Telecommunications

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) of today 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.