Transmission media
Twisted pair and its categories, RJ45 wiring, single-mode and multimode fibre, coaxial cable and radio: speed, reach and sensitivity to disturbance.
- 24 cards
- 6 topics
- Free
The outline, topic by topic
Each topic is weighted the way the exam weighs it.
- Concepts
- Questions
- Sort and link
- On the timeline
- Scenarios
- Understand
A few cards from the deck
Twisted pair: what is it?
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A copper cable whose wires are wound in pairs so that disturbances hit both the same way and cancel out; four pairs in an Ethernet cable.
It is the medium of local networks: cheap, limited to 100 metres. Categories 5e, 6 and 6a differ in the frequency they support, hence in speed.
Which medium should be chosen for a 15 km link between two sites?
- Single-mode fibre
- Multimode fibre, the most widespread
- Shielded category 6a twisted pair
- A good-quality coaxial cable
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Single-mode fibre
Multimode stops at a few hundred metres and copper at 100 m for Ethernet. Only single-mode crosses kilometres without intermediate equipment.
The maximum length of an Ethernet twisted-pair link is … metres.
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The maximum length of an Ethernet twisted-pair link is 100 metres.
Beyond that, the signal is too weakened and distorted. To go further, you insert an active device (a switch) or move to fibre.
Crosstalk: what is it?
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The signal of one pair spilling over onto the neighbouring pair.
Twisting limits it. Untwisting too much wire at the termination, or bending a cable too tightly, brings it back, and speed suffers.
Two buildings are 400 m apart and the copper link goes down at every storm. Likely cause, then medium to lay?
- Cable category too low for the speed; replace it with shielded 6a
- Crosstalk between the cable's pairs; redo all the RJ45 connectors
- Over 100 m and lightning; lay optical fibre
- A crossover cable where a straight one would do; lay a straight one
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Over 100 m and lightning; lay optical fibre
Copper is limited to 100 m, and a long cable between two buildings picks up storm surges and electrically joins two different earths. Fibre has no limit at this distance and no conduction.