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Telecommunications
Telecommunications
, communications over a distance using technology to overcome that distance. It usually means the transmission of words, sounds, pictures, or data in the form of electronic signals or impulses, sent either as an individual message between two parties or as a broadcast to be received at many locations. While broadcasting is far removed from private communications, a new range of one-to-one communication services (including video-on-demand, and other personal information and entertainment services provided over cable networks and so-called "webcasting" over the Internet) will blur the current clear distinction between the two.
Since its invention by Alexander
Graham Bell in 1876, the telephone has become the most familiar form of
telecommunications. More recently, voice telephony has been supplemented by a
range of computer-based telecommunication services. These have become popular
through the Internet and World Wide Web—vast computer networks that provide many
people with the means to exchange information.
II
HISTORY
It is now taken for granted in
developed nations that by pressing a few buttons people can talk to family,
friends, or business associates across the world. The technology that has led to
one of the most complex creations of the 20th century—the telephone network—has
evolved over the past hundred years or so.
The first electrical means of
communication was not the telephone, however, but the telegraph, which allowed
messages sent in code (usually Morse Code) to be received and printed at a
distant location. The age of commercial telegraphy dawned in 1839 when the
British pioneers William Fothergill Cooke and Charles Wheatstone opened their
line alongside the main railway route running west from London. A technically
simpler system of telegraphy was devised in 1843 by Samuel Morse, and after this
the spread of telegraph networks was rapid, with routes spreading across most of
the countries of the Old and New Worlds and then beneath the oceans that
separated them. By 1930 nearly 650,000 km (400,000 mi) of undersea cables had
been laid, linking the economic, political, military, and cultural institutions
of the world.
An even greater breakthrough was
made in 1876, when Alexander Graham Bell made the first telephone call to his
assistant with the words "Mr Watson, come here, I want you". Bell’s invention
sparked a series of innovations, ultimately culminating in today’s information
superhighway. Key steps along the way were:
In 1889 Almon Strowger developed an automatic switching system that could set up a telephone call without intervention by a human operator. Strowger’s motivation for this invention was to prevent his calls being diverted to a business competitor by his local operator. The impact of the invention was much wider as it provided the basis for the current telephone network.
In 1901 Guglielmo Marconi demonstrated that radio waves could be used to transmit information over long distances when he sent a radio message across the Atlantic. Radio is still one of the key transmission media today, and is the basis of many mobile services.
In 1947 William Shockley, John Bardeen, and Walter Brattain invented the transistor. This enabled the electronics revolution to take place and provided the basis for a computerized, rather than mechanical, telecommunications network.
In 1965 Charles Kao put forward the theory that information could be carried using optical fibres. These have subsequently been developed to provide a means of carrying huge amounts of information at very high speed. Optical fibres form the backbone of the global transmission network (seeFibre Optics).
The modern telephone network can be viewed as a globally distributed machine that operates as a single resource. Much of it uses interconnected computers. The network that most people use to carry voice traffic can also be used to transfer data in the form of pictures, text, and video images.
III
NETWORKS
Despite being very complex, global
telecommunications service is comprised of a few basic network components, which
are: (1) user equipment—telephones, computers, and all the other devices that
provide a means of accessing the network; (2) the access network—users are
connected to the main network by wireline or radio links; (3) the main
network—copper wire, microwave radio, and optical fibre cables connecting all
the nodes of the global network; (4) transmission equipment—the means by which
huge volumes of information (there are many millions of telephone and data calls
made every second) are carried over the network; and (5) switching equipment—the
hierarchy of local, long-distance, and international switches that allow any
user of the network to connect to any other user. Each of these components has
to consist of a combination of hardware and software.
A
Hardware
This usually covers items such as
telephones, transmitters, cables, interface devices, switches, and computers. In
the past, telecommunications have relied heavily on hardware, such as dedicated
switching elements, and on the logic providing its control functions. A
situation is now developing in which more of the system relies on elements
operating under computer (software) control. Because this software can be
upgraded, this makes it easy to add new, enhanced functionality later.
B
Software
This is code that instructs a
computer or network device. Until the 1980s, most of the operational
instructions used by a telecommunications network were hard-wired or pre-set.
The advent of digital systems and data networks has led to a much wider range of
network services. Software solutions are well suited to the complexity and
flexibility inherent in these services.
IV
TELECOMMUNICATIONS CONCEPTS
There are several ways of carrying
information between senders and users. The options chosen should reflect the
type of communication required. For instance, humans compensate for noise and
transmission errors when they talk to each other. Unexpected delays or echoes
cause problems in understanding, however. Computers have the reverse
characteristics—being tolerant of short delays and less so of transmission
errors. The following concepts underpin telecommunications networks.
A
Analogue and Digital Networks
Many older telecommunications
systems are analogue; the electrical signals conveying information vary
continuously in harmony with the sounds they represent. The quality of speech
across analogue networks is determined by the amount of the speech spectrum that
could be carried. Around 3 kHz was accepted as a reasonable compromise of cost
and quality for normal telephone calls.
The alternative way of transmitting information is with a straightforward electrical signal that is either on or off, as with Morse’s telegraph. Computers also communicate with discrete, digital (on/off) signals, and while these can be converted to tones for transmission over analogue communications, it makes more sense to send them back in their original digital form. Speech and other analogue communications can readily be converted into digital form, and back to analogue (see Digital-to-Analogue Converter and Analogue-to-Digital Converter). Most telecommunications networks today are "integrated" digital systems, ideally suited to computer networking and other multimedia applications such as speech (voice), data, text, fax, and video.
B Circuit-Switching and Packet-Switching The distinguishing feature of circuit-switching is that an end-to-end connection is set up between the communicating parties, and is maintained until the communication is complete. The public switched telephone network (PSTN) is a familiar example of a circuit-switched network.
Communication between computers, or between computers and terminals, always involves the transfer of data in blocks rather than continuous data streams. Packet-switching exploits the fact that data blocks can be transferred between terminals without setting up an end-to-end connection through the network. Instead they are transmitted on a link-by-link basis, being stored temporarily at each switch en route where they queue for transmission on an appropriate outgoing link. Routing decisions are based on addressing information contained in a "header" appended to the front of each data block. The term "packet" refers to the header plus data block.
C Congestion and Blocking In a packet-switched network, packets compete dynamically for the network’s resources (buffer storage, processing power, transmission capacity). A switch accepts a packet from a terminal largely in ignorance of what resources the network will have available to handle it. There is always the possibility, therefore, that a network will admit more traffic than it can actually carry with a corresponding degradation in service. Controls are therefore needed to ensure that such congestion does not arise too often and that the network recovers gracefully when it does.
In a circuit-switched network the competition for resources takes the form of "blocking". This means that one user’s call may prevent another user from getting access. Since the circuit is reserved by the user—irrespective of what is sent—for the duration of the user’s call, no one else has any form of access until the call is cleared. Traditional circuit-switched networks are designed to balance the amount of equipment deployed against a reasonable level of access for the users of that network.
D Performance A circuit-switched network, such as the PSTN, provides end-to-end connections on demand, as long as the necessary network resources are available. The connection’s end-to-end delay is usually small and always constant, and other users cannot interfere with the quality of communication.
In contrast, in a packet-switched network, packets queue for transmission at each switch. The cross-network delay is therefore variable as it depends on the volume of traffic encountered en route and if it exceeds a certain level, system performance can be badly impaired.
V
THE INTERNET
It is evident from the recent
growth in data communications that the telephone network is far from exhausted.
The Internet—an expanded group of computer networks that shares a common set of
protocols and address space—has grown over the past 25 years and now links
around 40 million people worldwide.
Originally conceived by the United States military as a means of removing reliance on central computers, the Internet has grown into a global community of users who use the facility to send and receive mail messages, to exchange files, and to access shared data. This has been enabled by the adoption of a family of protocols (known as TCP/IP), which allow most users to communicate with most remote services, irrespective of local equipment type.
The Internet provides a means for any computer to communicate with any other. The World Wide Web is an Internet-based application that exploits this capability to provide a global information service. It enhances the mail and file transfer features of the Internet in that it can support multimedia—that is, allow a user to access pages containing pictures, video clips, and sounds, as well as text. It also provides links that allow users to navigate between information held on different computers, simply by clicking on the screen. Many companies, organizations, interest groups, and individuals now have Web pages, mostly for promoting their services or offering information of general interest.
The basic tool for reading Web pages is called a browser (Netscape Navigator, Internet Explorer, and the older Mosaic are the most common examples). The browser is packaged as part of the software given when users register with an Internet service provider.
VI THE FUTURE No one can predict with certainty exactly how telecommunications will develop, but certain trends can be noted. The cost of communication is falling in real terms, making advanced applications more affordable. Broader competition in the marketplace will reduce prices further. Telephone companies (telcos) recognize their revenues from carrying calls will decline and are encouraging, successfully, many new valued-added services that combine communication with the supply of information or services. Most of these have yet to evolve, but electronic commerce, mobile commerce, and various information-on-demand services are already being developed.
Some communication services currently provided by wire are migrating to radio means for greater convenience and flexibility; this includes not just cordless telephones (seeCellular Radio) in the home and the workplace but also connecting these telephones to the network. Conversely radio and television programmes, traditionally broadcast over the airwaves, are moving on to cable networks (seeCable Television).
Fixed-mobile convergence is another trend, in which the distinction between conventional telephones and mobile networks will dwindle. Many people will carry a single "personal communicator" that functions as a cordless phone within the home, as their business extension in the workplace, and as a pocket mobile telephone elsewhere.