Comparison of bandwidth availability for various media.

The idea

Bandwidth is the range of frequencies a medium can carry. Different media offer very different bandwidths, which limits how much information they can transmit per second. Twisted-pair copper cable has a modest bandwidth and suffers greater attenuation at high frequency; coaxial cable offers more; fibre optic cable offers very large bandwidth because infrared or visible light has a very high frequency, and it is immune to electromagnetic interference. Radio and microwave links provide wide bandwidth but require line of sight or suitable aerial alignment. Comparing media means considering available bandwidth, signal attenuation, noise and interference, cost and practicality. A larger bandwidth allows more channels or a higher data rate, which is why fibre is preferred for high-capacity long-distance links.

What earns the marks

• Define bandwidth as the range of frequencies a medium can transmit, usually measured in hertz.
• Compare typical bandwidths: twisted pair is lowest, coaxial cable is greater, and fibre optic is greatest among guided media.
• Relate greater bandwidth to a higher possible rate of information transfer or more simultaneous channels.
• Discuss attenuation and interference: copper cables lose more signal at high frequency and pick up electromagnetic noise, whereas fibre is largely immune.
• Consider practical factors such as cost, installation, distance between repeaters and security of the signal.
• Use appropriate examples, such as comparing a telephone twisted-pair line with a fibre-optic backbone link.

Common mistakes and how to put them right

• Confusing bandwidth with the frequency of the signal itself.

  Put it right by describing bandwidth as the range of frequencies the medium can carry.

• Assuming fibre always has the highest bandwidth in every situation without considering the transmitter, receiver and distance.

  Put it right by comparing complete systems, not just the cable.

• Treating bandwidth and data rate as identical.

  Put it right by explaining that bandwidth sets an upper limit, while the actual data rate also depends on the modulation and coding used.

Exam tips

• Structure comparisons around named criteria such as bandwidth, attenuation, interference and cost.
• Use comparative language such as greater, lower or more immune rather than simply listing media.
• Link each medium's bandwidth to a consequence, such as the number of channels or the data rate it can support.