Cables and RJ45 plugs

## UTP cable Networks usually use **UTP (Cat5e, Cat6)** cable. Inside there are **8 wires** twisted together in pairs. The twisting protects against electromagnetic interference from outside. An **RJ45** plug is crimped onto the end of the cable. Hold the plug with its clip facing **down** and look at it from the side where the wires go in: the positions are numbered **1–8** from left to right. ## T568B — the most common | Position | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | |---|---|---|---|---|---|---|---|---| | **T568B** | white-orange | orange | white-green | blue | white-blue | green | white-brown | brown | | **T568A** | white-green | green | white-orange | blue | white-blue | orange | white-brown | brown | > 💡 How to remember it: T568B is the pair order **orange → green → blue → brown**, but the green pair "hugs" positions 3 and 6 with the blue pair squeezed in the middle. T568A just swaps the green and orange pairs. ## Straight and crossover cables Depending on which standard you use to crimp the **two ends** of the cable, you get two kinds of cable: | | End 1 | End 2 | What connects to what | |---|---|---|---| | **Straight-through** | T568B | T568B | Computer ↔ switch, switch ↔ router, printer ↔ switch | | **Crossover** | T568A | T568B | Computer ↔ computer, switch ↔ switch, computer ↔ router | **Straight cable**: every wire goes to the position with the **same number** on the other end (1→1, 2→2 … 8→8). **Crossover cable**: the send and receive wires are **swapped**: | End 1 (T568A) | 1 white-green | 2 green | 3 white-orange | 6 orange | |---|---|---|---|---| | Position on **End 2 (T568B)** | **3** | **6** | **1** | **2** | > 🧠 Rule: devices of the **same kind** (PC–PC, switch–switch) → **crossover**; **different kinds** (PC–switch) → **straight**. > > 💡 Most devices today have **Auto-MDIX**: they detect the cable type and adjust by themselves. So in practice a straight cable is almost always used, but it's still important to know the difference. ## Crimping steps 1. Strip 2–3 cm of the cable's outer jacket. 2. Untwist the pairs and straighten them. 3. Line up the colours following the standard. 4. Cut the ends evenly (leave about 1.3 cm) and push them all the way into the plug. 5. Squeeze hard with the crimping tool. Check it with a cable tester. > 🎮 Try it: **Logic games → 🔌 Cable crimping** > > 🧮 Problems: **Networks → Level 2**