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7.2: Reading- Networks and Telecommunications

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    Photo of fiber optic cables, orange background.

    One of the fastest growing technology areas is that of telecommunications (often referred to as telecoms). Organizations have realized that stand-alone computers present many problems: fragmentation of data, lack of control, insufficient integration and limited opportunity for teamwork. One of the major trends over the last decade has been the move not only to have a personal computer on the desk of virtually every knowledge worker, but to have that computer linked to the other computers in the organization.

    This chapter deals with the basic telecommunication devices, the types of computer networks and the telecommunications services available in South Africa. We conclude with a discussion of arguably the most interesting development in information systems of the last decade: the Internet.

    Computer Networks

    When a numbers of computers are connected together, they form a computer network. There are many ways of classifying computer networks.

    Networks according to size

    Networks sizes can range from tiny to very large.

    • Personal Area Network (PAN): consists of two to five computing devices. This not very common term would apply to the network typically found in the home, and may be based on wireless technology e.g. Bluetooth.
    • Local Area Network (LAN): the most common type of network. It consists of from about four up to as many as a couple of hundred of computers linked together with one set of cables, usually within the same building. Most LANs are controlled by a central file server that takes care of network communications, security control and the storage of data files. A student computer laboratory typically constitutes one LAN.
    • Metropolitan Area Network (MAN): a network infrastructure linking various local businesses within a large city area. This is now almost completely superseded by the Internet.
    • Wide Area Network (WAN): the opposite of the LAN. It links computers over large geographical areas. This network usually makes use of the public telecommunications network. The widely dispersed Automatic Teller Machine (ATM) network of a commercial bank is typically part of the bank’s WAN.
    • Value-Added Network (VAN): although not relating to size (but it rhymes with the others!), it refers to the provision of a network infrastructure service to other businesses. The service goes beyond the physical cabling and includes “value-added services” such as limited data and transaction processing or message routing. An example for the banking industry is the provision of an inter-bank Electronic Funds Transfer (EFT) and clearing service, linking the computers of different commercial banks (and, possibly, retailers) together.

    Network Topologies

    The network topology refers to the physical and logical way in which the computers in a network are connected together. Although there are a number of proprietary ways, the following three are the main topologies in common use (refer to Figure 1). Note that these topologies usually refer to a LAN configuration.

    • The star network is driven by one central computer to and through which all other computers communicate. Although this allows for central co-ordination and control, it requires a very reliable central computer and lots of cables.
    • The ring network consists of a continuous loop connecting all computers. Signals travel in a given direction and all computers have equal access to the data. A special version of the ring network is the token ring whereby a special code, the token, is passed around the ring. This token serves as the data holder and computers can send information only after grabbing an available i.e. empty token, adding their data and passing the token back onto the network.
    • The bus network is currently the most popular configuration. A central data cable is used, to which each computer (and other devices such as printers and routers) can be attached. Although bottlenecks can occur, its popularity stems from its inherent robustness: devices can be added or removed without affecting the rest of the network. Data clashes (two computers attempting to send information simultaneously) can prevented by a variety of means.
    Image showing a ring network, star network and bus network of computers
    Figure 1: Network topologies

    Telecommunication Devices

    Regardless of the network topology that has been implemented, the same basic equipment is used to connect the different computers and to ensure error-free data transmission between them.

    • Network cables are the physical wires by which computers are linked together. The most common types are:
      • Twisted pair: thin insulated copper wires, combined in one single cable. This is similar to the wire used for voice telephone connections.
      • Coaxial cable: (or coax) a thin copper wire inside a tube of insulation material, surrounded by a sheath or mesh of conducting wire, again insulated on the outside. This is similar to the wire used to connect antennas to video or TV equipment. Because there is less possibility of interference, it allows greater volumes of data to be transmitted in a given time – the amount or volume of data that can be transmitted over a network connection is referred to as bandwidth.
      • Optical fibre or fibre-optic: a translucent and flexible material through which laser light can travel over long distances. This fibre is much more difficult to work: it requires special connectors as well as lasers and sensors (with electronics-to-light converters) at each terminal. Although this technology is more expensive, laser light can be switched on and off a lot faster than electricity (and it travels ten times faster), resulting again in a much greater bandwidth.
      • Wireless: not all computer devices need a physical cable connection. Because of the cabling costs and hassles, engineers have explored many methods of transmitting data without the use of wires. For short distances, infra-red signals work well albeit slowly – the same technology as your VCR remote control. For longer distances and higher bandwidths, radio frequencies or other parts of the electro-magnetic spectrum are used. Satellite technology is increasingly being used for digital data transmissions, especially in conjunction with Global Positioning Systems (GPS).
    • Network interface cards (NICs) are necessary when computers are connected directly to other computers by means of digital network cables (as opposed to the situation when two computers are connected to each other via a telephone link). Their primary function is to make sure that there are no transmission conflicts with the other computers linked to the network, since data may be simultaneously sent and received by many different computers all linked to the same network. In addition, the network card usually fulfills an error-checking function, to ensure that uncorrupted data is received at its destination.
    • Multiplexers allow a single channel to carry data transmissions simultaneously from many sources, by merging them at one end of the channel and then separating the individual transmissions at the receiving end of the channel.
    • Front-end processors (FEP) are used in bigger networks that are centrally controlled by large computers – often mainframes. In order to give the expensive mainframe more “time” to concentrate on application processing, it needs to be relieved from the rather mundane task of network control. FEPs handle all or most communication processing such as error-checking, data conversion, packaging and transmission control.
    • Routers and bridges are computers dedicated to the translation of network protocols and standards between different networks. They are becoming important as more and more organizations are linking their own networks to those of other organizations. They may be using
      • different operating systems (Novell, Unix or Windows NT),
      • other technologies (coax or fibre-optic),
      • or different protocols (proprietary or public standards set for computer communications).
    • Finally, the modem allows a computer to communicate with another computer by means of the public voice telephone network, rather than by using digital cabling.
      Photo of a modem.
      The original model 300-baud Smartmodem

      This requires the conversion of digital computer signals (used inside the computer) into analogue sound signals (that can travel over the voice telephone lines) – this process is called modulation. At the other end of the line, these sound signals are converted back into digital signals – or demodulated. The word modem refers to this modulation/demodulation process. You may have heard this “modulated signal” when listening to a fax machine, which is really a scanner/printer/modem in one. Since the modem replaces the network card, it usually carries out similar error-checking functions to ensure the correct transmission of data.

    Trivial fact: More than 5000 satellites are orbiting the earth and most of them are involved in telecommunications.

    SA Public Telecommunications Services

    Because telecommunication services are a critical part of any country’s infrastructure, most governments have been very protective towards their telecoms. Paradoxically, this protectionism often resulted in high tariffs (monopolies!), thus reducing the overall competitiveness of local businesses. Recently more and more countries have started to privatize these services and allowed competition to drive prices down. The South African public telecommunication services are controlled by Telkom, although its legal monopoly is being phased out. The following are the main data network services provided by Telkom

    Public Switched Telephone Network (PSTN) Services

    The oldest data service provided by Telkom is the Datel service, which provides a connection between computers by means of the standard Public Switched Telephone Network (PSTN)—i.e., the same as the normal voice telephone traffic. This requires the use of built-in or external modem equipment that modulates the digital signal into an analogue audio signal (and demodulates it back at the receiving end). This service is quick and easy to set up since it is available anywhere where there is an ordinary voice telephone point. The main drawbacks are the limited transmission speed, high error rate and the lack of security. Customers may choose between a dial-up or leased line connection.


    Diginet is a dedicated digital data service from Telkom that provides reliable and efficient point-to-point (i.e. not switched) data connections. It differs from the Datel network in that the transmission path is entirely digital: a combination of fibre-optic, microwave and coaxial cable. Because the signal does not have to be translated into analogue form, no modem is required, resulting in a cost saving. However, its main advantages are the higher transmission rates and a substantial reduction in transmission errors. The standard Diginet service allows for 64 kpbs (kilobits per second) though an enhanced service called Diginet-Plus has been designed to transfer up to 1920 kpbs, which allows slow-scan TV and video conferencing signals to be transmitted in real-time.

    Public Switched Data Network (PSDN) Services

    Saponet is Telkom’s Public Switched Data Network (PSDN). The Saponet-P service relies on a packet-switching mechanism whereby all data transmissions are broken up into smaller, standard-size units or data packets. Each of these packets is then routed independently to their destination. The path traveled by the packet depends on the available capacity and bottlenecks. At the destination, the original transmission is reassembled out of the constituent packets. A Packet Assembler & Disassembler (PAD) is responsible for the breaking up of a message into packets and the opposite process of reassembling packets into a message at the destination. This PAD can be a separate hardware device or a software program.

    X.400 and Telkom400

    Telkom400 is a VAN on top of the X.400 infrastructure. It supports electronic message handling and electronic data interchange (EDI). EDI is the automated computer to computer application exchange of structured business data between different organizations. An international standard defines common business documents such as order forms, invoices or electronic funds transfer documents that are exchanged directly between the computers of the respective business partners.

    ISDN and ADSL

    Now that most of Telkom’s telephone exchanges have become digital, Telkom is able to provide new functions and services. One all-digital connection that has sufficient capacity (bandwidth) to support speech, video conferencing, facsimile, data and image transfer. This connection is called an ISDN line (Integrated Services Digital Network) and is currently available in selected metropolitan areas. Much more popular is the newer Asymmetric Digital Subscriber Line (ADSL) which allows a broadband connection (at least several hundred kilobits/second) over your standard telephone line while keeping the line available for voice telephone calls. It is called “asymmetrical” because the standard allows for much greater “download” than “upload” speeds; this reflects the typical home user pattern. Higher volumes and transmission speeds of up to 150 mbs – typically needed by mid-size and larger businesses – are available through Telkom’s ATM Express service.

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