Bandwidth of Transmission Medium

What it means for a medium. Just as a signal has its own bandwidth requirement (Section 10.6), the physical transmission medium carrying it -- a length of cable, or a slice of radio spectrum used for a wireless link -- can only support signals up to some maximum bandwidth of its own, beyond which the signal is degraded (attenuated or distorted) too badly to be usefully recovered at the far end. This available bandwidth differs enormously between different kinds of medium.

Twisted-pair cable. A pair of insulated copper wires twisted around each other -- the traditional medium for telephone local loops and for lower-speed data links -- has a comparatively modest usable bandwidth, of the order of no more than about a megahertz over a practical transmission distance, because resistive losses and cross-talk between adjacent pairs grow rapidly with both frequency and cable length.

Coaxial cable. A single central conductor surrounded by a cylindrical shield, with insulation in between, confines the electromagnetic field almost entirely inside the cable and so suffers far less loss and interference than a twisted pair at the same frequency; its usable bandwidth extends into the hundreds of megahertz, which is why it has long been used for cable-television distribution and for carrying many telephone or data channels multiplexed together over one physical line.

Optical fibre. A hair-thin strand of glass carries information as pulses of light (rather than an electrical signal) guided along the fibre by total internal reflection. Because light itself is an electromagnetic wave of extremely high frequency, an optical fibre can support a bandwidth reaching into the terahertz range -- many thousands of times larger than a coaxial cable of similar physical size -- which is why modern high-capacity, long-distance communication links are built on optical fibre wherever it is economically possible to lay it. …