Cables, fibre and Ethernet standards

Why the cable matters

Layer 1 problems, such as a bad cable or the wrong cable type, cause a large share of real network faults. Knowing what each cable can do, and how far, saves hours of guessing.

UTP copper cable

UTP means Unshielded Twisted Pair: 8 copper wires twisted into 4 pairs inside one jacket. The twisting cancels electrical noise from nearby cables and motors. The plug at each end is an RJ45 connector, and the socket on the wall or switch is an RJ45 port. Cables are sold by category (Cat). Higher categories are built to handle higher frequencies, so they carry faster speeds.

The maximum length of one copper Ethernet run is 100 metres, from the switch to the device, including patch cords. Longer runs need a switch in between or fibre.

Straight-through, crossover and Auto-MDIX

Copper Ethernet sends on some wires and receives on others. A PC, router or server sends on one pair; a switch or hub expects to receive on that pair. So a PC-to-switch link uses a straight-through cable (same wire order at both ends). Two devices of the same kind, such as switch to switch, PC to PC, or router directly to PC, need a crossover cable, where the send and receive pairs are swapped.

Most modern switches and NICs support Auto-MDIX: the port detects the cable type and swaps the pairs itself, so either cable works. Almost all gigabit ports handle it. You still need to know the rule for exams and for old equipment.

Fibre optic cable

Fibre carries pulses of light through a thin glass core. It is immune to electrical interference, cannot be tapped easily, and reaches much further than copper. It is used between buildings, between floors (backbone links), in data centres, and by ISPs for FTTH.

• Multimode fibre (MMF): a wider core, cheaper optics, used for distances of a few hundred metres, inside buildings and data centres. Usually an orange or aqua jacket.
• Single-mode fibre (SMF): a very thin core (about 9 micrometres) and laser light, reaching many kilometres. Used between campus buildings and by ISPs. Usually a yellow jacket.

Reading Ethernet standard names

A name like 1000BASE-T has three parts: the speed in Mbps (1000), BASE for baseband signalling, and the medium (T for twisted pair copper; S and L for short and long wavelength fibre).

SFP modules

Many switches have empty slots called SFP ports. You plug in a small transceiver module (SFP for 1 Gbps, SFP+ for 10 Gbps) that matches the fibre type and distance you need, such as an SX module for multimode. This lets one switch model serve many cabling situations.

Power over Ethernet (PoE)

PoE lets a switch send power down the same UTP cable as data, so IP phones, access points and CCTV cameras need no separate power socket. The standards, with the power a switch port can supply:

• 802.3af (PoE): up to 15.4 W per port. Enough for IP phones and simple cameras.
• 802.3at (PoE+): up to 30 W. Common for access points and PTZ cameras.
• 802.3bt (PoE++): up to 60 W (Type 3) or 90 W (Type 4). For powerful APs, displays and small devices.

Some power is lost in the cable, so the device receives a little less than the port supplies. Check the switch total PoE budget too: a 24-port switch may not power all 24 ports at full power at once.