Essay on Computer Networks and Communications

A local area network is a system of linked PCs and other devices such as printers. LANs can have a server computer holding files used by more than one computer, and providing storage capacity to the other computers in the network. A wide area network is a network of computers that are dispersed on a wider geographical scale than LANs. They are connected over the public telecommunications network. A WAN will normally use minicomputers or powerful PCs. Computers used in organizations are usually part of a connected group of computers known as networks. In this section, we look at different types of computer networks. Later in the chapter, we look at how computers communicate with each other.

A computer network is made up of a number of connected computers each with its own processor, for example, a number of connected PCs. Networks are popular because they provide a number of users with access to resources (eg data files, printers, software). Therefore, a network allows computing resources to be used more efficiently between a group of users. There are two main types of network, a local area network (LAN) and a wide area network (WAN).

Local area networks (LANs):

A local area network (LAN) is a network of computers located in a single building or on a single site. The parts of the network are linked by computer cable rather than via telecommunications lines.

Network topology refers to the physical arrangement of items (nodes) in a network. A node is any device connected to a network: it can be a computer or a peripheral device such as a printer.

There are several types of LAN system configuration. For example, in a bus structure (diagram follows), messages are sent out from one point along a single communication channel, and the message is received by other connected machines.

Each device can communicate with every other device and communication is quick and reliable. Nodes can be added or unplugged very easily. Locating cable faults is also relatively simple.

Other types of LAN which architecture is most appropriate depends upon a number of factors- such as which files are required to be accessed by many users. The number of printers to be shared and the relative power of the computers.

Local area networks have been successful for a number of reasons. First of all, personal computers f sufficient power and related software were developed, so that network applications became possible. Some organizations which could not afford a mainframe or minicomputer with terminal links have been to afford a LAN with personal computers.

Wide area networks (WANs)

Wide area networks (WANs) are networks on a number of sites, perhaps on a wide geographical scale. WANs often use minicomputers or mainframes as the ‘pumps’ that keep the data massages circulating, whereas shorter distance LANs normally use PCs for this task.

A wide area network is similar to a local area network in concept, but the key differences are:

The geographical area covered by the network is greater, not being limited to a single building or site.
WANs will send data over telecommunications links.
WANs will often use a larger computer as a file server.
WANs will often be larger than LANs, with more terminals or computers linked to the network.
WAN can link two or more LANs.

Client-server computing:

Client-server computing is a configuration in which desktop PCs are regarded as “clients” that request access to the server available on a powerful server PC, such as access to a file, e-mail, or printing.

The term client-server is a way of describing the relationship between the devices in a network. With client-server computing, tasks are distributed among the machines on the network.
A client is a machine that requests a service, for example, a PC running a spreadsheet application which the user wishes to print out.

A server is a machine that is dedicated to providing a particular function or service requested by a client. The server includes file servers, print servers, and e-mail servers.

Computer Communication:

A computer linked in a network needs to be able to communicate with each other to allow the sharing of resources. Computers in one network may also require the ability to occasionally communicate with computers in a different network. Communication may involve the transfer of data from one part of a system to another, for example, transaction values may be posted from the receivable ledger module of an accounting system to the general ledger module, or could involve one-off messages, for example through the use of electronic mail.

Data links:

When all data processing is done in the same office, the transmission of data between input and output devices and the central processor is usually provided for using an internal cable.

When the input or output device is located away from the computer, so that it has to be transmitted along with a telecommunications link (for example a telephone line) there are additional items of data transmission equipment that have to be used, and the way in which the data is to be transmitted has to be resolved.

A data link might typically connect the following:

A computer and a remote terminal (keyboard and VDU). A computer may have a number of remote terminals linked to it by data transmission equipment.
Two computers located some distance from each other (for example a mainframe and a PC, which would use the link to exchange data).
Several processors in a network, with each computer in the network able to transmit data to any other.

Data Transmission Terminology

This section explains some common terms used in the context of data communications.

Modems and digital transmission

New technology systems capable of delivering substantial quantities of data at great speed. For data transmission through the existing ‘analog’ telephone network to be possible, there has to be a device at each end of the telephone line that can convert (MOdulate) the data from digital form to analog form, and (DEModulate) from analog form to digital form, depending on whether the data is being sent out or received along the telephone line. The conversion of data is done by devices called modems. There must be a modem at each end of the telephone line.

Digital means ‘of digits or numbers’. Digital information is information in a coded (binary) form. Information in analog form uses continuously variable signals. It is enough for you to appreciate that there is a distinction between the two and that digital methods are more advanced.

Bandwidth:

The amount of data that can be sent down a telecommunications line is in part determined by the bandwidth. Bandwidth is the range of frequencies that the channel can carry. Frequencies are measured in cycles per second, or in Hertz. The wider the bandwidth, the greater the amount of data the channel can carry. (See ‘Broadband’ later in this section).

Integrated Services Digital Network (ISDN)

Faster telecommunications networks such as ISDN and ADSL have been developed. Digital networks for mobile phones also exist.

An important development of the last few years is the spread of Integrated Services Digital Networks (ISDN). Data can be transmitted significantly faster over ISDN than over standard tel-communications lines.

Asymmetric Digital Subscriber Line (ADSL)

ADSL (Asymmetric Digital Subscriber Line), offers data transfer rates of up to 50 Mbps, considerably faster than ISDN. ADSL allows information to be sent out over ordinary copper wires and simultaneous use of the normal telephone service is possible.

Broadband:

A fast, always-on connection to the internet is sometimes referred to as Broadband. ADSL is an example of broadband technology. Broadband means a relatively high capacity, and therefore relatively fast, communications link. The term broadband is usually used to describe an Internet connection that is always on’, meaning it is not necessary to dial-up to establish a connection.

Network cards and connections:

Computers and other devices on a network are connected to each other using a computer cable (known as coaxial cable). This cable is plugged into all devices (eg computers, printers) on the network into a slot on the device (similar to a telephone connection slot). This connection connects the cable to a network card that holds the circuitry required for network communication.

Ports:

A port is a socket on a computer into which you plug a peripheral device such as a printer. Ports can be serial, parallel, or Universal Serial Bus (USB) as explained below. USB ports allow faster data transfer and are expected to eventually completely replace serial and parallel ports.

Interfaces:

The term interface is frequently used in computer communications contexts, but it has at least three different meanings – as shown in the following table.

Mobile communications:

Networks for mobile telephone communications have grown since the development of mobile phones, also known as ‘cellular phones’, in the late 1980s.

Digital networks have been developed, which are better able to support data transmission than the older analog networks. Digital networks offer higher transmission speeds and less likelihood of data corruption. Internet access through mobile phones is available, and combined handheld computer/cellular phones have been developed.