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Wired Technologies

Wired Technologies

Wired communication refers to the transmission of data over a
wire-based communication technology.

Wired connections are by far the most common. The main media in use are coaxial cable, twisted pairs and fibre optics. For each of these, specific network technologies or specifications have been designed. The medium must have properties that will ensure a reasonable error performance for a guaranteed distance and rate of date delivery (i.e. speed). It must also support two-way or multiway communications.

Fibre optics systems use mainly infrared light for data transmission. At each end, fast opto-couplers and diodes are used to translate the signal to electrical levels. Most fibre-based systems can only transmit one way, so a pair of fibres are required to implement a two way system. Hubs are also required at each junction as fibre systems can only operate point-to-point. Fibres are used mainly in areas where high speed, security and/or electrical isolation are important.

Coaxial cable systems were very popular in the early days of networking, mainly because it made use of cheap and commonly available 75 or 50 Ω coaxial cable, nominally used for video and radio frequency applications. In a coaxial system, nodes are connected together via a single backbone, that is, the cable is laid out as a single line, passing through all stations, this is also known as bus topology.


A resistive 75 (or 50) Ω terminator R is placed at each end of the cable to absorb all reflections. The nodes act on receive as high impedance signal pickoffs, and on transmission, as current drives into the (resistive) line. To all extents, this line looks to all the devices connected just like a purely resistive load of R/2 Ω; such a known load resistance allows the transmitters to use simple current generators to inject a fixed amount of current into the line. The voltage generated on the line is of the order of half a volt for a typical Ethernet network, this allows receivers to use dynamically adjustable thresholds sensors to determine zero crossings, and also to detect voltage overloads caused when two or more station transmitters are attempting to drive the line together. This provides a simple form of collision detection. To avoid ground loops, the coaxial cable shield connection is grounded at only one point; network adapters must therefore incorporate isolation hardware to float all the power supplies and other circuits directly connected to the cable, adding somewhat to their cost.

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