Реферат: Wireless connectivity

Caching

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)

Prefetching

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

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

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)

Dynamic URLs

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. With digital signaling, it is possible to encrypt the data stream and provide easier error control. CDPD is a robust protocol that is connectionless and corrects errors at the receiver side without asking the source to retransmit the errored packet.

Other digital techniques presently being tested and utilized by the carrier companies are:

·  Time-division multiple access (TDMA)

·  Extended time-division multiple access (ETDMA)

·  Code-division multiple access (CDMA)

·  Narrowband advanced mobile phone service (N-AMPS)

In the case of ETDMA the bandwidth can be increased by a factor of 15, making it much more acceptable for today's application needs.

Radio-based wireless connectivity

The most widely sold wireless LAN products use radio waves as a medium between computers and the WEB or each other. An advantage of radio waves over other forms of wireless connectivity such as infrared and microwaves is that they propagate through walls and other obstructions with little attenuation. Even though several walls might separate the user from the server or an access point to the Web, users can maintain connections to the network, thus supporting true mobility. The disadvantage for radio frequencies is that governments manage the region and not all the spectrum can be used everywhere; hence, techniques such as FHSS and DSSS (as described ) must be used.

There are three regions of the E-M spectrum utilized by these waves:

·  902-928 MHz

·  2.4-2.484 GHz

·  5.725-5.850 GHz

Presently Metricom is operating a two way radio based multi-user data communications system is San Francisco called Ricochet. The architecture is shown below:

The concept is to use wireless access points and network radio relays approximately one half mile apart to facilitate connectivity between users. The radios operate in the license-free 902-928 portion of the radio spectrum using FHSS. The underlying network protocol is TCP/IP, allowing it to interact with the Internet seamlessly.

An important goal for wireless communications has been to make the application layer transparent to the underlying protocol (TCP/IP) in order to have more acceptability by the Web users. To understand the kind of standards developed for wireless networks, it helps to see the affected layers in an OSI (Open System Interconnect) model. The bottom two layers are the ones of interest to us. At the very bottom is the Physical layer. This layer defines the electrical characteristics of the actual connection between network nodes. For wired networks, it covers topics like voltage levels and type of cabling. But for wireless networks, it addresses areas such as frequencies used and modulation techniques, including spread-spectrum technologies.

The next layer up is the Data Link Layer. It deals with how the network is shared between nodes. The Data Link Layer defines rules such as who can talk on the network, how long they can occupy network resources. This layer can be further divided into two separate layers (shown below).

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