Twisted pair
Ethernet copper cable has four pairs of wires, and the two wires in each pair are twisted together. The twisting makes outside interference affect both wires equally, so the receiver can cancel it out. It also reduces crosstalk (signals leaking from one pair into another). Most cable is UTP (unshielded); STP/FTP types add foil or braid shielding for electrically noisy environments.
UTP vs. STP
| UTP (unshielded twisted pair) | STP / FTP (shielded) | |
|---|---|---|
| How it reduces noise | Twisting alone | Twisting plus foil around each pair and/or a braid around the whole cable |
| Names you will see | U/UTP | F/UTP (overall foil), U/FTP (foil per pair), S/FTP (braid + foil per pair) |
| Cost and handling | Cheaper, thinner, easier to bend and terminate | More expensive, stiffer, needs shielded plugs and jacks |
| Grounding | None needed | The shield must be grounded at the patch panel, or it can make noise worse |
| Typical use | Homes and most offices | Factories, near motors or lift machinery, and many Cat6a/Cat7 installations in Europe |
💡 Do not confuse STP the cable (shielded twisted pair) with STP the protocol (Spanning Tree Protocol, a CCNA topic that prevents switching loops). They share the same letters but have nothing in common.
Categories
| Category | Rated bandwidth | Typical use |
|---|---|---|
| Cat5e | 100 MHz | 1 Gbps at 100 m (also 2.5 Gbps) |
| Cat6 | 250 MHz | 1 Gbps at 100 m; 10 Gbps up to about 55 m |
| Cat6a | 500 MHz | 10 Gbps at 100 m: the usual choice for new installations |
| Cat8 | 2000 MHz | 25/40 Gbps up to 30 m, inside data centres |
The 100 m limit applies to the whole channel: the patch leads plus the cable in the wall.
The RJ45 connector
The clear plastic plug on the end of an Ethernet cable is called RJ45 (strictly, an 8P8C plug: 8 positions, 8 contacts). It holds all 8 wires, one for each pin, and clicks into the matching socket (a jack) on a PC, switch or wall plate.
- RJ45 vs. RJ11: a telephone or DSL plug (RJ11) is narrower, with 4 or 6 positions. It fits loosely in an RJ45 socket but will never work for Ethernet.
- Plugs must match the cable: Cat6a and shielded cables use larger or shielded plugs. A low-cost Cat5e plug on Cat6a cable can ruin the link's performance.
- The latch: a broken latch lets the plug work loose. This is a classic cause of a link that drops when someone bumps the desk.
RJ45 pinouts
| Speed | Pairs used |
|---|---|
| 10BASE-T, 100BASE-TX | Two pairs: pins 1-2 transmit, 3-6 receive (on a PC) |
| 1000BASE-T and faster | All four pairs, transmitting and receiving at the same time |
Straight-through vs. crossover
On 10 and 100 Mbps links, a PC transmits on pins 1-2, and a switch expects to receive on pins 1-2. So a PC-to-switch cable can be wired the same way at both ends (straight-through). Two devices of the same type (PC to PC, switch to switch) both transmit on the same pins, so the classic rule was to use a crossover cable (T568A at one end, T568B at the other).
- 1. Different device types: a PC-to-switch link works with a straight-through cable.
- 2. Same device type: a switch-to-switch link traditionally needs a crossover cable, so that one side's transmit pins reach the other side's receive pins.
💡 Auto-MDIX, found on almost every modern port, detects the cable type and swaps transmit and receive internally, so either cable works. Nearly all gigabit ports support it, which is why you rarely need a crossover cable today. You will still see the rule in exams, though.
Which cable should I use?
- Patch leads at home or in the office: Cat5e or Cat6 UTP is enough for 1 Gbps (and 2.5 Gbps).
- New cabling in walls: Cat6a, so the building is ready for 10 Gbps for years to come.
- Electrically noisy places: shielded (F/UTP or S/FTP) cable with properly grounded panels.
- Longer than 100 m, or between buildings: not copper at all. Use fibre instead.
The cable also affects the link speed both ends agree on. A damaged pair often shows up as a gigabit link stuck at 100 Mbps.
Learn more: Speed and Duplex
- Ethernet copper cable has four twisted pairs. UTP is the norm; shielded types are for electrically noisy places.
- Cat5e and Cat6 handle 1 Gbps; Cat6a is the usual choice for 10 Gbps at 100 m.
- RJ45 (8P8C) connectors hold all 8 wires; wiring follows T568A or T568B.
- Auto-MDIX means straight-through cables work almost everywhere today.
Check yourself
You are cabling a new office and need 10 Gbps on runs of up to 100 m. Which category is the usual choice?
You need to connect two old switches that do not support Auto-MDIX. Which cable should you use?