Transmission media
The transmission medium is the physical path between transmitter and receiver in a communicationsnetwork. The media that have been used in local networks include twisted-pair wire, coaxial cable and
optical fibre. In addition, forms of electromagnetic propagation, through the atmosphere, can be employed
for building-to-building connections or over large geographical areas.
The various media can be described using the set of characteristics described below.
• physical description: the nature of the transmission medium
• transmission characteristics: include whether analogue or digital switching is used, modulation
technique, capacity, and frequency range over which transmission occurs
• connectivity: point-to-point or multipoint
• geographic scope: the maximum distance between points on the network
• noise immunity: resistance of the medium to contamination of the transmitted data
• relative cost: based on costs of computers, installation, and maintenance.
Transmission media or channels have the following transmission characteristics:
• bandwidth: this is an electrical characteristic of the transmission line or circuit. It indicates the
range of frequencies (measured in Hertz) which can be successfully trans-miffed over the line
• baud rate: the number of single elements or condition changes per second. This defines the
signalling rate on the transmission line. A signal element is a discrete voltage, phase or frequency value
• channel capacity: this is the maximum rate at which it can carry information without error. For
digital information, this is measured in bits per second and: capacity = baud rate x number of bits
per signal element
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