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• Message: Information being communicated.
• Channel: The medium through which data is transmitted.
DATA
Communication Channel
Node 1 DATA Node 2
Figure 6.1 Communication System
Figure 6.2 shows a collection of interconnected nodes that communicate through some channel that forms a computer
network. Each participating node in a network has a unique Media Access Control address (MAC address). For example,
if you are using WINDOWS® you can find your computer’s MAC address (physical address) by executing the following
command from the command prompt:
ipconfig /all
What is the MAC address of your computer?
Computer System 1
Mobile
Printer
Computer
System 2
Computer Computer
System 3 System 4
Figure 6.2 Computer Network
One or more nodes in a computer network may provide resources and services to other nodes. An application program
that provides a service in a computer network is called a server. For example, an FTP server installed on a computer
enables the transfer of files to various nodes in the network. In informal use, the node that hosts a service is sometimes
called a sever, even though it does not confirm to the technical meaning of the term.
6.2 Evolution of Networking
In 1876, Bell developed a communication paradigm based on telephone lines. Initially, each telephone was connected
by a dedicated wire to another phone it wished to communicate with. However, the network soon became a mess
of cables. In 1877, his Bell Telephone Company established the Public Switched Telephone Network (PSTN), allowing
several home phones to connect to a dedicated office that would establish a dedicated connection between the
interested parties. This form of end to end communication remained the primary mode of communication for the
next hundred years. Establishing a dedicated connection (also called path or circuit) between the two end-users often
requires passing through several intermediate switches en route. So, setting up a telephone circuit is called circuit
switching. Figure 6.3 shows a telephone network.
222 Touchpad Informatics Practices-XII

