tion 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:
ТЗ Time-division multiple access (TDMA)
ТЗ Extended time-division multiple access (ETDMA)
ТЗ Code-division multiple access (CDMA)
ТЗ 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.
В
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 EM spectrum utilized by these waves:
ТЗ 902-928 MHz
ТЗ 2.4-2.484 GHz
ТЗ 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. p> 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).
ТЗ The Medium Access Control (MAC) layer. p> ТЗ The Logical Link Control (LLC) layer. p> The first f...
