Cabling a LAN¶

Exercise cabling-a-lan-annex.pdf · PDF

Wi-Fi exists. So why does every office still pull kilometres of cable through its walls and ceilings? Look up in the classroom: the access point on the ceiling has one cable, and that one cable is enough.

In Meet RouterOS you talked to your group's router. Today you build the road your packets will use: the cables of your group's LAN, made with your own hands, tested, and plugged in. Every network you configure in the coming weeks runs on top of these cables, so they have to work.

Why still cables?¶

Three reasons a cable beats Wi-Fi in an office:

• It is faster and more stable.
• It does not share the air with the neighbours.
• It can carry power to a device. That is why the access point on the ceiling needs one cable, not two.

You met the categories in Networks: Cat 5e and Cat 6 both carry 1 Gbit/s up to 100 m. Past 100 m, a copper link is out of standard.

Inside the cable¶

UTP (unshielded twisted pair) is a cable with eight copper wires in four pairs, each pair twisted, with no metal shield.

Each pair has its own colour: orange, green, blue, brown. In each pair, one wire is solid and the other is white with a stripe of that colour.

Why twisted?¶

On 19 July 1881, Alexander Graham Bell got the patent for the twisted pair. Telephone wires ran next to electric tram lines, and they hummed. If you twist the two wires of a pair, the noise hits both of them equally. The receiver listens only to the difference between the two wires, so the noise cancels out. The same trick, more than 140 years later, is inside this cable.

The T568B order¶

The plug is an RJ45. Its technical name is 8P8C: 8 positions, 8 contacts.

The order used in almost every patch cable is T568B. Hold the plug with the clip down, the gold contacts up, and the cable towards you. Pin 1 is on the left.

Three things to notice:

• Striped, solid, striped, solid... all the way.
• The green pair is split: it sits on pins 3 and 6, around the blue pair. This is the mistake everyone makes once.
• 10 and 100 Mbit/s Ethernet use only pins 1, 2, 3 and 6 (orange and green). 1 Gbit/s uses all four pairs. So a cable with a broken blue or brown wire still "works", but only at 100 Mbit/s.

Straight-through and crossover¶

T568A is the other standard: the orange and green pairs swap places.

Today's ports detect the cable and swap the pairs by themselves. That feature is called Auto MDI-X. So you always make straight-through cables, T568B on both ends.

Crimp it, test it¶

You make one cable alone; the group shares the crimping tool. The steps:

1. Strip about 2.5 cm of the outer jacket. Do not cut into the wires inside.
2. Untwist the four pairs and straighten the eight wires. Put them in T568B order, flat, between thumb and finger.
3. Cut them straight, about 13 mm from the jacket. Do not untwist more than that.
4. Push the wires into the plug until you see eight copper dots at the front. The jacket must be inside the plug, under the plastic clamp.
5. Read the colours through the plastic, left to right. Correct? Crimp once, hard.
6. Do the same on the other end. Test the cable.

flowchart LR
    S["<b>Strip</b><br>2.5 cm of jacket"] --> O["<b>Order</b><br>T568B, flat"]
    O --> C["<b>Cut</b><br>13 mm, straight"]
    C --> P["<b>Push</b><br>8 copper dots<br>at the front"]
    P --> K["<b>Check, crimp</b><br>colours through<br>the plastic"]
    K --> T["<b>Test</b><br>1 to 8 on<br>both boxes"]

Reading the cable tester¶

A cable tester has two boxes, one for each end. It sends a signal down each wire in turn.

A plug cannot be crimped twice. If a wire has no contact, cut the plug off and redo that end. The first plug usually goes in the bin: that is normal.

Where people get stuck¶

Cable the group's LAN¶

Now the cables go to work. The plan for each group of 3:

ether1  -> nothing yet (label it WAN: the way out, later in the unit)
ether2  -> PC of student A
ether3  -> PC of student B
ether4  -> PC of student C

flowchart TB
    subgraph LAN ["Group G's LAN (an island: no Internet yet)"]
        R["<b>MikroTik</b>"]
        A["PC of student A"]
        B["PC of student B"]
        C["PC of student C"]
        R ---|"ether2 · your cable"| A
        R ---|"ether3 · your cable"| B
        R ---|"ether4 · your cable"| C
    end
    R -.-|"ether1 · WAN<br>empty for now"| W(["Wall socket"])

Unplug your PC from the wall socket and plug it into your group's router with a cable you made. Then check the link on your PC:

ip -br link
cat /sys/class/net/NAME/speed

Replace NAME with your wired interface from the first command. It starts with en, for example enp3s0, but it is not always that one.

On the router, the port light is on. In WinBox, Interfaces shows the flag R (running) on that port.

Then look at ip -br address. Your PC has no IP address, or one starting with 169.254. Nobody is giving addresses in this LAN yet. The light is on, the link is up, and nothing works: no address, no ping. A cable is layer 1. The next layers (a bridge, addresses, a DHCP server) come in the next session.

Finally, draw the physical map of your group's LAN on the Exercise sheet: router, ports, cables, PCs, and who sits where. Then put the PCs back on the wall socket. The cables stay with the router, labelled.

Key takeaways¶

• UTP: eight copper wires in four twisted pairs, no shield. The twist cancels noise (Bell's patent, 1881); untwist at most 13 mm.
• The RJ45 (8P8C) plug in T568B: white-orange, orange, white-green, blue, white-blue, green, white-brown, brown. The green pair is split on pins 3 and 6.
• Straight-through (same order on both ends) is what you make; crossover is history thanks to Auto MDI-X.
• A cable tester must show 1 to 8 in order on both boxes. A cable that links at 100 Mbit/s instead of 1000 has a broken pair.
• ip -br link shows LOWER_UP when layer 1 works; /sys/class/net/NAME/speed shows the speed. One copper link: at most 100 m.

Quick check¶

Next: these cables carry nothing yet. Turn the router into the real router of 10.G.0.0/24, with a bridge, an address, a DHCP server and DNS, in Wired LAN with RouterOS.