The video call that crackles near the lift
In an old office, the video calls on one floor keep dropping. The fault is always worst near the lift.
A technician follows the data cable. It runs for 20 metres tight against the lift motor's thick power cable.
The power cable is leaking noise into the data cable.
Key words
| Word | What it means |
|---|---|
| Interference | unwanted noise that damages a signal |
| Electromagnetic interference | noise made by electrical equipment, such as motors and power cables |
| Crosstalk | a signal leaking from one wire into the wire next to it |
| Static | a build-up of electric charge that can add noise or damage equipment |
| Shielding | a metal layer around wires that blocks noise |
| Segregation | keeping two things apart, such as data cables away from power cables |
| Susceptibility | how easily a cable is affected by interference |
| Standard | an agreed set of rules for how work is done |
| Twisted pair | two wires twisted together to cancel out noise |
| Network | computers and devices joined together so they can share data |
| Packet | a small piece of data sent over a network |
| Signal | a pattern of electricity, light or radio waves that carries data |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can name three types of interference, say how to reduce each one, and explain why data and power cables are kept apart.
Starter
Have a go
Starter
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Why power and data cables are kept apart
A power cable has a changing current. That makes an invisible field around it. A copper data cable lying next to it picks up some of that field as noise.
This noise can change a one into a zero. The result is lost packets and a slow link.
This is segregation: data cables are kept apart from power cables.
Three types of interference
| Type | Where it comes from | An example |
|---|---|---|
| Electromagnetic | Motors, lights, power cables, radio transmitters | A cable next to a lift motor |
| Static | A build-up of charge, often from carpet, dry air and friction | A crackle on a link, or a damaged part |
| Crosstalk | Signals leaking between wires in the same cable | Two pairs in one cable disturb each other |
The cells students fill in are left blank in this preview.
How to reduce interference
Shielding: use a cable with a metal foil or braid around the wires. It blocks noise from reaching the signal.
Twisted pairs: the wires are twisted so noise hits both equally and cancels out. A tighter twist cuts crosstalk too.
Keep apart: put data cables a safe gap from power cables.
Light instead of copper: fibre carries light, which electrical noise does not touch.
Running cables side by side
Cables that run side by side for a long way pick up the most noise. So where data and power must run the same way, keep a set gap between them, and put them in separate trays.
Where they must cross, cross at a right angle, 90 degrees. This keeps the time they are close very short.
Data cables of the same kind can run in parallel together, neatly.