Ethernet cables, often called network cables or LAN cables, are meticulously wired internally to ensure reliable data transmission between devices. At their core, an Ethernet cable features 8 pins connected to 4 twisted pairs of wires.
The Foundation: Pins, Pairs, and Signals
The standard connector used for Ethernet cables, typically an RJ45, presents 8 contact points, or pins. These pins are the interface where the cable connects to network devices like computers, routers, and switches. Inside the cable jacket, these 8 pins are wired to four pairs of insulated copper wires. These pairs are twisted together along the length of the cable.
Ethernet technology transmits data by representing signals using the difference in voltage in a twisted pair. This differential signaling helps to reject noise and interference, ensuring data integrity over distance.
Wiring for Different Ethernet Speeds
The way these twisted pairs are utilized depends on the speed of the Ethernet connection.
For common older standards like 10 Mbps (Megabits per second) and 100 Mbps Fast Ethernet, only two of the four twisted pairs are actively used for data transfer. As noted in the reference: "For 10 Mbps and 100 Mbps, one pair is used to transmit data, a second pair receives data. The other two pairs are unused." This means that for these speeds, two pairs carry the outgoing data signals, and two pairs carry the incoming data signals, while the remaining two pairs are present in the cable but not electrically connected for data transmission.
Faster Ethernet standards, such as Gigabit Ethernet (1000 Mbps) and beyond, utilize all four twisted pairs simultaneously for both transmitting and receiving data, enabling much higher bandwidth.
Common Wiring Standards: T568A and T568B
To ensure interoperability between devices and cables, two primary wiring standards exist: TIA/EIA-568-A (T568A) and TIA/EIA-568-B (T568B). These standards define the specific order in which the wires from the four twisted pairs are terminated onto the 8 pins of the RJ45 connector.
The key difference between T568A and T568B is the swapping of the orange and green wire pairs.
Here's a simplified look at the pinouts (viewed from the front of the RJ45 connector, latch down):
| Pin | T568A Color | T568B Color | Pair | Function (10/100 Mbps) |
|---|---|---|---|---|
| 1 | White/Green | White/Orange | 3 | Transmit+ |
| 2 | Green | Orange | 3 | Transmit- |
| 3 | White/Orange | White/Green | 2 | Receive+ |
| 4 | Blue | Blue | 1 | Unused / Bi-directional |
| 5 | White/Blue | White/Blue | 1 | Unused / Bi-directional |
| 6 | Orange | Green | 2 | Receive- |
| 7 | White/Brown | White/Brown | 4 | Unused / Bi-directional |
| 8 | Brown | Brown | 4 | Unused / Bi-directional |
- Straight-Through Cable: The most common type, used to connect unlike devices (e.g., computer to switch). Both ends are wired using the same standard (either T568A on both ends or T568B on both ends).
- Crossover Cable: Used to connect like devices directly (e.g., computer to computer, switch to switch) without an intervening switch or router. One end is wired using T568A, and the other end is wired using T568B. This swaps the transmit and receive pairs so that the transmit pins on one end connect to the receive pins on the other. (Note: Modern network equipment often has auto-MDI/MDIX capability, which automatically adjusts for cable type, making crossover cables less necessary).
The Importance of Twisting
The reason the pairs are twisted is crucial for minimizing electromagnetic interference (EMI) and crosstalk (signal bleed between pairs). Twisting the wires causes any induced noise voltage to be roughly equal but opposite in each wire of the pair, effectively canceling out the noise when the differential voltage is read by the network device.
In summary, Ethernet cables are wired with 8 pins connected to 4 twisted pairs. While the core mechanism involves using voltage differences across these pairs for signaling, the specific utilization of the pairs varies by speed, with 10/100 Mbps using only two pairs (one for transmit, one for receive) and faster speeds using all four. The specific order of wires on the pins follows standard schemes like T568A and T568B.
