TOPIC 6.1.: DATA TRANSMISSION

PART 1

COMMUNICATION CHANNELS

A communication channel is the link between two computers. It could be a cable of some kind, a telephone line, a microwave or a communication satellite link.

CABLING SYSTEMS

Type of cabling has a major bearing on a network's speed, performance, cost and practicality (a very thick cable being much harder to lay in or along walls).

Twisted pair, like telephone wire, is the cheapest but has slow transmission rates and suffers from electronic interference.

Coaxial cable is high quality, well insulated cable which can transmit data much faster and more accurately than twisted pair.

There are two types of coaxial cable: -

1. Baseband - carries one signal at a time. A bit value of 1 or 0 is sent by the presence or absence of a voltage in the cable. Baseband signals can travel very fast, but can only be sent over short distances. Over about 1000 feet special booster equipment is needed.

2. Broadband - can carry multiple signals on a fixed carrier wave, with the signals for 0 and 1 sent as variations on this wave. Data, audio and video transmission can take place simultaneously.

Fibre optic cable represents the latest technological development, being very fine cable that can carry several hundred thousand voice communications simultaneously.

SATELLITE TRANSMISSION

Using a satellite dish and a communication satellite, it is possible to send signals over long distances, such as from Britain to America.

Communications satellites are placed about 22,000 miles above the earth is geosynchronous orbit (meaning that they rotate with the earth).

MICROWAVE TRANSMISSION

Microwaves are similar to radio waves, and can be used to transmit data between microwave stations.

Transmission distance between stattions is limited to about 30 miles because of the earth's curvature, since microwaves travel in a stratight line.

Microwave stations are usually located on hilltops or towers.

TRANSMISSION MODES: ASYNCHRONOUS AND SYNCHRONOUS

In asynchronous transmission mode, individual characters are transmitted at irregular intervals - for example, as they are typed by a user.

The bits representing the character are enclosed by start and stop bits, and a parity bit is usually added to the character code to provide a check against incorrect transmission.

A complete character therefore needs ten bits to be transmitted; a start bit, seven bits for the character code, a parity bit and a stop bit.

This mode of transmission is used for lower speed transmission, for example on a local area network consisting of a number of linked PCs.

In synchronous transmission mode, timing signals synchoronise the transmission at the sending and receiving end so there is no need for start and stop bits for each character, only at the beginning and end of the whole block.

This mode of transmission is more suitable for longer transmissions such as remote job entry, but requires more expensive and sophisticated equipment.

SIMPLEX, HALF DUPLEX AND FULL DUPLEX TRANSMISSION

The direction of transmission may be either:

1. Simplex - transmission can only take place in one direction. This type of transmission could be used for example when the sending device such as a temperature sensor never requires a response from the computer.

2. Half Duplex - transmission can take place in both directions but not at the same time, as for example in a citizen's band radio. This type of transmission is often used between a central computer and terminals.

3. Full Duplex - data can be sent in both directions at the same time. Most interactive computer applications use full duplex transmission.

TRANSMISSION RATE

The speed at which data is transmitted is measured in bits per second.

Baseband coaxial cable, for example, can transmit up to 10M bits per second over short distances.

Baud rate is another measure of speed, and is the number of times per second that the signal changes.

At speeds up to 2400 bps, usually one bit is transmitted per signal change so baud rate and bits per second are the same thing, but to achieve higher speeds more than one bit is transmitted per signal change, and so the bps rate will exceed the baud rate.

DETECTING TRANSMISSION ERRORS

Interference or noise in a communication line may cause bits to be wrongly received.

This event can be detected by transmitting a parity bit with the code for each character, and a checksum with each block of data.

DATA COMPRESSION

Data compression is frequently used when transmitting large quantities of data, thereby reducing the nuber of blocks transmitted and thus reducing the cost as well as the probability of transmission errors.

It basically works by replacing repeated bytes by one copy of the byte plus a count of the repetitions.

Graphic images can also be compressed. To compress a black and white image, coded as a two dimensional array of 0s and 1s, the array may simply be replaced with a set of integers representing the length of a series of 0s, then 1s and so on.

Compression ranges from 90% or more down to 5%, depending on the type of file and the amount of repetition of characters or blocks of space, colour etc.

Utility programs such as PKZIP and PKUNZIP for compressing and decompressing files are readily available.

PROTOCOL

In order to allow equipment from different suppliers to be networked, a strict set of rules (protocols) has been devised covering standards for physical connections, cabling, mode of transmission, speed, data format, error detection and correction.

Any equipment which uses the same communication protocol can be connected together.

Manufacturers are gradually incorporating some of these standards in some of their products, but it is unlikely that total standardisation will be achieved for some time, if ever.

Where two devices have different protocols they can sometimes communicate via a 'protocol conversion computer'.

THE OSI SEVEN LAYER MODEL

OSI stands for Open system interconnection and a model for OSI (implying the equipment from any manufacturer can be connected to any other manufacturer's equipment) has been under development since 1977.

A hierarchy of seven layers has been identified as below:

1. APPLICATION LAYER

• This is the highest layer, closest to the user.
• It supports the transfer of information between end-users, applications programs and devices.
• Several types of protocol exist in this layer, covering specific and general applications such as accounting, entry control and user identification.

2. PRESENTATION LAYER

• The aim of protocols in this layer is to ensure that different devices using data in different formats can communicate with each other, for example handling conversion between ASCII and EBCDIC.
• It may also carry out encryption to ensure data security during transmission.

3. SESSION LAYER

• The session layer is the user's interface into the network.
• When a user requests a particular service from the network, the session layer handles the dialogue.

4. TRANSPORT LAYER

• This layer handles the data transmission between host computers, dealing with addressing and error controls to ensure a high quality data transmission service for the network.

5. NETWORK LAYER

• The function of the network layer is to perform the routing of information around the network, connecting adjacent nodes in the network and also carrying out accounting functions to enable the network owner to charge users.

6. DATA LINK LAYER

• The physical data transmission media used in a network are subject to interference which can corrupt data, and this layer handles data transmission errors.
• The techniques used for receipt and acknowledgement of data by a receiver are handled in this layer.

7. PHYSICAL LAYER

• This layer is concerned with standards for the mechanical, electrical and procedural aspects of interface devices; for example, the number of pins a network connector should have.
• It is concerned with how the binary data is transmitted along the communication channel.

GATEWAYS

The term 'wide area network' applies to both national and international networks and there are hundreds of such networks worldwide, with most major networks being interconnected through a system of gateways.

A gateway is a computer which acts as a point of connection between different networks.

PART 2

MANAGING A COMPLEX NETWORK

• The distance along a network that a signal may travel before degrading is limited.
• The higher the transmission rate, the less distance can be traveled.
• Devices help manage these problems, and help with the routing too.
• Repeaters are simple devices to boost signals so they can travel over longer distances.
• Bridges may also be used to boost the signal so that you end up with a longer physical network.
• Intelligent bridges are used to pass packets from one network to the next, or to filter packets so preventing them from using up valuable bandwidth on parts of the networks that do not need to be affected.

LOCAL AREA NETWORK

• A LAN (Local Area Network) is a collection of computers and peripherals connected together by cables, and generally confined to one building or site.
• A LAN offers a means of pooling resources and information between a number of users. A central mini or mainframe computer with a number of 'dumb' terminals attached is one form of LAN, but the term is frequently used to describe a number of linked PCs, often with a more powerful computer controlling the network.
• A local area network has several advantages over a collection of standalone microcomputers:

        - sharing of resources such as disk storage, printers and possibly a large, powerful computer;

        - sharing of information held on disk drives accessible by all users;

        - sharing of software;

        - ability to communicate with other users on the network. This is not important when all the computers are in the same room but can be very useful when they are distributed round a large building.

Components OF A LAN

• All LANs will have at least three basic components:

            1. workstations - PCs or terminals.

            2. file server or disk server - a special PC or larger computer where shared software resources are stored, including the network software which monitors network operation.

            3. cabling and connection hardware - this includes the cables that link the computers together, and a special interface card or printed circuit board which has to be inserted into each computer on the network to give it a unique identity and allow it to interact with other components of the network.

• In addition, LANs may have additional hardware such as printers and extra disk storage. Network printers have to be connected to a computer designated as the print server - this may well be the same computer that is designated as the file server.
• Each workstation on the network may have its won processing power and may have its own disk storage (floppy or hard disk) and its own printer.
• Consideration will have to be given to backup facilities; some LANs have two file servers, with data being copied at regular intervals (say every few minutes) from one file server to the other, so that if one goes down, the other can take over with the loss of only a few minutes' work.
• For making regular backups of files stored on the hard disk, a tape streamer consisting of a compact tape cartridge unit with a high storage capacity may be used.

NETWORK TOPOLOGIES

• The topology of a network is its physical layout - the way in which the computers and the other units are connected.
• There are three basic layouts: star, bus and ring.

STAR NETWORK TOPOLOGY

• Each node in a star network is connected to a central microcomputer which controls the network.
• Network signals travel from the server to the station along each individual station's cable.
• A polling system is commonly used - the file server polls each station in turn to see if it has a signal to send.
• The server then handles signals as they are received.

ADVANTAGES OF A STAR NETWORK

• If one cable fails, the other stations are not affected.
• Consistent performance even when the network is being heavily used.
• Reliable, market-proven system.
• No problems with 'collisions' of data since each station has its own cable to the server.
• Easy to add new stations without disrupting the network.

DISADVANTAGES OF STAR NETWORK

• May be costly to install because of the length of cable required. The cabling can be a substantial part of the overall cost of installing a network.

BUS NETWORK TOPOLOGY

• In a bus network, all the devices share a single cable. Information can be transmitted in either direction from any PC to any other. The problem here is that several stations may want to transmit down the same line simultaneously, and there has to be some strategy for deciding who gets in the line.
• A popular scheme called Ethernet uses a collision system known as 'carrier sense multiple access with collision detection' (CSMA-CD).
• Before a station begins to transmit, it checks that the channel is not busy; if it is, it has to wait before transmission can begin.
• Once it begins transmission, it listens for other nodes also beginning transmission.
• If the transmitted message collides with another, both stations abort and wait a random period of time before trying again.
• This system works well if the channels are not too heavily loaded. On the other hand if sixteen students sit down at sixteen computers all at once and all try to load software from the network's hard disk, the whole system more or less grinds to a halt!

THE ADVANTAGES OF A BUS NETWORK

• easy and inexpensive to install as it requires the least amount of cable;
• easy to add more stations without disrupting the network.

THE DISADVANTAGES OF A BUS NETWORK

• the whole network goes down if the main cable fails at any point.
• cable failure is difficult to isolate.
• network performance degrades under a heavy load.

RING NETWORK TOPOLOGY

• In a ring network, a series of computers is connected together and there is no central controlling computer.
• Each computer may communicate with any other computer in the ring, with messages being specifically addressed to the destination computer.
• Using the 'token ring' system, a 'message token' (actually a unique character sequence) is passed from node to node, and each node has a designated time at which it can remove the token and either add a message together with the addresses of the receiving node and sending node, and some control bits) or take a message from it.
• A receiving device acknowledges the receipt of a message by inverting a 1-bit field.

THE ADVANTAGES OF A RING NETWORK

• there is no dependence on a central computer or file server, and each node controls transmission to and from itself.
• transmission of messages around the ring is relatively simple, with messages traveling in one direction only.
• very high transmission rates are possible.

THE DISADVANTAGES OF A RING NETWORK

• if one node in the ring breaks down, transmission between any of the devices in the ring is disrupted.

WIDE AREA NETWORK

• A wide area network (WAN) is a collection of computers spread over a wide geographical area, possibly spanning several continents.
• Communication may be via microwave or satellite link, or by ordinary telephone lines supplied for example a combination of many different types of communication link between various computers along the route between sender and receiver.
• The use of global networks (including the Internet) has increased enormously over the past few years owing to:

            - changeover of telephone networks from old-style analogue to high-speed digital technology;

            - reduction in cost of connecting to and using networks;

            - improved compression techniques which allow faster transmission of text and graphics;

• We are well on the way to seeing the implementation of the information superhighway, which will use a combination of telephone lines, satellite links, computer networks and multimedia to bring interactive two-way communication to many homes.
• The distinction between a television, a microcomputer and a telephone may become blurred when you can dial up and download videos or TV programs on request, send electronic mail and browse the World Wide Web all from the same piece of equipment.
• A network that is capable of supporting voice, video and computer data is called as an Integrated Service Digital Network (ISDN).
• An ISDN needs high-speed digital connections with a high bandwidth to send video data (e.g. for video conferencing) in real time.

PUBLIC AND LEASED LINES

• A WAN may use either the public telephone lines, dialing up as required, or if communications are very frequent, a leased line.
• This can be permanently connected and is not charged by the minute, but the subscription charges are obviously heavy.

MODEMS

• Telephone lines were originally designed for speech, which is transmitted in analogue or wave form.
• In order for digital data to be sent along a telephone line, it must first be converted into analogue form and then converted back at the other end.
• This is achieved by means of modem (MOdulator DEModulator).
• Modems can transmit data at speed between 300 and 40,000 bits per second. With newer digital lines, a modem is not required.

MULTIPLEXORS

• A multiplexor combines more than one input signal into a single stream of data that can be transmitted over a communication channel. This increases the efficiency of communication and saves on the cost of individual channels.
• At the receiving end, the multiplexor (sometimes called the de-multiplexor) separates the single stream of data into its separate components.
• Often a 'front-end processor' or 'communications processor' is connected between the multiplexor and the main computer in order to handle the communications, leaving the main processor free for other tasks.

MULTIPLEXING

• Multiplexing is sending different signals over the same link.
• There are two main methods of multiplexing: time-division and frequency-division.
• If different times are used it is called time-division multiplexing, or for different frequencies, it is called frequency division multiplexing.
• In time division multiplexing, either the bits or characters being transmitted from several terminals are interleaved during transmission. Terminals are polled in sequence, and each terminal is given a time slot whether or not it has anything to transmit. The multiplexor at the other end separate the bits and passes them to the computer.
• Each signal is given a tiny slice of time, usually very brief so other users, who receive information during their time slices, do not realise that others are using the same link too.
• Frequency division multiplexing transmits signals at exactly the same moment in time, but on different carrier frequencies, as described earlier.
• It is identical to fm radio. Many different stations are sending out signals over the airwaves.
• In frequency division multiplexing, there are several carrier waves each having its own frequency, and each carries one lot of data.
• The multiplexor at the other end can therefore distinguish between each set of data and separate them out.

THE INTERNET

• The Internet is a collection of thousands of computer networks throughout the world.
• The linked networks may be either LANs or WANs, most of them owned by individual organisations.
• Some WANs such as CompuServe act as Service Providers and members of the public or businesses may join these networks in return for a monthly charge.
• Millions of people subscribe to one or other service provider, thereby gaining access to approximately 4 million host computers and a hug amount of information as well as services.
• The entire Times and Sunday Times are available on the World Wide Web, a collection of pages stored on computers throughout the world containing information of every kind, and with hypertext links that jump to other parts of the Web.

ADVANTAGES OF THE INTERNET AS A SOURCE OF INFORMATION

• You can access the Internet from anywhere and a huge amount of information is provided to you from all over the world. The use of laptops and Internet access using televisions and mobile phones has meant that users can also access the Internet in a non-traditional setting.
• Information is up to date. By the time books are written, edited and printed the information they contain can be dated. Information on the Internet can be continually updated. However, you do still need to make sure that any information you use is up to date.
• Multimedia can be used. Information can be presented in the most interesting way possible, using video clips, animation, sound etc.
• You can access huge amounts of information. Encyclopedias, dictionaries, newspapers, magazines and many research papers are available on the Internet. You do not even need to leave your house to access them.
• Search engines are available to help you find the information you are looking for. Searching in traditional books is very slow; using a search engine and the Internet make browsing very easy.
• Using e-mail you can access experts all over the world on certain subjects. If you have a question about something on a website you can send an e-mail to its author and they can send you their reply using e-mail.

DISADVANTAGES OF THE INTERNET AS A SOURCE OF INFORMATION

• The equipment and connection needed are relatively expensive. Computers, modems (or ISDN adapters), telephone and/or Internet service provider (ISP) costs must all be met. The costs, however, are decreasing quite rapidly.
• You need some knowledge to perform searches successfully. If searches are constructed incorrectly, then you will get either no information or too much, most of it irrelevant.
• The negative aspects of the Internet such as:

        - Hacking which means gaining access to a computer system illegally; the people who do this are called hackers.

        - Breaches of copyright (copying games, software, music). Just because material is on public display on the Internet does not mean that you can copy and distribute it. Some of the material is copyright free, but the majority is owned by someone, who is said to own the copyright in the material.

        - Spreading rumour is very easy to be done using the Internet. You only have to tell a few people in a chat room and a rumour will soon spread.

        - Internet addiction is possible to get hooked on using the Internet in the same way that you can get hooked on gambling.

E-MAIL

• An electronic mail system may be used within an organisation using a local area network, or it may be used for communication all over the world via a service provider such as CompuServe.
• Typically the basic services provided by an electronic mail service include:

        - authentication of sender for security purposes;

        - notification to sender if message cannot be delivered;

        - the ability to send the same message to several people;

        - recording of times of dispatch and receipt of messages;

        - message filing and retrieval;

        - automatic accounting and billing of users.

RESOURCE: P.M. Heathcote, [A Level Computing, 3rd Edition], Letts Educational Ltd., 1996.