Increasing availability of low-cost computers and associated equipment of varying capabilities promoted the trends towards distributed data processing. Developments in communication techno-logy has made it possible to interconnect such geographically dispersed computing resources of different kinds and makes.
A computer communication network is an interconnection of a collection of several computers from which the user can select the service required and communicate with any computer as a local user. A computer communication networks can be viewed as a collection of nodes with computing resources and nodal-switching computers that facilitate communication through a set of transmission links (McGoven, 1989, pp. 2-3).
Users can access the network through terminals attached to a node and messages traverse these network through the switching nodes. Since computer communicate using digitized signals instead of electric signals, it requires different transmission facilities than those used for transmitting analogue electrical signals. It is, however, a common practice to use conventional telecommunication links for data transmission by converting digital signals into analogue signals using a process called “Modulation” and the device that performs the conversion is called the MODEM (MOdulator – DEModulator).
The current trends are towards the development of an Integrated Service Digital Network (ISDN), which can handle all telecommunications requirements (i.e. voice, data, telemetry, facsimile and video) without discrimi-nation.
Data Communication is an integral part of the modern information storage and retrieval systems in terms of their online access. In the initial stage, the information networks operated in an off-line mode, where in, a query was loaded into a computer which was later matched with the database for relevant bibliographic records.
The search results consisting of such records on the subject of query was generated as an output. This process was not satisfactory for effective and efficient retrieval of relevant records. Further progress in computer and communication technology has made it possible to carry out this process in an online interactive mode wherein a user can access an online host via a micro computer from a remote location and can define and redefine his query based on the search results obtained till he is fully satisfied with the final outcome.
The use of communication technology for information retrieval activities gained momentum with the creation of several large databases made available online for shared use through vendors like DIALOG, STN, Datastar, etc. By mid-1970’s, several organization had started offering online searches from remote terminals on a variety of online databases. The article, before describing data communication and networks and their use in the libraries and information centres, touches upon their basic concepts, and the types of hardware involved which are considered necessary for users to know.
A GENERAL MODEL OF THE DATA COMMUNICATION PROCESS
A simple data communications system typically links input/output devices at remote locations with one or more central processors. Interface elements such as MODEMS and front-end processors are used to accommodate different data communication environments. MODEMs are used to permit the system to switch back and forth from digital data to analog signals that can be transmitted over traditional voice telecommunication lines.
A front-end processor is a computer used to monitor and control the data transmission channels and the data being transmitted. The entire data communications activity is controlled by program instructions stored in communications processors and central processors.
DATA COMMUNICATIONS – BASIC CONCEPTS
The term “data communication” is used to describe the transmission of computerized records having structured format which are generally unintelligible in the transmitted form unlike messages and word processing communications which consist of unformatted text. Basic concepts, procedures and standards that are associated with high-speed data communication are given below:
The unit for measuring the speed of data transmission is known as baud. It refers to the number of signal elements transmitted each second. Common speeds available to terminal users accessing remote systems are 300 and 1200 bauds. Faster transmission speeds of 2400, 4800, 9600 and even 24,000 and 96,000 bauds are also available. The speed of transmission depends upon the band-width of the channel being used to transmit the data, the bandwidth is measured in Hertz or cycle per second. The number of bits transmitted per second may not be same as a baud for this very reason.
Serials vs. Parallel Data Transmission
Within a computer, it is usual to employ parallel data paths which transmit 8, 16, and 32 bits simultaneously. Parallel data buses are employed to achieve much higher transmission speeds where the cost of additional wires or tracks on circuit boards is not significant. While the parallel interfaces allocate dedicated functions to the wires, the serial lines have to carry data and accompanying infor-mation multiplexed in a bit-by-bit form according to a communication protocol. It is common to employ parallel data transfer buses between computers and printers (18 pins, 24 pins, etc.) and disk drives to achieve higher speed and accuracy. Serial transmission is preferable for all long-distance data transmission as the cost of an interface for a long parallel transmission cable becomes prohibitive.
Data transmission occurs in one of three modes:
� Half Duplex (HDX)
� Full Duplex (FDX).
The choice for selection of transmission mode rests upon host computer system being accessed. Simplex transmission is suitable only for device such as printers which never transmit information. In full duplex mode, data can be transmitted in both directions along the telecommunication channels simultaneously. In full duplex mode a key pressed on the keyboard results in a series of bits which is transmitted down the channels to the host computer and then ‘echoed back’ for display on the screen or console which, in turn, serves as a check on the character actually received by the computer. In half-duplex mode data can be transmitted along the channels in one direction at a time; consequently the terminal displays characters that were transmitted. Thus the appearance of characters on the screen has no assurance that the host computer has received the transmitted data correctly.
Information may be sent on a line in one of the two modes – asynchronous or synchronous. Asynchronous or start-stop transmission is simpler and is used usually when terminals access remote computer systems. In this case a start code is prefixed and a stop code is suffixed to each character being transmitted. Thus an ASCII character of 8 bits is transmitted as a string of 10 or 11 bits; so a speed of transmission of 300 baud is roughly equivalent to 30 characters per second (cps).
In Synchronous transmission, data octets are transmitted in a continuous sequence without start and stop pulses. Each set of synchronous character in a continuous starting of bits which are delimited by using a number of synchronization characters at the beginning of the block and by counting at groups of 8 bits octets after the final synchronous character.
There are various ways by which terminals and computer systems can be connected to other computer systems. Dial-up access employs a telephone line for dialing up the host computer system directly. Although cost effective, this method can not be used for fast and accurate transmission of data. A leased line may be used to transmit large amount of data at a very high speed. A data net-work, is a practical alternate for remote access. The charges for using such a network are usually dependent on the amount of data transmitted and not on the distance.
Computers connected to the transmission lines may establish a path by either circuit switching, packet switching or message switching. Circuit switching connects the two machines via a line and this line is used exclusively by the two machines as long as they communicate. In packet switching, blocks of messages to be transmitted between machines are formed into a packet with sources and destination addresses synchronizing, error detection and control bits and placed on the channel. Packets are routed using the address information. In message switching, all packets are sent to a central computer by all other machines. The central computer, stores and forwards the messages to the appropriate destination addresses. It is more economical to use packet switching for data communication.
Data transmission protocols are sets of commonly agreed rules that are followed to interconnect and communicate between computers in a network. A protocol defines the communication procedures and encoding used to interconnect the systems. A number of such protocols are now available and are in vogue. A universally used standard method of interconnecting user terminals to computers is RS 232-C (proposed by Electronic Industries Association, USA). The RS 232-C interface consists of 25 connections, voltage levels, signal transmission rates, timing information and control information. In 1976, CCITT (The International Telegraphy and Telephonic Consultative Committee) also introduced the X.25 standard for the interface between terminal and host computer in a packet switched data network. CCITT introduced another standard X.75 in 1978, for the interlinking of packet-switched data network.
International Standard Organization (ISO) has suggested a layered approach where each layer addresses itself to one aspect of the communication problem. This approach allows each layer to be independently developed. CCITT X.25 standard defines the first three layers of ISO. This standard has been integrated in the network architecture of many vendors. A few important standards and specifications available for various aspects of data transmissions are given below (McGoven, 1989, p. 81):
� Physical Transmission Links: RS-232-C, RS-422, RS-423 & X.21
� Data Link Control (DLC): Async, Bisync, SDLC , ISO HDLC
� Communication Path Control: X.25 Packet Switching Procedures
� System and User Control: IBM SNA, DEC DNA, ISO OSI Standards
Network topologies address issues like number of network nodes, concentration of terminals and devices to various locations and how effectively they could be interconnected. Different patterns of interconnections amongst computers in a network are known as network topologies. Computers in different locations may be interconnected in a Mesh Network having multiple message paths between nodes; Star Network having dedicated channels between each station and the control hub. All communication between stations must pass through the hub; Bus Network having a linear topology and station attached by tabs; Tree Network which are bus networks consisting of a series of branches converging indirectly to a control point and offering only one transmission path between any two stations; and Ring Network in which each node is connected to its two adjacent nodes and messages are circulated around the closed ring.
A loop network is a ring network in which one master station control transmission. Each of these topologies have their advantages and disadvantages. The local requirements and computer configurations involved may be considered while choosing a specific topology (Stallings, 1984).
Communication media are the physical channels through which information is transmitted between computers in a network. Media may be classified as bounded (i.e., wires, cables and op-tical fibres) or unbounded (i.e., ether or airwaves through which radio, Transmitted data can move along simplex, half-duplex or full- duplex lines depending on the needs and protocols involved.
Twisted – pair of wires are inexpensive media used in voice grade telephone lines. They are used for low speed transmission of signals of the order of 1200 bps. Coaxial cables can be used for high-speed data transmission over distance of several kms. Coaxial cables have wide bandwidth of the order of 400 MHz. They may be used in LAN at transmission rates of about 1 Mega bps. Fibre Optics cables are glass fibres that provide high quality transmission of signals at very high speeds of nearly 1000 Megabits per second (Mbps) for distances up to 25 miles.
Radio wave in the very high frequency band (VHF) at about 300 MHz is used for communication between computers in inaccessible locations or for short-range communications. Micro-waves are used for wide bandwidth line-of-sight communication. Rates of transmission up to 20 Giga bps is possible with this media. Communication satellite acts as microwave relay station is the sky. Transponders on the satellite are used to receive, amplify and retransmit signals sent from an earth station. The main advantage of satellite is its wide coverage of a large area and thus it may be used from inaccessible location. A transponder has a very large capacity and can handle about 400 channels, each channel having 64 kbps speed.
A MODEM is a MOdulation-DEModulation device that converts the discrete stream of digital “on-off” electric pulses used by computers into the analog wave patterns used for transmission of the human voice. Demodulator (recovers) the digital data from the transmitted signal. A special type of MODEM called an acoustic coupler is often used in libraries and information units with portable machines, but internal and external direct-connect MODEMs are generally used at permanent stations.
A multiplexer is a device used for transmission of several messages over a single channel using predetermined frequencies within the full bandwidth. The multiplexer operates on the principle that individual channel may require only a small account of actual transmission time; thus the multiplexer acts almost as a timeshare computer allocating use of the single communication lines on a priority basis. A multiplexer is capable of accepting as many as 45 separate channels for transmitting data on a single communication line.
Concentrators (Data Switch)
A concentrator is a switch that allocates a particular input to a particular output. While a multiplexer combines the inputs on a high-speed line; the concentrator allocates a particular input to the line for the duration of its information transfer. The allocation of the output line to a specific input depends on when the request is made, the speed of the input line, and the class or importance of the request.
Front-end processors are installed to handle communication related functions for a mainframe computer so that the later can be fully used for processing applications, be it an inquiry to a database, a printing job or updating a master file, etc… The aim of the front-end processor is to provide an inter-face between the mainframe computer and the network so that the data is passed to and from the mainframe efficiently. The front-end processor is responsible for supervision of the input/output controllers or channels attached to the network, for providing buffering and partial processing of in-coming and outgoing data, for the assembling and dissembling of messages and for error handling.