COMMUNICATIONS NETWORKS
Telephone calls, fax messages, e-mails, computer data – and often radio and television signals as well – all travel from place to place through a complex, worldwide telecommunications network. All the different forms of telecommunications are turned into signals that can travel through the network. They always travel between two points in the network (for example, two telephone receivers or fax machines), and are directed through it by electronic circuits at telephone exchanges. This sort of telecommunications network is called a switched network. Radio and television networks, where signals are sent to many receivers from one transmitter, are known as broadcast networks.
All the telecommunications devices (telephones, fax machines and home computers) connected to normal telephone lines are linked by the lines to a local telephone exchange. Each line has its own unique telephone number which the exchange uses to find it. All the local exchanges in one area are linked to a main exchange, which in turn is linked to other main exchanges to form a national network. Also linked into the network are special exchanges for mobile telephones and Internet service providers. Most information (speech, fax messages and computer data) travels to and from the local exchange in analogue form and between local exchanges in digital form.
There are several different ways of linking together telephone exchanges on a network. Some links are underground cables, either in the form of electrical cables or optical-fibre cables, in which signals are carried by light. Some links are made with microwaves. International links across oceans are made via satellites in orbit around the Earth, and through cables stretched across the sea bed.
THE INTERNET
The Internet is a vast computer network that stretches right around the world, made up of hundreds of millions of computers. Data can travel from any computer on the network to any other computer. The Internet began in the 1960s, when research agencies in the USA built their own communications network. Other organizations, such as universities, gradually joined. As home computers became cheaper and more popular in the 1990s, the Internet began expanding rapidly, with anybody being able to use it via a telephone line.
Internet use falls into two main areas – e-mail and the World Wide Web. With e-mail, it is possible to send a text message (with other data files, such as photographs, attached if needed) almost instantly to any other Internet user at their e-mail address. The World Wide Web (the “Web”) is a huge information-gathering system. It allows one computer connected to the Internet to ask for and copy files from another computer. The files are stored in standard form so that any computer can read them.
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