15.01.2013 Views

U. Glaeser

U. Glaeser

U. Glaeser

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

its information. Intuitively, this MAC protocol should be able to avoid collisions, but collisions still do<br />

take place. The reason is that transmissions travel along the transmission medium at a finite speed. If<br />

one user senses the medium at one point and finds it free, it does not mean that another user located at<br />

another point of the medium has not already begun its transmission. This is referred to as the effect of<br />

the finite propagation delay of electromagnetic signal along the transmission medium. This is the single<br />

most important parameter that causes deterioration of performance in contention-based local area<br />

networks [11,19].<br />

Design of local area networks has also been significantly impacted by the availability of transmission<br />

media with higher data rates. As the data rate of a transmission medium increases, the effects of propagation<br />

delay becomes even more visible. In higher speed local area networks such as Gigabit Ethernet,<br />

and 100-BASE-FX, the medium access protocols are designed such that to reduce the effects of propagation<br />

delay. If special attention is not given to the effects of propagation delay, the performance of highspeed<br />

local area networks becomes very poor [15,19].<br />

Metropolitan area networks essentially deal with the same issues as local area networks. These networks<br />

are generally used as backbones for interconnecting different local area networks together. These are highspeed<br />

networks and span a relatively larger geographical area. MAC protocols for sharing the same transmission<br />

media are based on controlled access. Two most common examples of metropolitan area networks<br />

are fiber distributed data interface (FDDI) and distributed queue dual bus (DQDB). In FDDI, the transmission<br />

medium is in the form of two rings, whereas DQDB uses two buses. FDDI rings carry information<br />

in one but opposite directions and this arrangement improves reliability of communication. In DQDB,<br />

two buses also carry information in one but opposite directions. The MAC protocol for FDDI is based on<br />

token passing and supports voice and data communication among its users. DQDB uses a reservationbased<br />

access mechanism and also supports voice and data communication among its users [19].<br />

Wireless and Mobile Communication Networks<br />

Communication without being physically tied-up to wires has always been of interest and mobile and<br />

wireless communication networks promises that. The last few years have witnessed unprecedented growth<br />

in wireless communication networks. Significant advancements have been made in the technologies that<br />

support wireless communication environment and there is much more to come in the future. The devices<br />

used for wireless communication require certain features that wired communication devices may not<br />

necessarily need. These features include low power consumption, light weight, and worldwide communication<br />

ability.<br />

In wireless and mobile communication networks, the access to a communication network is wireless<br />

so that the end users remain free to move. The rest of the communication path could be wired, wireless,<br />

or combination of those. In general, a mobile user, while communicating, has a wireless connection with<br />

a fixed communication facility and rest of the communication path remains wired. The range of wireless<br />

communication is always limited and therefore range of user mobility is also limited. To overcome this<br />

limitation, cellular communication environment has been devised. In a cellular communication environment,<br />

geographical region is divided into smaller regions called cells, thus the name cellular. Each<br />

cell has a fixed communication device that serves all mobile devices within that cell. However, as a mobile<br />

device, while in active communication, moves out of one cell and into another cell, service of that connection<br />

is transferred from one cell to another. This is called handoff process [7,16].<br />

The cellular arrangement has many attractive features. As the cell size is small, the mobile devices do<br />

not need very high transmitting power to communicate. This leads to smaller devices that consume less<br />

power. In addition, it is well known that the frequency spectrum that can be used for wireless communication<br />

is limited and can therefore only support a small number of wireless communication connections<br />

at a time. Dividing communication region into cells allows use of the same frequency in different cells as<br />

long as they are sufficiently apart to avoid interference. This increases the number of mobile devices that<br />

can be supported. Advances in digital signal processing algorithms and faster electronics have led to very<br />

powerful, smaller, elegant, and versatile mobile communication devices. These devices have tremendous<br />

mobile communication abilities including wireless Internet access, wireless e-mail and news items, and<br />

© 2002 by CRC Press LLC

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!