Page 31 - Intelligent Communication Systems

CHAPTER 3 / COMMUNICATION NETWORK STRUCTURE I 5
3.2.1 Computer Network

A computer network is composed of networks and computers and is used to inter-
connect computers that are widely distributed. The computer network contributes to
the functionality, usability, reliability, and efficiency of the distributed computers.
In the first step, the centralized computer system was developed. Here, a cen-
tral computer and terminals are linked and various kinds of application programs
are provided, such as inventory management and process control. In the second
step, the distributed computer network was developed. In this system, two or more
distributed computers are interconnected via the network, with terminals linked to
each computer. Each computer has its own functions, such as inventory manage-
ment and database management. A request from a terminal is distributed to the
computer, which fulfills its request.
The Advanced Research Project Agency (ARPA) is a typical example of a dis-
tributed computer network. The ARPA project, which started in 1969 under the
sponsorship of the U.S. Department of Defense, interconnected the computers of
universities and research institutes. Out of this activity, the packet switched net-
work, the TCP/IP protocol, and the network architecture were invented.
The ARPA network has become the backbone of the Internet. The TCP/IP
protocol, which was invented by the project, has been used as the de facto stan-
dard of the Internet. Mail message-handling services, such as electronic mail, and
bulletin boards were introduced in the ARPA network.

3.2.2 Network Architecture

A computer and a terminal exchange information via the computer network.
Therefore, the network should be efficient, fast, and reliable with respect to the
transmission of data. To transmit data between computer and terminal, between
peer computers, and between networks, protocols have to be developed and stan-
dardized. All kinds of computers and terminals should be linkable in the network.
The network is also expanded by interconnecting to other networks. In this way,
the network structure will change dynamically day by day.
To achieve this, the network architecture—such as protocols and network
topology—has to be defined and standardized. The network architecture should be
such that any kinds of components, such as terminals, computers, and networks,
can be interconnected without any restrictions.
ARPA was the network architecture invented first in the world. Since then,
computer manufacturers such as IBM, Digital Equipment, Hitachi, Fujitsu, and
NEC have announced their own architecture for their computer network. For exam-
ple, SNA was the architecture developed by IBM, and DECNET was developed
by Digital Equipment.
These architectures differed from each other, so standardization was proposed
and conducted, mainly by ISO and CCITT. The OSI reference model was proposed
and standardized as an architecture for computer networks. When making stan-
dard protocols, the following points are taken into consideration.