A communication network, consists of a set of equipment and facilities that provide a service, the transfer of information between users located at various geographical points. For example telephone networks provides telephone service, computer networks, television broadcast networks, cellular networks and Internet.
Communication network and transportation networks have become essential infrastructure in every society.
The capacity to transfer high volume of data for a long distance almost immediately are the features of network-based services. For example Internet provides e-mail, information search and retrieval, various forms of electronic service.
EVOLUTION OF NETWORK ARCHITECTURE AND SERVICES
A communication service which is used of transfer of information. Different services differ in the details of how and in what form information is transferred.
Let us consider three example networks
i) Telegraph networks
ii) Telephone networks
iii) Computer networks
TELEGRAPH NETWORKS AND MESSAGE SWITCHING
In the year 1987 Samuel B. Morse demonstrated a practical telegraph that provided the basis for telegram service, the transmission of text message over long distance. Here the text was encoded into sequence of dots and dashes. Each dot and dash which is used to be represented by short and long pulses of electrical current over a copper wire. By relying on two signals, telegraphy made use of digital transmission system. In Morse code the pause between letter was 3 dots and for word is 5 dots.
In store-and-forward process, the message is completely received and stored at an intermediate point and then forward to the next node depending on the availability of link. The process of selecting an appropriate link is known as routing. If the information is message, then it is known as message switching. In simpler terms, message switching is a switching method that sends data from point-to-point with each intermediate node storing the data , waiting for a free transmission channel and forwarding the data to the next point until the destination is reached.
In the beginning communication channel used to carry information sent by one person. Naturally transmission rate (in bits/second) was less. To increase the transmission rate multiplexing was developed. Multiplexing is a technique used to place multiple signals on a single communication channel. Multiplexing partitions a channel into many separate channels, each capable of transmitting its own independent signal, thereby enabling many different transmission over a single medium.
One approach of multiplexing involves modulation. Binary symbols can be transmitted by sending a sinusoidal signal of a given frequency for a given period of time. Multiple sequence of binary symbols could be transmitted simultaneously by using multiple pairs of frequencies for the different telegraphy signals. This concept is used in Modems.
We know characters can be represented by ASCII ( American Standard Code for Information Interchange) , is a 7 bit code. The equipments can communicate with ASCII code.
The telegraph service involves the transmission of text messages between geographically far places. To provide the service , the architecture of the telegraph network contains the following key elements or functions.
i) Digital transmission takes in the network that is information is represented either 0 or 1. It can make use of ASCII code also. The transmission medium can be copper wire and radio.
ii) To identify the messages exactly, messages are covered by beginning and ending by sequence of characters. This is known as framing.
iii) There should be destination address that is to whom it want to deliver. Routing procedure determines the path that a message follows across a network of telegraph stations interconnected by digital lines
TELEPHONE NETWORKS AND CIRCUIT SWITCHING
In 1876 Alexander Graham Bell developed a device that can transmit voice signals. The device that is known as Telephone. The telephone network provides a two-way, real-time transmission of voice signals across a network.
Telephone service became popular due to its voice service and expert operator with knowledge of Morse code is not required like in the case of telegraph. Here voice signals are converted into equivalent electrical signal and passed through cable.