Page 1076 - The Mechatronics Handbook

control will be too slow to be of any use. Therefore, some preventive mechanisms have been developed
to maintain the information traffic inside a computer network to a comfortable level. Such techniques
are implemented at the sender side by ensuring that only as much information traffic is allowed to enter
the network as can be comfortably handled by the networks [1,20,22]. Based on the users’ needs and
state of the technology, providing faster communications for different types of services (voice, video,
data, and others) in the same computer network in an integrated and unified manner has become a
necessity. These computer networks are referred to as broadband integrated services digital networks
(BISDNs). BISDNs provide end-to-end digital connectivity and users can access any type of communi-
cation service from a single point of access. Asynchronous transfer mode (ATM) is expected to be used
as a transfer mechanism in BISDNs. ATM is essentially a fast packet switching technique where infor-
mation is transmitted in the form of small fixed-size packets called cells. Each cell is 53 bytes long and
includes a header of 5 bytes. The information is primarily transported using a connection-oriented
(virtual circuit) environment [3,4,8,12,17].
Another aspect of wide area networks is the processing speed of switching nodes. As the data rates of
transmission media increase, it is essential to have faster processing capability at the switching nodes.
Otherwise, switching nodes become bottlenecks and faster transmission media cannot be fully utilized.
When transmission media consist of optical fibers, the incoming information at a switching node is
converted from optical form to electronic form so that it may be processed and appropriately switched
to an outgoing link. Before it is transmitted, the information is again converted from electronic form to
optical form. This slows down the information transfer process and increases the delay. To remedy this
situation, research is being conducted to develop large optical switches to be used as switching nodes.
Optical switches will not require conversion of information from optical to electronic and vice versa at
the switching nodes; however, these switches must also possess the capability of optical processing of
information. When reasonably sized optical switches become available, use of optical fiber as transmis-
sion media together with optical switches will lead to all-optical computer and communication networks.
Information packets will not need to be stored for processing at the switching nodes and that will certainly
improve the delay performance. In addition, wavelength division multiplexing techniques are rendering
use of optical transmission media to its fullest capacity [14].

Local and Metropolitan Area Networks
A local area network has a limited geographical scope (no more than a few kilometers) and is generally
limited to a building or an organization. It uses a single transmission medium and all users are connected
to the same medium at various points. The transmission medium may be open-ended (bus) as shown
in Fig. 38.3(b) or it may be in the form of a loop (ring) as shown in Fig. 38.3(c). Metropolitan area
networks also have a single transmission medium that is shared by all the users connected to the network,
but the medium spans a relatively larger geographical area, up to 150 km. They also use a transmission
medium with relatively higher data rates. Local and metropolitan area networks also use a layered imple-
mentation of communication protocols as needed in wide area networks; however, these protocols are
relatively simpler because of simple topology, no switching nodes, and limited distance between the
senders and the receivers. All users share the same transmission medium to exchange their information.
Obviously, if two or more users transmit their information at the same time, the information from
different users will interfere with each other and will cause a collision. In such cases, the information of
all users involved in a collision will be destroyed and will need to be retransmitted. Therefore, there must
be some well-defined procedures so that all users may share the same transmission medium in a civilized
manner and have successful exchange of information. These procedures are called medium access control
(MAC) protocols.
There are two broad categories of MAC protocols:

• Controlled access protocols
• Contention-based access protocols

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