Page 23 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes architecture represents a systemization of the various kinds of protocols needed to build a
network.
Computer manufacturers have developed different protocols as needed. This means that each
type of computer needed to support different protocols. This also necessitated large development
and maintenance costs. All computer manufacturers, therefore worked together to standardize
and systemize protocols to link their models and thereby reduce the development and
maintenance costs. This was how each manufacturer built own network architecture. Since the
concept of the network architecture was first introduced to connect the computers of the same
manufacture, the process has become easier. However, from user’s perspective, the ideal form of
network architecture is one, which enables machines of all manufacturers to connect to each
other. Therefore, the need of standardization of network architecture arose.

2.2 Layering the Communications Process

Open Systems Interconnection (OSI) was set up as an international standard for network
architecture to reduce their design complexity. Hence, most of the networks are organized as a
series of layers or levels. Layering the communications process means breaking it down the
communication process into smaller and easier to handle interdependent categories, with each
solving an important and somehow distinct aspect of the data exchange process. Each layer has
to offer specified services to the higher layers. Thus, layer on one computer carries on a
conversation with corresponding layer on another computer in the network. The rules and
conventions used in such communications are collectively known as the layer protocol. The
entities comprising the corresponding layers on different computers are called peers, which
communicate using the protocol. Between each pair of adjacent layers an interface exists that
defines primitive operations and services the lower layer offers to the upper one.
The International Organization for Standardization (ISO) took the initiative in setting up OSI.
OSI has two meanings. It refers to:

Protocols that are authorized by ISO
OSI basic reference model
OSI reference model divides the required functions of the network architecture into several
layers and defines the function of each layer.
The group of layers and protocols is called the network architecture. These groups of layers are
provided with enough information to allow a software/hardware implementation, which
correctly obeys the appropriate protocol.
The objective of this detail is to develop an understanding of the complexity and sophistication
that this technology has achieved, in addition to developing the concept for the inner workings
of the various components that contribute to the data communications process. The details of the
implementation and the specification of the interfaces are never part of the architecture because
they are not visible from the outside.
The functions of layered architecture may be comprehended with an example of conservations
taking place between two persons with different language of communication, say, English and
French. A three-layered architecture as shown in Figure 2.1 explains the concept. Dotted lines
from peers to peers indicate virtual connections.

Two persons (peer processes in layer 3), one speaking English and the other speaking
French, want to communicate.
They are using a translator (peer processes at layer 2).

A secretary (peer processes at layer 1) facilitates each translator for message transmission.

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