Wired Channels: Wire Pairs, Coaxial Cable and Optical Fibre – Telecommunication Channels (Senior 3 Physics) | Ganzaa

Telecommunication Channels

Lessons

Senior 3 · Physics

Telecommunication Channels

4 lessons

Lesson 3 of 4

Wired Channels: Wire Pairs, Coaxial Cable and Optical Fibre

25 min

Three cables, one job

Look behind a television, a computer or a telephone socket and you will usually find one of three kinds of cable. A telephone or network cable holds thin copper wires twisted in pairs. The aerial or satellite-dish lead of a television is a round, stiff coaxial cable. And the line that brings fast internet to a building is often an optical fibre, a strand of glass. In S2 ICT you met these as network media. In this lesson you will see the physics that decides what each can do: how the signal travels, how it is protected from noise, and how quickly it fades.

Definition

Wired (guided) channel: a channel in which the signal is guided along a physical path: a pair of wires, a coaxial cable or an optical fibre.

Interference: noise picked up from outside sources, such as motors, power lines or neighbouring cables.

Wire pairs

A wire pair carries the signal as a varying electric current: the current flows out along one wire and back along the other. Bare parallel wires act like an aerial and pick up interference. Twisting the pair helps: each twist swaps the positions of the two wires, so the noise voltages picked up in neighbouring twists are in opposite directions and largely cancel. Wire pairs are cheap and easy to install, and they carry telephone lines and local computer networks well, but their attenuation rises steeply at high frequencies, so they suit short distances or modest data rates.

Coaxial cable

A coaxial cable has a central copper conductor inside a plastic insulator, surrounded by a metal braid or foil and an outer jacket. The two conductors share the same axis, which is what "coaxial" means. The signal travels in the space between them, and the earthed outer braid acts as a shield: outside electrical noise is stopped by it, and the signal cannot leak out. Coaxial cable carries much higher frequencies than a wire pair, so it is used for television aerial and satellite-dish leads, cable television and connections between radio transmitters and their aerials.

Optical fibre

An optical fibre carries the signal as pulses of infrared light, kept inside the glass core by total internal reflection at the boundary with the cladding (unit 8, lesson 2). Light is not affected by electrical interference, the glass is so pure that the signal fades very slowly, and light can be switched on and off billions of times a second, so a single fibre carries enormous amounts of data. Fibres are thin and light, carry no electric current (so they cannot cause sparks or shocks), and cannot be tapped without breaking them. They are harder to join and need converters at each end: a laser or LED to turn electrical pulses into light, and a photodiode to turn light back into electrical pulses.

	Wire pair	Coaxial cable	Optical fibre
Signal	Electric current	Electric current	Pulses of light
Bandwidth (data rate)	Low	Medium	Very high
Attenuation	High	Medium	Very low
Interference	Reduced by twisting	Blocked by the metal shield	Not affected
Cost and handling	Cheapest, easy to join	Moderate	Cable cheap, but joints and converters cost more
Typical uses	Telephone lines, computer networks	TV aerial and dish leads, cable TV	Long-distance links, undersea cables, fast internet

Worked example 1: how long does light take?

The core of a fibre has refractive index 1.48. How long does a pulse take to travel 1000 km along it?

v = \frac{c}{n} = \frac{3.00 \times 10^{8}}{1.48} = 2.03 \times 10^{8}\,\mathrm{m\,s^{-1}} \qquad t = \frac{d}{v} = \frac{1.00 \times 10^{6}}{2.03 \times 10^{8}} = 4.93 \times 10^{-3}\,\mathrm{s}

About 5 ms: fast, but slower than radio through air, because light in glass travels at only about two thirds of its speed in a vacuum.

Worked example 2: attenuation along a wire pair

In an illustrative wire pair, the signal power halves every 15 km. What fraction of the power is left after 60 km?

60 km is $\frac{60}{15} = 4$ halvings:

\left(\frac{1}{2}\right)^{4} = \frac{1}{16}

Only one sixteenth of the power arrives, so long copper links need repeaters at intervals. Fibre may run many tens of kilometres before a signal needs boosting.

Worked example 3: how many calls fit in a fibre?

One digital phone call needs 64 000 bits per second (lesson 2). How many calls could share a fibre carrying $1.0 \times 10^{10}$ bits per second?

\frac{1.0 \times 10^{10}}{64\,000} = 156\,250\ \text{calls}

Warning

Common mistakes and safety

Saying an optical fibre carries electricity. It carries light; the electrical signal is converted at each end.
Saying the braid of a coaxial cable carries a separate signal. It is the return conductor and the shield.
Never look into the end of an optical fibre or its connector. The infrared light is invisible but can be strong enough to harm the eye. Cut fibre ends are sharp glass splinters: collect them on tape, never with fingers.

Tip

Twist, shield, light. The three cables fight noise in three ways: a wire pair twists so noise cancels, a coaxial cable shields it out, and a fibre uses light, which electrical noise cannot touch.

✦

Practice

Name the three main types of wired channel and the form the signal takes in each.
Explain how twisting a pair of wires reduces interference.
Label the four layers of a coaxial cable, and explain the job of the metal braid.
Explain why light stays inside the core of an optical fibre. Which must be larger, the refractive index of the core or of the cladding?
A fibre core has refractive index 1.50. How long does a pulse take to travel 600 km?
In an illustrative cable, the signal power halves every 10 km. What fraction remains after 50 km?
Give three advantages of optical fibre over copper cable for linking two towns.
A school wants to connect two buildings 80 m apart through an area with large electric motors. Evaluate whether a wire pair or an optical fibre is the better choice.

Show answersHide answers

Wire pair (electric current), coaxial cable (electric current), optical fibre (pulses of light).
Each twist swaps the positions of the two wires, so the noise induced in neighbouring sections is in opposite directions in the loop and largely cancels.
Central copper conductor, plastic insulator, metal braid, outer jacket. The braid is the return conductor and, being earthed, shields the signal from outside interference and stops it leaking out.
Total internal reflection at the core and cladding boundary; the core's refractive index must be larger.
$v = \frac{3.00 \times 10^{8}}{1.50} = 2.00 \times 10^{8}\,\mathrm{m\,s^{-1}}$ ; $t = \frac{6.00 \times 10^{5}}{2.00 \times 10^{8}} = 3.0 \times 10^{-3}\,\mathrm{s}$ (3.0 ms).
5 halvings: $\left(\frac{1}{2}\right)^{5} = \frac{1}{32}$ .
Any three: much higher data rate; much lower attenuation, so fewer repeaters; no electrical interference; no sparks or shocks; hard to tap; thin and light.
Over 80 m either could carry a school network. But the motors produce strong electrical interference, which a fibre ignores completely, and fibre also avoids any electrical connection (and danger) between buildings. Fibre is the better choice despite its higher cost of connectors; a shielded wire pair would be the cheaper compromise.

Key points

Wire pairs and coaxial cables carry electric currents; optical fibres carry pulses of light by total internal reflection.
Twisting reduces interference in wire pairs; the earthed braid of a coaxial cable shields it; fibres are immune.
Fibres have the widest bandwidth and lowest attenuation, so they carry long-distance and undersea traffic.
Light in a fibre travels at $v = \frac{c}{n}$ , about $2 \times 10^{8}\,\mathrm{m\,s^{-1}}$ .

Sources and credits

Photo: Optical fiber cable-06ASD.jpg by Asurnipal, Wikimedia Commons, licensed under CC BY-SA 4.0.

Written by Ganzaa: the lesson text, the cable diagram, the table, worked examples and practice exercises. The wire-pair attenuation figures are illustrative.

1Three cables, one job

2Wire pairs

3Coaxial cable

4Optical fibre

Three cables, one job

Wire pairs

Coaxial cable

Optical fibre