Here is a term paper on the ‘Network System’ especially written for school and college students.
Communication in the real sense means exchange of information, ideas, thoughts, etc., between two persons through the medium of special codes called languages; although non-human species do also communicate among themselves. The problem of human communication acquires a new dimension when it involves two persons who are physically far apart, not being able to talk to each other directly.
In the early days, drum-beating, smoke- signalling and even whistling have been used to bridge the communication gap in the long-distance communication; where two persons are not within normal hearing distance. With the discovery of telegraph, interactive communication using codes became possible.
The final breakthrough came in 1876 when Alexander Ghraham Bell established the system of voice transmission using electric signals — opening a new branch of science and technology called Tele-communication.
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On the other hand, with the development of computer systems, a need was felt to inter-connect different computer systems with each other for various reasons, such as:
1. To provide better service to customers by pooling the resources available at different centres to a center convenient to the customer. The Computerized Railway Reservation Service is a classic example. For example, you can buy a ticket for travelling from Puri to Calcutta from the centres situated in your locality; which earlier could only be done, directly, from the counter at Puri only.
2. Controlling geographically dispersed units/branches of a business organisation for better coordinated operation. Earlier, when the different offices, located in different regions, could be connected by telephone or fax, now these can be connected directly from computer to computer data exchange or communication vial Electronic Mail system.
3. Sharing specialised knowledge like the library network linking different libraries. By sitting at a computer, which has the connection facility, one can know what books or magazines are available at which library, dispersed all over the world.
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4. Sharing costly equipment like say laser-printer and making better utilisation. There are generally more than one computer in an office and each requiring printing facilities. Instead of having one printer per computer, one printer through network system can be used by all.
5. Better utilization of costly computer set up by providing many inter-active terminals to different users. A computer with large processing power may remain partly utilized because only one person can work in it. When interconnected, a number of persons can work simultaneously and utilize the full processing power.
6. Inter-active communication across computers with the facility of Electronic Mail [E-mail], Bulletin Board Service [BBS], etc. With the internet coming into existence, it is now extremely simple to communicate across the globe and obtain or give information world wide, by sitting at home, any where in the world.
7. Acquiring distant computing facility instead of buying a large computer, through Remote Time Sharing terminal. For example, MECON in Bangalore uses the DEC 1090 Computer System installed at Indian Institute of Science at Bangalore. The facility of Remote Job Entry becomes possible, like, Indian Space Research Organisation being connected to the same DEC 1090 of IIS, Bangalore.
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Incidentally, in case of computers, even sitting next to each other, interconnection is necessary to share the data and other services between them. It is interesting to note that earlier the technological development in telecommunication and computer systems proceeded parallely following their own individual paths.
But, in 1970s, it was realized that joint collaboration between the two disciplines would benefit each other and that has actually happened, with communication through wires and non-wires, and between computers, attaining newer horizons almost every year. This joint movement became most useful after the microcomputers came into use with their superior capabilities at lower cost.
Digital vs. Analog Transmission:
The electric signals used for transmitting data or verbal-communication can be of two types, Analog or Digital. In analog-transmission, electro-magnetic waves are transmitted, where as in digital-transmission electrical pulses are used as signals, which are in binary form.
The electromagnetic waves are like sound waves having electro-magnetic properties. The transmission rate in analog transmission is expressed in frequency, which is measured by Hertz, where 1 Hz stands for 1 cycle per second; its capacity being specified by its bandwidth. The digital signals comprise combination of zeroes and ones, called binary numbers. The capacity of a digital transmission is measured by the number of bits transmitted per second or bps.
Communication Channels:
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The fundamental requirement to ensure communication between two or more computers, whether for data transmission [movement of data from one place to another] or exchange of message, is to inter-connect them providing a path from one to the other and vice versa, and this path is called a channel.
The transmission or communication channel is a path which carries electric signals generated by one computer to another in such a manner that the latter can receive, decode, and use it. The channel may be a simple wire or may be a non-wire path through the atmosphere, or satellite linkage. The former is called bounded media and the latter is termed unbounded media.
The channels generally used are:
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These are the wires commonly used in the local telephone system. Two wires forming a pair are twisted together at 6 turns per inch to eliminate electro-magnetic interference, which contribute to noise during communication. It is the cheapest channel and can be used over short-distance without loss of signal level.
Generally, a group of wires is bunched together to form a cable. In twisted-pair wires, the bandwidth is around 4 Khz for telephone lines, though upto 50 Khz bandwidth rate is possible. For digital transmission the lowest rate is 9600 bps for less than 100 feet distance.
2. Co-Axial Cable:
It consists of a single metal conductor covered alround by a layer of insulating material, which is again covered with a fine wire net called screen and finally a jacket of protective insulating material is put on it. The screen is connected to the earth to prevent external electro-magnetic interferences to distort the signals.
It is used for both digital and analog transmission. It can be used over fairly long distance. It is much superior but at the same time costlier than the twisted-pair cables. The bandwidth available with co-axial cables is 300 MHz for analog transmission and 10 Mbps [Mega-bits per second] for digital pulses. Co-axial cables are used for inter-city trunk telephones at 300 MHz [STD].
3. Microwave:
Data is also transmitted using the non-wire path with the help of microwaves, which are electro-magnetic waves with wavelength [and frequency] varying from 1 mm [300,000 MHz] to 10 cm [3000 MHz]. At this frequency range, the waves which goes to the sky are not reflected back by the ionosphere and those which travel along the ground, called ground-waves become ineffective after a few feet.
The space-waves which travel though the atmosphere at a level immediately above the ground behave like light waves — travels at straight lines, reflected by polished surface, etc. These are extensively used in video transmission as well as telephone system, in which a broad range of frequencies called UHF (ultra high frequency 300 Mhz – 3000 MHz) and VHF (very high frequency 30 – 300 MHz) are used; coming in the range called infra-red. It allows a bandwidth of 40 -110 GHz for analog waves and 16 Gbps [Gigabits per second] for digital signals.
4. Radio Wave:
These are also electro-magnetic waves, but in the frequency range of 3 KHz to 30 MHz. The greatest advantage is that these are reflected by the ionosphere and so can be used to cover longer distances without repeater station. Although it is extensively used for audio-transmission, in Europe these are also used for Radio Teletext Services.
5. Optical Fibre:
These hollow cables are formed by using flexible strands of fibre-glass to provide transmission channels in which light beams are used for transmission of data. These can carry several gigabits of information per second, the distance being unlimited. The fibre-glass itself being insulating material, do not radiate energy, provide electro-magnetic immunity, and safety with low loss of transmitted data’s energy level. These are cables with small diameter and quite lighter.
As a comparative figure, a 0.125 mm diameter optical fibre cable, with overall diameter of 3.5 mm with protective layer, has almost the same capacity as that of 900 pair of twisted-pair wires which will have a diameter of about 800 mm. The possible bandwidth is 3.3 GHz for analog transmission and 4 Giga-bps for digital transmission.
Bandwidth:
Any electro-magnetic wave can be broken down into a number of symmetrical waves called pure tones having different frequencies. In transmission technology, bandwidth indicates the difference between this upper and lower frequencies, that is, the range of frequencies of signals that can be transmitted over the channel using electro-magnetic waves without hardly any loss in power-level.
Higher the bandwidth, the greater the volume of data that can be transmitted per unit time. Depending on bandwidth, channels are classified as narrow-band, voice-band, and broad-band types. As the name suggests, narrow-band has a very low bandwidth, in which data can be transmitted at a rate of about 5 to 30 character per second [cps] and these are used in telegraph transmission using morse-codes.
The next higher level, called voice-band are used in normal telephone lines with the frequency range of 300 – 3400 Hz for voice transmission and their capacity is about 1000 cps. The channel with highest bandwidth is called broad-band or wide-band channel, which is being increasingly used even in telephone systems; which has a capacity of about 100,000 cps.
All the three classifications stated above are used in analog transmission, where electromagnetic waves are used as signals. In broad-band transmission, the signal to be sent is integrated with a special high frequency wave called carrier-wave and this is transmitted, the actual wave being isolated at the receiving end from the carrier-wave.
The integration can be done by Amplitude Modulation, Phase Modulation or Frequency Modulation [called FM transmission which is being used in India in radio broadcasting to a limited extent, all the other being AM transmission]. If you switch on a radio long before the transmission starts, often the steady tone generated by the carrier wave can be heard.
There is another way of transmitting signals and this is called baseband transmission, in which either the electro-magnetic wave or digital pulses are transmitted without any change in the original form as generated, but this is possible only over a short distance. For long distance transmission, the digital signals are converted to analog waves and transmitted by carrier waves.
However, long distance digital transmission are now being developed, under the system called ISDN or Integrated Service Digital Network. In order to use digital networks for voice transmission, which is analog wave, the analog signals are converted to digital pulses by what is called Pulse Coded Modulation [PCM].
In digital data transmission, the speed of transmission, the data-transfer rate, is expressed in number of bits per second, which is called bit-rate. There is another unit for denoting the speed of transmission, called baud-rate, which denotes the number of times the electro-magnetic state of a data transmission line changes per second.
For example, if the magnitude of the signal value changes from positive to negative once in a second, the Baud rate will be one. If the bit-rate of each signal element is equivalent to 1 bit of information, then both Baud rate and Bit-rate are equal, otherwise not. The relation between the two depends on the coding method used. Quite often, by mistake, these are described as same.
As you know, a group of bits are used together to enable different codes to be developed to represent various characters and generally, 8-bits are used together; which is called a byte. Suppose the codes for capital A is 01000001. Now these 8 bits can be sent in a transmission channel in two ways. We can have 8 wires to transmit the 8 bits simultaneously and so the receiver will know that “A” has been transmitted.
On the other hand, we can use only one wire and send the bits one after another, starting with the MSB [Most Significant Bit] and the receiver will know by the sequence of bits received that “A” has been sent. This method is called Serial Transmission, with the earlier one being called Parallel Transmission, where all bits forming a character move in parallel.
Leaving aside other factors, parallel transmission is much costlier because of the large number of wires required. All long distance transmissions are serial transmission — parallel transmission being used for short distances, such as from the computer to the parallel printer attached to it.
Modulation:
Since the computers we use are digital computers, the signals generated by them are binary coded pulses, where as, the transmission network which is in existence through telephone lines handle voice-band analog waves. Hence to send data from one computer to another with broadband transmission, these digital signals have to be converted to appropriate electro-magnetic waves.
Along with digital to analog conversion, the original signal [baseband] is mixed with a carrier frequency by Amplitude, Phase, or Frequency Modulation. This is done by a device called modulator. At the receiving end, the analog signal is again converted back to digital pulses and separated from the carrier wave by using a demodulator. The combined device of modulator-demodulator is called a modem in short.
Transmission Mode:
When the data is transmitted from the sender to a receiver, there has to be definite ways for the receiver to know what it has received — there are two ways by which the receiver can properly decode the data received; this is called transmission mode. In one mode, the clock timing signals of the sender are received and perfectly synchronised by the receiver’s system and then the data flows from sender to receiver in regular and correct manner.
This method can be used for both serial and parallel transmission. In serial transmission the bits are synchronised by the protocols defined by CCITT [Consultative Committee on International Telephone and Telegraph] in X.25 series and in parallel transmission the bytes are synchronised by the protocol like BSC [Binary Synchronous Communication] X3.28 series developed by ANSI [American National Standard Institute].
This mode of transmission is called Synchronous Transmission. Under this mode, each block of code transmitted is preceded by a number of special signals called synchronisation signals. The transfer rate is very high, but storage buffers are required in the channels to receive the data transmitted. It does not call for any additional characters to be sent to identify the data.
In Asynchronous Transmission, the serial for bits and the parallel for bytes, the bits or bytes are sent as individual units, their being no fixed interval or synchronous time. To identify characters, each of them is provided with a “start” and “stop” bit, which indicates the beginning and end of the code sent; a parity bit is also attached — in other words, each code carries its own synchronising signals. The signal is sent in irregular time intervals.
In Asynchronous transmission, additional number of bits over arid above the actual data bits have to be sent for identification and this is called “overhead”. So, it is not suitable for transmission of large data at regular intervals. It is generally used for small volume of data because of its simplicity, the complex process of synchronisation being absent.
Another advantage of asynchronous transmission is that it does not require local storage at terminals — it is transmitted as soon as the signal is generated. The transmission from a mouse or to a printer are all asynchronous transmissions. Because of the overhead, to compute the data transfer rate in characters per second, the bits per second rate is to be divided by the number of bits used to form a character, which includes the overhead bits.
Circuit Elements:
Since communication involves two sources, the ideal circuit condition is when both can simultaneously communicate to each other, like we do in our local telephones. When one person is saying something in the mouth-piece, his ear-piece of the set is in attendance to receive what the other person would say. This is called full-duplex circuit — providing a simultaneous two-way flow of signals.
The other alternative could be, when a single circuit is used, both being not able to talk to each other simultaneously — only one can talk at a time with the other listening. This is called a half-duplex system which is generally followed in International Subscriber Trunk Dialing System [ISTD], etc., — the signal flows in both direction but only one at a time. There is a third choice, when one can only talk and cannot listen — the signal flows in one direction only. A classic example would be the loud-speakers blaring at us. This is called a simplex circuit.
Communications Systems:
In a communication system, for data transmission, either you can have a number of terminals which are connected to a computer or a system where several computers are interconnected; in some cases these computers having their own terminals. The simplest method of connection between a terminal and a computer is by using a direct link between the two for point-to-point communication.
But since the cost of the communication links are quite high, a number of terminals are usually connected to a single line, the computer being at the other end. But in such a case, only one terminal can be in communication with the computer at a time using the link, each being identified by a unique address.
To ensure that only one terminal communicates at a time, a system called polling is used. The system of having more than one terminal connected to a line is called multi-point or multi-drop system. Often multiplexing is used with multi-drop system to achieve economy and ease in communication.
Computer Networks:
In network terminology, the points of inter-connection between links are called nodes, where a number of links denoting different routes converge. The node can be a computer system or a mere junction point, a sort of exchange where different links meet. At a node, a link containing an incoming message is connected to any one of a number of outgoing links; but only one at a time.
This connection of temporary need-based nature is called switching and there are two types of switching systems, called circuit switching and packets switching. In circuit switching, a direct connection is established between the incoming and outgoing link, allowing the message to flow as it is.
For example, in our ordinary local telephone system if you are making a phone call from a number to another number, the link is made directly by circuit switching and whatever communication takes place move from one end to another directly.
In packet-switching system, the message is divided into a number of small packets, which form the physical unit of transmission, the total message being a logical unit of transmission. Each packet is transmitted from one node to another whenever the required link is free without establishing actual physical connection between the sender and the receiver. The packets are stored at the nodes till these are transmitted, minimizing transmission costs.
If each node, comprising computer systems, are all connected to each other, then for each n nodes there will be (n-1) connections, the total number of links being n(n-1). However, since connection between say Node 1 and Node 2 is same as that between Node 2 and Node 1, the total number of inter-connections required would be [n(n-1)]/2 links for full interconnection. To eliminate the need for so many costly links, switching system is used.
Multiplexing:
From economic considerations, a large number of low-speed signals can be combined together and then sent across a high-speed transmission channel — at the receiving end the individual signals being again seggregated. This process of combining different signals is called mutilplexing, as multiplex means many- to-one. Its reverse is demultiplexing. Three basic methods of multiplexing used are called Space Division Multiplexing [SDM], Frequency Division Multiplexing [FDM], and Time Division Multiplexing [TDM].
In Space Division Multiplexing, the individual channels are grouped together to form one channel with a high bandwidth. The twisted-pair wires, co-axial cables, or even fibre-optic cables are bunched together to form a cable of large diameter, carrying a number of different signals. This is extensively used in local telephone systems.
In Frequency Division Multiplexing, a higher bandwidth channel is divided into a number of smaller bandwidth channels. The individual signals are sent simultaneously using different frequencies for the carrier wave, as is done in TV transmission.
In Time Division Multiplexing, the low-speed channels are given a small time in turn to use the main bandwidth channel completely — it is almost like time- sharing of computer systems.
Polling:
In a multi-drop system, which contains a number of terminals connected to a computer, the technique of finding out which terminal is going to transmit data is called polling. In one method, called roll-call polling, the computer asks each terminal in turn whether it wants to transmit or not and acts accordingly.
In the other method, called hub-polling, the computer sends a “poll-train” to a terminal, which then forwards it to the next. The terminal which has received the poll-train can transmit data to the computer.
Communication Protocol:
The rules, regulations, procedures relating to the functioning of a communication system using a network is called Communication Protocol. It is an attempt to ensure that equipments of different manufacturers of different countries can be used without any problem for communication; making them compatible to each other.
The protocol generally provides for:
1. How to establish connection and end it during data transmission, called a session.
2. How the messages are to be framed for onward transmission. Well, when we send a simple letter, we use envelopes to secure it, and so is done with the data transmitted.
3. How errors are detected and what steps are taken to correct them by re-transmission, but without duplication, etc.
4. How to find out which message has been sent by whom and for whom.
5. How the transmission continues during a-session.
Network Protocols:
It is absolutely essential for various devices including computers used in network operations to be compatible to each other, so that effective communication can take place between different systems. With microcomputers manufactured by different organisations, using components of different make, the system of compatibility prevails forming a single class of IBM PC Compatibles, where all MS DOS based application programs can be used without modifications.
Similar attempts have also been made in the area of communication. The initial efforts prior to 1985 to have compatibility only at hardware level for networked system was not successful and now the total system compatibility is based on the combination of appropriate hardware and software; almost similarly as ROM BIOS ensures compatibility between IBM PC Compatibles even with different hardware.
In order to make interconnection of different communication and computer systems compatible to each other, the International Standards Organisation [ISO] has developed network protocols which is called Open System Interconnection [OSI].
Under these recommendations, the total communication system including hardware and software have been classified into seven different interdependent layers, each layer having a distinct role to play and providing the levels at which compatibility between different systems can be achieved. It is called a layered protocol.
Each layer is dependent on the adjacent layers only and communicate with it. From the user’s application point of view, the topmost layer is called the Application Layer and the bottom-most layer is culled the Physical Layer.
The layers, starting with the bottom most are:
This is the layer at which direct physical connection between different nodes take place through links — between the source and destination which are the two extreme nodes, there can be many nodes forming the transmission channel.
It defines the mechanical and electrical characteristics of the physical equipments like voltage, pin-connections and the media used for transmission. The recommendations provide for serial and parallel transmission, using half-duplex and full-duplex lines for point-to-point and multi-point configurations. In this layer bits of strings are transmitted from one node to the other.
This layer does coding of data at transmission end and its decoding at receiving end. Here the messages are converted to frames or blocks of data, which are provided with address of source and destination and error-checking procedures; the layer defining the access control mechanism.
It is responsible for error-free transmission, for which it may retransmit a frame, also ensuring that duplication does no takes place. It controls the speed of transmission to ensure that the receiver gets the frames at the right speed even though the sender sends it at higher or lower speeds.
3. Network Layer:
It defines the switching mechanism which is usually packet-switching and also defines selection of the best route for transmission. Here the frames are converted into packets, which are actually transmitted over the network. It controls movement of data from one node to another along the way from source to destination and vice versa and ensures verification of the receipts of the packets sent to the destination.
It selects the route along which the packets are to be sent, without congesting the line. It also ensures that packets are sent in proper sequence. When the packets are send via different routes, it becomes quite complex to ensure that at destination these are available in sequence.
This layer, which is the first layer of the Network Operating System, provides how connection between source and destination will be made and broken, giving an impression that actually the transmitter and the receiver are directly connected to each other — called virtual connection.
Suppose you want to access a particular data file from your computer, which is stored in the main computer, then when you use the necessary commands the file becomes available to you. You would feel that you are directly working with the file, but physically it would not be so and this is where the transport layer comes into play. It provides internetwork routes.
5. Session Layer:
This is the second layer of the Network Operating System, which manages the resources allowing interactive communication. It ensures that you have proper authority to get the data file which you have requested for and then establishes the set up so that two parties can communicate with each other.
It decides the nature of inter-connection like whether it will be half-duplex or full-duplex. If a link breaks down during a session it arranges for alternate transmission route, ensuring that session continues, till it is broken down by the persons using it. At this layer, the applications can use names to communicate with devices.
It is quite possible that the two computers inter-connected, are of different types using different machine codes. The function of this layer, which comes under software, is to ensure compatibility between different systems between source and destination. The difference between two interconnected system may be relating to printer configuration, display adapters, file-format, etc. It also carries out code to code conversions.
It is the layer where dialogue between the sender and receiver takes place after connection has been made. It comprises the various utilities used for various operations under networked conditions.
The Data Link Layer [Layer 2] and the Physical Layer [Layer 1] specify the hardware requirements. The topology used for networking and the speed of data transfer are determined in these layers. There are three general protocols recommended by IEEE [Institute of Electrical and Electronic Engineers] for hardware levels which are Ethernet [802.3], Token-bus [802.4], and Token-ring [802.5]; forming the 802 series of classifications.
The Network Layer, Transport Layer, and the Session Layer, which are provided by network software, work in unison to form what is called the subnet level, which control the hardware. They establish and maintain the virtual connection, which is a temporary link, between the source and destination.
The point-to-point communication services are generally interfaced at the Session Layer, where as, all application programs of network are interfaced at the Presentation Layer. The Application Layer is where all program execution takes place. In 1985, when the IBM PC Network was developed, it called the subnet part of the software NETBIOS, a short form for Network BIOS and this has virtually become the industry standard for Local Area Networks.
Let us try to understand the roles played by each layer by taking up a model of interconnecting two persons through international courier service. Suppose an Indian Manager wants to communicate with a Japanese Manager for the purchase of an equipment. Whatever the Indian Manager wants to know is the business of the Application Layer, where the message begins.
Now a translator translates the letter in English to Japanese — it is done at Presentation Layer and the letter is made ready. Then the Secretary of the Indian Manager, puts the letter in an envelope, writing the address of the destination and source [Session Layer]. The despatch section, enters the details of the envelope in a register and gives it to a courier, who may have many, letters going to Japan and other different destinations and so decides the route along which this envelop will go [Network layer].
The envelope is put in the courier-bag for Singapore on way to Japan [Physical Layer]. At the next destination, that is at Singapore [an intermediate node], the courier-bag is unloaded [Physical Layer], receipt of the envelope in proper condition is confirmed to the courier’s office at India [Data Link Layer], put in another courier-bag for moving it from Singapore to Japan [Network Layer], hands over the courier-bag to the person concerned [Data Link Layer], and the courier-bag is put on the aircraft and moves to Japan [Physical Layer].
At Japan the mailbag is unloaded [Physical Layer], the envelope is checked and found alright and a confirmation is sent to Singapore. It is delivered to the proper office, who sends a confirmation to India [Transport layer] and enters it in a Receipt register [Session layer]. The envelope is opened and the letter is taken out [Presentation Layer] and then given to the Manager who reads it [Application layer] and understands what is required.
This is a very simplified example, but basically such sort of activities take place, and almost instantly through communication links. When you make an International Subscriber Trunk Dialing call to USA from India, almost similar activities take place along the route, ultimately allowing you to talk in half-duplex mode.
Local Area Network (LAN):
In an effective information system it is necessary to ensure fast and flexible movement of data in an economical manner with low error-rate between independent computing units which are interconnected to form a network. When the area of such networking is physically restricted within the business premises of an organisation, it is called a Local Area Network or LAN.
The unique characteristic which identifies a LAN is that the transmitted signal, which is digital, comes within the category of baseband transmission, that is, the signal generated is transmitted in its original form without any alteration and at any moment of time, only one signal can flow in the network. The network service is controlled through time-sharing or time-division multiplexing.
For necessary operation, a LAN has to provide for the following specifications:
1. A medium through which the data flows — twisted-pair wire, coaxial cable, etc.
2. Network Interface Card [NIC] which connect the computer and the communicating medium.
3. A topology, defining the configuration of physically inter-connecting the stand-alone machines — how these are connected to form the net-, work system as a whole.
4. An access protocol which specifies how the individual units would use the network for transmission.
5. A set of programs, collectively called Network Operating System to ensure error-free operation of the system with full security and protection.
Naturally, a LAN has to be reliable, serviceable, and available to its members when required. Under LAN, the computers inter-connected retain and use their own computing power, resulting in effective distributed processing.
The reasons or advantages for using LAN are:
1. Security:
Extensive security is possible under the LAN setup relating to use of programs, data, etc., by different users.
2. Distributed Processing:
By maintaining the computing ability of the individual computers connected to the network system, the processing can also be done in these units in a dispersed manner.
4. Inter-Unit Communication:
It becomes possible for each user to interact with other with messages through Electronic Mail or E-mail.
5. Reduced Cost:
Cost can be minimized in hardware, as well as software used by sharing them among all users.
6. Flexibility:
The network system can be expanded to cover additional units or it may even be inter-connected with other networks.
In a LAN system, a particular computer with higher capacity and speed is generally treated as a File Server where the files are stored and which manages the files, handles communication, allocates resources, controls operations, etc., — it has the network operating system installed in it.
The other computing units are called Work Stations, with their own operating system of MS DOS or OS/2, where the processing are carried out. Incidentally, LAN forms peer-level network, where all the computers in it can communicate with each other. This is unlike the terminals of the mainframe or minicomputer, which can only communicate with the host and not among themselves.
Network Topology:
The physical method of inter-connecting the individual computer units is called the topology and it is one of the way of classifying LANs. Topology indicates the routes along which the data moves from one unit to the other.
The different topologies available are:
Under this system, all the computers are connected in parallel to each other thorough a common connecting cable called bus/which is open at the two ends, as shown in Figure 1. One of the network system using this topology is known as Ethernet system.
It is identical to the bus topology except that the two ends of the bus are connected to form a closed path or a ring of network as shown in Figure 2. The Cambridge Ring uses this topology.
3. Star Network:
Under this topology, all the computers are connected to a central hub which control switching, communication, and other functions. Its configuration is shown in Figure 3.
Access Control:
Broadly classified, there are two basic types of Access Control, called Token and Contention Systems. In the token system, an access-granting signal called token is sent to each Work Station in turn, which can transmit only when it is in possession of the token.
In the contention system, each Work Station listens to find out whether the network is being used by any other computing unit. If no one is transmitting, it starts transmission, else waits. Taking into account the access systems and different topologies, there are three different types of LANs, which have been defined by IEEE under category 802.
Ethernet [IEEE 802.3]:
The access control system used in the Ethernet system is called CSMA/CD [Carrier Sense Multiple Access / Collision Detection], which belongs to the contention scheme and it uses bus topology. The individual computing units monitor the network and transmits when no one else is transmitting. In case of a collision, both stops transmitting and then retransmits after a random time.
Token Bus [IEEE 802.4]:
This system also uses bus topology, but the access control system is based on explicit access-right or token, which is passed in sequence to all the units one by one. A unit can transmit only when the token is with it.
Token Ring [IEEE 802.5]:
This type uses the token system with ring topology. The token goes round the ring with each unit receiving and retransmitting it, if no data is to be sent. Transmits when the token is with it.
EPABX Network:
This acronym stands for Electronic Private Automatic Branch Exchange, which earlier used to be called PBX or Private Branch Exchange and is installed at the business premises. Such systems are extensively used to extend the telephone system in an office for internal use with limited external lines.
Previously these exchanges used to be manually operated connecting different lines which dealt exclusively with voice systems in analog form. With innovations over the years, now such exchanges use automatic digital switching system and deals with both data and voice.
In case of voice communication, the analog signal representing voice are converted to digital signals using Analog-to-Digital [A/D] Converters, proper connection made as demanded, and then reconverted to the analog signal with Digital-to-Analog [D/A] Converters; providing the final connection between-two telephones.
The combination of A/D and D/A is called CODECS, which is the short form of Coder and Decoder. However, with the latest technology, the analog waves are converted to digital signals by a technique called Pulse Coded Modulation or PCM.
In view of the facility of digital switching available in the EPABX, these are now being used to form the hub of a Local Area Network with star topology, which deals with both voice and data communication. The EPABX network uses circuit-switching system as against packet-switching system used in pure data networks, where the message is converted into packets before transmitting, their being no direct physical link between the sender and the receiver.
ARCnet [Attached Resource Computer Network] is a baseband, token system network using star or linear [bus] topology. It is the cheapest LAN system for short distance operation.
Wide Area Network (WAN):
As the name suggests, WAN interconnects computers which are geographically far apart, and hence the medium used for the system is provided by the traditional Telecommunication Network built up for the telephone system. Such networks use broadband transmission, where a number of signals can be transmitted simultaneously. Modern telecommunication networks can carry different types of analog and digital signals for voice and data.
Under the WAN system, each computer has to have a modem for converting the digital signals to analog waves and then integrating it with carrier waves for transmission — the ultimate connection is from computer to computer. The topology for WAN is described as mesh or tree configuration.
The transmission takes place using different intermediate nodes along different links. With latest technological developments, the telephone network system is being changed to be fully digital under the protocol developed for Integrated Services Digital Network [ISDN].
Wide Area Networks have extensive application in the areas of:
Communications between computing units.
Interaction between computer terminals and the host computer.
Open air audio and video signals.
Close circuit TV for safety, fire control, etc.
Satellite control and weather forecasting, etc.
The advantages are:
High speed communication.
Physical independence – distance is not a factor.
Versatility – can accomodate any expansion/extension.
Transmission of audio, video and data signals simultaneously.
Flexible control system – can be centralised or decentralised.
The protocols used in network systems are developed either by CCITT [Consultative Committee for International Telephone and Telegraph] or by IEEE [Institute of Electrical and Electronic Engineers]. X.25 is a protocol defined by CCITT, which describes a method of routing, a physical connection, and a mechanism for providing links between different locations [nodes].
It provides for the lower three layers of the OSI model, which are Physical Layer, Data Link Layer [HDLC — High Level Data Link Control], and Network Layer. It is for full-duplex synchronous transmission. The X.21 protocol defines the standards of the physical level using RS-232 interface, which is the standard serial port in a Personal Computer.
Some of the devices commonly used with networks are:
Repeater:
It is a device which is used to amplify an analog signal or regenerate a digital signal, as the signals get weakened when transmitted over long distances. It interfaces at physical layer.
Bridge:
It is a device which is used to link different LANs of identical types and is interfaced at data-link layer. Connecting different networks is called internetting.
Gateway:
It is a device which is used to access resources of a mainframe or minicomputer by connecting a LAN to a host. It is also used for inter-netting totally dissimilar networks. It is interfaced at layer 3, that is, the network layer.
Router:
It is a device which provides interconnection at the Transportation Layer [Layer 4], which is for control of end-to-end transmission.
Some of the wide area networks in use in India are:
RABMN [Remote Area Business Message Network]:
Its objective is to provide facility for interconnection between all computers in the country especially those which are in remote areas. It is under the control of Department of Electronics [DOE],
VIKRAM:
It is a packet switched public network of Department of Telecommunication [DOT], with switching nodes at metropolitan cities.
NICNET:
It is to be used for grass-root level data collection at the village levels, under control of National Informatics Centre.
INDONET:
It is a commercial network with computer centres at Bombay, Calcutta, Delhi, Madras, and Hyderabad. Managed by CMC.
CALIBNET:
It is for linking 38 libraries in Calcutta under National Informatics Centre for Science & Technology.
BANKNET:
It is for linking the five Reserve Bank of India offices and about 300 banks of the country.
COALNET:
It is for Coal India Ltd.
SAILNET:
It is for Steel Authority of India Ltd.
SIRNET:
It is for linking forty laboratories of the CSIR.
ERNET:
It is for linking academic and research institutes under control of DOE.
Distributed Data Processing (DDP):
Prior to 1980s, when the concept of networked system was yet to be fully developed, all computer processing jobs used to be done in a centralised manner — the mainframe or minicomputer being the host where the processing used to be done being controlled from a number of dumb terminals, which were not in a position to communicate between themselves.
With the development of network systems, under the platform of MS DOS or OS/2, peer-level communication became possible, that is, each computer within the network being able to communicate with the other, the technique of Distributed Data Processing evolved.
Under this processing system, inter-connected but physically separated databases, positioned in different geographical locations in different computers, which retain their own processing powers, can function as one comprehensive unit as viewed by the end-user — this is being done by making them a part of a network, which uses the national communication network with their own modems.
If two or more computers are inter-connected we have a network system, but, if these inter-connected computers function as a single computer logically, it forms a distributed data processing system. For example, an airline may have its passenger reservation system’s databases physically distributed at Calcutta, Delhi, and Madras with the main system at Bombay.
Now it is quite infrequent for a person at Bombay or Madras asking for a Calcutta-Guwahati ticket. If DDP is not implemented, with the host computer at Bombay with the database, terminals would be required at Calcutta and Guwahati and they would have to regularly communicate with Bombay. But with a DDP system, a computer at Calcutta will have a portion of the database of the Calcutta-Guwahati circuit — the processing being done where the activity is taking place.
Reduction in transmission cost.
Faster processing of transactions.
Better system of control at local centres.
Increased system reliability.
Better protection from the total system being damaged.
As an example, we can refer to the Hewlett-Packard Company of USA, whose DDP system uses 10 mainframe computers, about 1000 minicomputers and about 40,000 Personal Computers, spread over different countries.
Since DDP systems are networked systems, some of the concepts of physical inter-connection has been borrowed from LAN systems. In the hierarchical system, which is an extension of the star topology, a central host computer communicates with and exercises control over a number of satellite [or node] computers at the second level.
A satellite may in turn, act as a host to a third level of sub-ordinate computers or terminals.
Generally, the host is a main computer with the satellites being minicomputers at the second level and Personal Computers at the third level, as shown below:
The lateral system is an extension of ring topology, where each computer in the system has equal status — no single computer acting as a host or main computer.
Each computer generally have Personal Computers or Terminals attached to them, as shown below:
Real-Time System:
A real-time system is a special case of on-line system, or interactive system, where the response time is of critical importance. Once an input is fed in, the output has to be available in quickest possible time. Apart from industrial control systems, the passenger reservation system of airlines, railways, etc., are typical applications of real-time systems.
When the number of terminals of a real-time system is more than one, the concept of time-sharing becomes important. The system operates in such a manner, that each terminal user feels that he has the system at his total disposal; but in reality, the processing, time is allocated to each user in turn, giving a time-slice of the processing time.
More and more real-time systems are coming into use in commercial applications. A package called SABRE is an airline reservation system which is being used by many. A typical example of an airline system is given in Diagram, 1.6.6, where two computers are used to provide back-ups.
Another area of application is in cash-less Electronic Fund Transfer [EFT] systems, one aspect of which is popularly known as Automated Teller Machine [ATM]. In this system a bank customer, after identifying himself with a special coded card, can operate the terminal to make withdrawal or deposit.
Videotex:
It is a generic term used to refer to a system where the contents of a specific database, containing useful information in both text and graphics form, are made available to different non-expert viewers through TV broadcasting, for them to view in their own TVs. The user has to use a special decoder. In the interactive system, the user can query for different information contained in the database. Videotex is sometimes called view data.
In Europe, there is also a Radio Data System, in which encoded digital information is transmitted along with ordinary radio programmes without any mutual interference. The listener has to use a Radio-data decoder along with his radio to listen to the information or if he wishes, he can view it in his TV with a special attachment.
Hypertext:
It is a kind of text, usually provided as help or for information, which contains some special words embedded in it. This special words are usually given in different colour or reverse video. When any of these words are selected by mouse or keyboard, another page comes on the monitor screen containing some text relating to the word selected.
For example, suppose you are looking at a screen where something is written about animals and it contains a word “tiger”. When you select this word tiger, it will open up another page containing details about tigers. Hypertext, are thus, linked pages of texts containing some key words which leads to different linked pages. It is also called linked-texts.
Internet:
Around 1969, the Defence Department of the Government of USA, to ensure better coordination between their research projects, decided to interconnect the computer systems of different laboratories and universities of America and Europe by a network. These establishments contained different varieties of computers and the job was not at all easy.
This network was named as DARPANET — Defence Advanced Research Project Agency Network. Forty institutions with fifty computer installations were thus connected. Later on it was renamed ARPANET. Ultimately, this network has today grown to become an international network called Internet.
In this Internet, documents in the form of text was only available. At the beginning of this decade, a new dimension has been added where by pictures are also available along with text and this is being called World Wide Web. Internet information are arranged in the form of hypertext.
E-Mail:
With the help of network system, a new mailing system has come up which is called Electronic Mail or E-Mail. The service is provided by different organizations, who maintain a computer with large storage capacity. The subscribers are given their unique address, who have a computer with modem.
Anyone wishing to send any message to another, sends it using the telephone system using the modem. If the computer of the receiver is not switched on, the message is stored in the computer of the organiser. When the receiver’s computer is switched on, a message comes informing that he/she has mail, which can be received.
BBS:
A similar kind of facility is used to create another kind of service called Bulletin Board Service or BBS. The service provider maintains different kind of information in his computer and any subscriber can see those information by connecting via a modem. It is the electronic form of standard Notice Boards, where one does not have to go to read the notices.