Network Fundamentals · Topic 3 of 17
Physical Interfaces and Cabling Types
Compare copper, multimode fiber and single-mode fiber, understand shared media versus point-to-point Ethernet, and see how PoE uses twisted-pair cabling.
- 5 min
- Exam objective 1.3
- UTP copper reaches 100 metres per run and can carry PoE.
- Fiber is immune to electrical interference and does not carry PoE.
- Multimode fiber is for shorter distances. Single-mode fiber is for longer distances.
What it is
Every wired link needs a physical medium and an interface that fits it.
The medium decides how far the signal can travel, how fast it can go and how much it costs. The interface decides what kind of cable or module plugs in.
This topic compares the cable types a CCNA candidate should know: copper, single-mode fiber and multimode fiber. It also looks at the two Ethernet connection styles, shared media and point-to-point, and at how PoE fits in.
How it works
Copper carries electricity, fiber carries light, and the link style decides whether devices share the wire.
Copper
The most common copper cable is unshielded twisted pair (UTP) with RJ-45 connectors. It contains four pairs of wires, and the pairs are twisted to reduce interference.
Copper is cheap and easy to install, and it can also carry power for PoE.
The standard reach of Ethernet over twisted pair is 100 metres for each cable run. Category ratings show how much speed the cable supports.
For example, Category 5e supports 1000BASE-T. Category 6a supports 10GBASE-T at the full 100 metres, while Category 6 supports it only over shorter runs, around 55 metres.
Copper is affected by electrical interference and by signal loss over distance. It is not a good choice for long runs or for areas with heavy electrical noise.
Two wiring styles are used:
| Cable | Connects | Example |
|---|---|---|
| Straight-through | Unlike devices | PC to switch |
| Crossover | Like devices | Switch to switch |
Most modern ports use auto-MDIX to detect the wiring and adjust, so either cable usually works.
Fiber
Fiber carries light through a thin glass strand instead of electricity through a wire.
It is immune to electrical interference, supports long distances and very high speeds, and it does not carry power for PoE.
There are two types:
- Multimode fiber (MMF) has a wider core. Light travels along several paths at once. It is cheaper and is used for shorter runs, such as within a building or between buildings on a campus. Distances are in the range of hundreds of metres.
- Single-mode fiber (SMF) has a very narrow core. Light travels along one path. It carries signals much further, up to many kilometres, and is used for long links between buildings, cities or sites. The optics are more expensive.
Exact distances depend on the transceiver and the speed. The relationship stays the same: single-mode goes further than multimode, and multimode goes further than copper.
Fiber connects to switches through transceivers such as SFP and SFP+ modules. The module must match the cable type and the other end. A multimode module will not work properly on single-mode cable.
A quick comparison:
| Feature | Copper UTP | Multimode fiber | Single-mode fiber |
|---|---|---|---|
| Signal | Electrical | Light | Light |
| Typical reach | Up to 100 m | Hundreds of metres | Kilometres |
| Interference | Affected | Immune | Immune |
| Cost | Lowest | Medium | Highest optics |
| PoE | Yes | No | No |
Reach runs copper, then multimode, then single-mode, from shortest to longest. A narrow core means single-mode.
Shared media and point-to-point
Early Ethernet used shared media. Many devices connected to one cable or to a hub, and they took turns using it.
Only one device could send at a time, so the devices used CSMA/CD to detect and recover from collisions. This is half duplex: a device sends or receives, but not both at once.
Modern Ethernet uses point-to-point links. Each switch port connects to exactly one device.
There is no sharing, so there are no collisions and the link can run in full duplex, sending and receiving at the same time. Because there is no contention, CSMA/CD is not needed on full-duplex links.
PoE concepts
PoE sends power on the same copper pairs that carry data.
The switch senses whether the connected device needs power before it provides any, so an ordinary PC is not harmed. PoE needs twisted-pair copper, since fiber cannot carry power.
What to remember for the exam
Eight facts cover the cabling objective.
- UTP copper reaches 100 metres per run and can carry PoE.
- Fiber is immune to electrical interference and does not carry PoE.
- Multimode fiber is for shorter distances. Single-mode fiber is for longer distances.
- Fiber ports use transceivers such as SFP, and the type must match the cable.
- Straight-through connects unlike devices. Crossover connects like devices. Auto-MDIX can hide this.
- Shared media is half duplex and uses CSMA/CD.
- Switched point-to-point links can run full duplex with no collisions.
- PoE is delivered over twisted pair, not over fiber.
Collisions on a modern switch port are not normal. They point to half duplex, a hub or a duplex problem.
Common mistakes
These five slips come up often.
Mixing up multimode and single-mode. Single-mode has the narrow core and the longer reach. Plugging the wrong module into a link causes poor or no connectivity.
Forgetting the 100-metre limit. A copper run longer than the standard limit can cause errors even if the link comes up.
Assuming collisions happen on every Ethernet link. On a full-duplex switch port, collisions do not occur. They point to half duplex, a hub or a duplex problem.
Expecting PoE on fiber. Fiber has no electrical conductor, so a powered device on a fiber link needs a separate power source.
Counting on the cable type to fix everything. A higher category cable does not help if the run is too long or the ports are mismatched.
Check yourself
Work through these three prompts.
How do multimode and single-mode fiber compare in core size, reach and cost?
Multimode has a wider core, reaches hundreds of metres and is cheaper. Single-mode has a very narrow core, reaches up to many kilometres and has more expensive optics.
Why does a switched full-duplex link not need CSMA/CD?
Each switch port connects to exactly one device, so there is no sharing and no collisions. With no contention, CSMA/CD is not needed.
When would you choose copper over fiber, and when is fiber the only sensible choice?
Choose copper for short runs up to 100 m where low cost, easy installation or PoE matters. Fiber is the sensible choice for long runs or areas with heavy electrical noise, and it is immune to interference.
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