Copper: twisted pair
Most office networks use UTP (unshielded twisted pair) copper cable with RJ-45 plugs. Inside are four pairs of wires twisted together; the twists cancel out electrical noise.
| Category | Common use | Top Ethernet speed |
|---|---|---|
| Cat 5e | Older offices | 1 Gb/s (1000BASE-T) |
| Cat 6 | Most new offices | 1 Gb/s, or 10 Gb/s up to about 55 m |
| Cat 6a | Data centres, Wi-Fi access points | 10 Gb/s at the full 100 m |
Remember: Twisted-pair Ethernet has a maximum length of 100 metres per run. Longer than that and the signal arrives too weak: you get errors and lost frames.
Which wires send and which receive
10 and 100 Mb/s Ethernet use two pairs: pins 1 and 2, and pins 3 and 6. A PC or router sends on 1,2 and receives on 3,6. A switch does the opposite. Gigabit Ethernet uses all four pairs in both directions.
| Cable | Wiring | Joins |
|---|---|---|
| Straight-through | Pin 1 to pin 1, 2 to 2 ... | Unlike devices: PC to switch, router to switch |
| Crossover | Pins 1,2 swapped with 3,6 | Like devices: switch to switch, router to router, PC to PC, PC to router |
A device that sends on 1,2 is called MDI; one that sends on 3,6 (a switch) is MDI-X. Modern ports have auto-MDIX: they notice the wrong cable and swap their pins themselves, so either cable works. Auto-MDIX only works while the port's speed and duplex are both set to auto.
Fibre
Fibre cables carry light instead of electricity. They go much further than copper and are immune to electrical noise, so they connect switches between floors and buildings. A switch takes fibre through a small plug-in module called an SFP.
| Multimode (MMF) | Single-mode (SMF) | |
|---|---|---|
| Core | Wide (50 or 62.5 µm) | Very thin (9 µm) |
| Light source | LED or cheap laser | Laser |
| Typical distance at 1 Gb/s | About 550 m (1000BASE-SX) | 10 km and more (1000BASE-LX) |
| Jacket colour (usually) | Orange or aqua | Yellow |
Common mistake: Optics and fibre must match: SX optics need multimode fibre, LX optics need single-mode. Plug the wrong fibre in and the link stays down.
In the network below the two switches are joined by a 220 m multimode fibre. Click the cable between them to inspect it. show interfaces status shows each port's type:
In NetLab
▶ SW1 SW1>enable SW1#show interfaces status | include Port|Fa0/1_|Gi0/1 Port Name Status Vlan Duplex Speed Type Fa0/1 connected 1 a-full a-100 10/100BaseTX Gi0/1 connected 1 a-full a-1000 1000BaseSX SFP