In Ethernet standards, we use copper cables called UTP cables (Unshielded Twisted Pair).
“Unshielded” means that the wires have no metallic shield, which makes them vulnerable to electrical interference.
This is why the wires are “twisted” together, as it helps protect against electromagnetic interference (EMI).
UTP cables and RJ-45
UTP cables have either 2 pairs of cables twisted together (4 wires), or 4 pairs of cables twisted together (8 wires).
This depends on the Ethernet standard used:
- 10BASE-T and 100BASE-T use 2 pairs (4 wires)
- 1000BASE-T and 10GBASE-T use 4 pairs (8 wires)
We use RJ-45 (Registered Jack) connectors at the end of a copper Ethernet cable (UTP):
We plug them in RJ-45 interfaces/ports.
A switch (image above) usually have 24 or more RJ-45 ports, so many end-hosts can connect to it.
The two other ports on the right are called SFP ports, but this will be for another article.
Straight-through and Crossover cables
10BASE-T and 100BASE-T UTP Cables
As I said above, 10BASE-T and 100BASE-T Ethernet standards use 2 pairs of wires (4 wires in total).
Routers, firewalls, and PCs transmit data on pins 1 and 2 and receive data on pins 3 and 6.
Switches and hubs do the contrary, as they transmit data on pins 3 and 6 and receive data on pins 1 and 2:
This is an example of a straight-through cable.
We use a straight-through cable to connect 2 devices of different types, such as router and switch/hub, firewall and switch/hub, pc and switch/hub, server and switch/hub, etc…
By the way, when two devices transmit and receive data at the same time, we call this a full-duplex transmission. As the wires to transmit and those to receive data are separate, no collision will occur.
On the other hand, when we need to connect 2 devices of the same type (switch/switch, router/router, pc/router, pc/pc, firewall/router, hub/hub, switch/hub), we have to use a crossover cable.
In fact, since devices of the same type receive and transmit on the same pins, the transmission simply wouldn’t work.
In a crossover cable, the pins are reversed on each end:
However, most current network devices have a function called Auto MDI-X that allows them to recognize when the wrong cable is being used and automatically adjust their logic to make the connection work.
1000BASE-T and 10GBASE-T UTP Cables
1000BASE-T and 10GBASE-T Ethernet standards use 4 pairs, so all 8 wires.
Each pair is bidirectional, which means it is only used for transmitting or receiving data:
Resources:
Jeremy’s IT Lab: Free CCNA | Interfaces and Cables | Day 2 | CCNA 200-301 Complete Course