What are various types of lines for communications over the telephone network?

Telecommunications is the act of sending and receiving data over a distance. There are various applications of data transmission between separate computers. Airline reservation systems provide a good example for this; a great number of computers of different types communicate to handle transactions in a correct and speedy manner. Large banks are also users of such a technology. Electronic mail can be sent via the global computer network from your computer to one on the other side of the world; this may be pure text or even voice mail, in which the actual voice message of the sender is digitized, put in a file, transmitted, and decoded for the receiver to listen at the other end. Electronic bulletin boards are 'public' files available for all interested users to read and 'post' messages via the network. Bulletin boards about any area of interest that one can think of have been established all over the world. Fax machines enable the transmission of pages of text and picture information; a microcomputer equipped with a fax board can practically act like a fax machine, and receive and send such messages to/from its memory. Huge databases containing all kinds of information are connected to the network, and one can 'query' (i.e. send messages containing questions to) them and receive messages containing the required information. 'Laptop' computers, designed to be able to operate even when away from a fixed power source, can be connected to cellular phones, mobile devices which use special stations called cells placed around the city, to gain access to a network.

There are two kinds of communications media through which the transmission can take place: Physical lines or microwaves. Physical lines can be twisted-pair wires, which make up most of the world's telephone network, coaxial cables, which have a much higher data carrying capacity and are used, for instance, in cable TV networks, or fiber optic cables, in which data can be sent in the form of laser beams through hundreds of very thin, transparent fibers in enormous speeds and amounts. Data in the form of microwave signals can be sent or received by terrestrial ('on the Earth') microwave stations or communications satellites in space. Satellites are generally more expensive than terrestrial stations, but since the range of terrestrial stations is practically limited to a few tens of kilometers, it may be more economical to send up three satellites to geosynchronous orbit (where they seem to be fixed points from Earth, since they rotate at the same speed as the Earth around its axis) if one wants to have all of the planet's surface in range.

The capacities of communications media are measured in bps (bits per second) that can be transmitted. Ordinary telephone lines have a capacity of around 2400 bps. If one wants faster (wideband) transmission, coaxial or fiber optic cable or microwave should be used.

Transmission types can be classified according to the direction of the flow of information on the medium. In simplex transmission, information can travel in only one direction. In half-duplex transmission, data can go in both ways, but not at the same time. In full-duplex transmission, messages can go in both directions simultaneously.

Signals can be analog (i.e. a continuous wave) or digital (i.e. streams of discrete impulses corresponding to 0's and 1's.) The phone system in most parts of the world is analog, so data from/for computers has to be translated between its intrinsic digital form and analog form if the phone network will be used for computer communication. Modems are devices which perform this duty of modulation (digital -> continuous) and demodulation (continuous -> digital). When most of the world's telephone network has been replaced with digital lines, widespread use of ISDN (integrated services digital networks) technology, which enables various different kinds (graphics, text, voice, etc.) of information to be transmitted within the same framework, will be possible.

Data transmission can be parallel (with all bits of a byte traveling in their own channels simultaneously) or serial (where the bits travel one after another through a single channel.)

Computer networks can be classified according to the size of the geographical area that they cover. A network which spans an area with a diameter of at most, say, 2 km, is a local area network (LAN). Many organizations (including some departments of our University) have LANs connecting most of the computers in their buildings. Bigger networks, connecting computers in different cities, or even countries, are called wide area networks (WANs). Big public (e.g. academic, as opposed to commercial) networks are usually connected to each other as well, through one or more computers which are linked to both, so if one has a connection to a planetwide network like the Internet, or Bitnet, one can access data and communicate with other people through one’s computer no matter where they are located. In general, a collection of interconnected networks is called internetwork or just internet (do not confuse with the Internet). Several computers in our University are connected to such international networks, and students can obtain accounts in those computers and learn the world of computer networks for themselves. Today, the most popular internetwork is the Internet. The main applications of the Internet are email, news, remote login, file transfer, and the World Wide Web (www). The www made it possible for a site to set up a number of pages of information containing text, pictures, sound, and even video, with embedded links to other pages. By clicking a link, the user is transported to the page pointed to by that link. There are various pieces of software to view the Internet such as Mosaic, Netscape, etc.

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