Page 1073 - The Mechatronics Handbook

user in order to communicate. In fact, in wide area networks, two communicating users will rarely be
directly connected with each other. This implies that the users will be sharing the transmission links for
exchanging their information. This is one of the most important aspects of computer networks. Sharing
of resources improves utilization of the resources and is, of course, cost-effective as well. In addition to
sharing the transmission links, the users will also share the processing power of the computers at the
switching nodes, buffering capacity to store the information at the switching nodes, and any other resources
that are connected to the computer network. A user who is connected to a computer network at any
switching node will have immediate access to all the resources (databases, research articles, surveys, and
much more) that are connected to the network as well. Of course, access to specific information may be
restricted and a user may require appropriate authorization to access the information.
The information from one user to another may need to pass through several switching nodes and trans-
mission links before reaching its destination. This implies that a user may have many options available to
select one out of many sequences of transmission links and switching nodes to exchange information. That
adds to the reliability of the information exchange process. If one path is not available, not feasible, or not
functional, some other path may be used. In addition, for better and effective sharing of resources among
several users, it is not appropriate to let any user exchange a large quantity of information at a time; however,
it is not uncommon that some users may have a large quantity of information to exchange. In that case,
the information is broken into smaller units known as packets of information. Each packet is sent toward
its destination as a separate entity and then all packets are assembled together at the destination side to
re-create the original piece of information [2].
Due to the resource sharing environment, users may not be able to exchange their information at any
time they wish because the resources (switching nodes, transmission links) may be busy serving other
users. In that case, some users may have to wait for some time before they begin their communication.
Designers of computer networks should design the network so that the total delay (including wait time)
is as brief as possible and that the total amount of information successfully exchanged (throughput) is
as large as possible.
Many aspects must be addressed for enabling networks to transport users’ information from one point
to another. The major aspects are:
• Addressing mechanism to identify users
• Addressing mechanism for information packets to identify their source and destination
• Establishing a connection between sender and receiver and maintaining it
• Choosing a path or a route (sequence of switching nodes and transmission links) to carry the
information from a sender to a receiver
• Implementing a selected route or path
• Checking information packets for errors and recovering from errors
• Encryption and decryption of information
• Controlling the flow of information so that shared resources are not over-taxed
• Informing the sender that the information has been successfully delivered to the intended desti-
nation (acknowledgment)
• Billing for the use of resources
• Ensuring that different computers running different applications and operating systems can exchange
information
• Preparing information appropriately for transmission over a given transmission medium
This is not an exhaustive list of items that need to be addressed in computer networks. In any case, all
such issues are addressed by very systematic and detailed procedures. The procedures are called communi-
cation protocols. The protocols are implemented at the switching nodes by a combination of hardware and
software. It is not advisable to implement all these features in one module of hardware or software because
that will become very difficult to manage. It is a standard practice that these features be divided into different

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