a document is loaded and displayed on a mobile device the links in a hiarchial fashion are used to prefetch the relevant documents and cached. In a hyperobject application the system will use its knowledge of the relevancy and the position of various objects in order to anticipate and prefetch other objects. Prefetching can only be done if the system resources allow it. For example as a user is viewing the first page of a document the relevant objects for that document are being prefetched into the cache given the wireless link is up and functioning. Prefetching hides the latency of the link and it will also filter the burstiness by spreading the traffic over a longer time.
Data reduction can be dynamically decided by the user for various high bandwidth applications such as video transmissions. A video stream delivers certain number of frames per constant unit of time. As the number of frames are reduced it adversely affects the quality of the video but the bandwidth needed is also reduced; hence the user can dynamically find a balance between what the available resources and the desired video quality. The same principal can be applied to the sound and also any real-time stream of data over the wireless link.
Mobile WWW Browsers
Web infrastructure as it exists today can not easily accommodate mobile clients because of the fact that almost all information resides statically in HTML documents. The dynamic information that the Web supports is returned to the client without incorporating any user context or is incorporated explicitly using forms-based interfaces that require user input on the client. Extensions to the Web have been created to include:
· A network server that maintains mobile computing contexts within a client-specific domain.
· An asynchronous callback mechanism to notify Web clients when a user's dynamic computing environment changes.
· A syntax for referencing dynamic information in URLs and documents. (Voelker)
Active documents are HTML documents that allow the Web client to automatically react to changes in mobile computing environment. If the information in an active document that the client is displaying becomes invalid then the client can be notified of that change so that more relevant information can be displayed. Variables such as location can be updated as the mobile user roams from one cell area to the next. Active documents are written just like any other HTML file with only a minor addition. A subscribe command is embedded in an HTML comment line. By having the subscribe command embedded in a comment line backward compatibility can be preserved thus allowing regular Web browsers to view the documents. (Voelker)
Ordinarily URLs are links to set static documents on the Web. Dynamic URLs will reference a user to a different document based upon other variables such as the location variable. Dynamic URLs exist in active documents in order to receive the variables from the client. When a user selects a dynamic URL in a document the client browser is responsible for resolving all references to dynamic variables within the URL. When all variable references have been resolved the result is a standard URL that the client then sends to the server. (Voelker)
Data over Cellular links
The analog cellular telephone system uses FM (Frequency Modulation) radio waves to transmit voice grade signals. To accommodate mobility this cellular system switches radio connection from one cell to another as the mobile user moves from one cell to another (roaming). Every cell within the network has a transmission tower that links mobile callers to a Mobile Telephone Switching Office (MTSO). The MTSO which is owned and operated by the cellular carrier in each area provides a connection to the public switched telephone network. The public telephone networks acts also as gateways to the Internet.
Most modems that operate over wireline telephone services will also interface and interoperate with cellular phones; however modem software optimized to work with cellular phones minimizes battery usage. There are problems with modem communication over cellular links. The first problems occurring were the hand-off problems or roaming. As a mobile user moves from one service area to the next a hand-off occurs from one service area to the next. The hand-off would disrupt the call for 100 to 200 ms. This is just enough to disrupt the carrier detect (CD) cycle; hence the modem assumes that one of the callers has disconnected and it hangs up. This problem can be overcome similar to fax modems over cellular links. The modem will delay 400 ms before hanging up giving the hand-off enough time to take place. Some data might be affected but error detection and error correction procedures (CRCs) will detect and correct the data bits that have been corrupted. But all these techniques lower the effective throughput of our communication system and the effective throughputs achieved with cellular modems hover around 19200 bits/s. (Bates Gregory 1995)
To establish a dedicated wireless data network for mobile users a consortium of companies in the United States developed the Cellular Digital Packet Data (CDPD) standard. CDPD overlays the conventional analog cellular telephone system using a channel hopping technique (previous section) to transmit data in short bursts during idle times in cellular channels. CDPD operates full duplex meaning simultaneous transmission in both directions in the 800 and 900 MHz frequency bands. The main advantage of the analog cellular system is widespread coverage. Since CDPD piggybacks on this system it will also provide nearly worldwide coverage. The main advantage with CDPD is that it uses digital signals making it possible to enhance the transmission of data.