Well Explained Note on RJ45
by Joses Maombi Chukwuemeka
1. What is an RJ45 Connector?
The RJ45 (Registered Jack 45) is a standardized physical network interface used primarily for Ethernet networking. It’s the clear, rectangular plug you see at the end of most network cables (CAT5, CAT5e, CAT6, etc.), containing 8 gold‑plated contacts. It’s designed to connect computers, routers, switches, and other network devices.
2. Physical Characteristics
- 8P8C design – 8 Positions, 8 Contacts. All 8 positions are typically wired.
- Transparent housing – Allows visual inspection of wire order.
- Locking tab – Secures the connector in the port; prevents accidental disconnection.
- Crimp‑on style – Most common for field termination; requires a crimping tool.
3. Wiring Standards: T568A vs T568B
Two main pinout schemes exist for terminating twisted‑pair cables with RJ45 plugs. They differ only in the color order of two pairs (orange and green). Functionally identical if both ends use the same standard.
| Pin |
T568A Color |
T568B Color |
Signal (100BASE‑TX) |
| 1 |
White/Green |
White/Orange |
TX+ |
| 2 |
Green |
Orange |
TX- |
| 3 |
White/Orange |
White/Green |
RX+ |
| 4 |
Blue |
Blue |
Unused (or PoE) |
| 5 |
White/Blue |
White/Blue |
Unused (or PoE) |
| 6 |
Orange |
Green |
RX- |
| 7 |
White/Brown |
White/Brown |
Unused (or PoE) |
| 8 |
Brown |
Brown |
Unused (or PoE) |
Quick tip: A “straight‑through” cable uses the same standard (A‑A or B‑B) on both ends. A “crossover” cable uses T568A on one end and T568B on the other.
4. Common Cable Types Using RJ45
- CAT5e – Up to 1 Gbps (1000BASE‑T), 100 MHz bandwidth.
- CAT6 – Up to 10 Gbps (limited distance), 250 MHz, thicker conductors.
- CAT6a – 10 Gbps up to 100 meters, 500 MHz, better shielding.
- CAT7/8 – Higher frequencies; often use shielded RJ45 connectors (GG45 or TERA, but backward‑compatible).
5. Applications Beyond Ethernet
While synonymous with networking, RJ45 is also used for:
- Serial console connections (RS‑232 via RJ45 on many network devices).
- Analog/digital telephony (some PBX systems).
- Power over Ethernet (PoE) – delivering DC power over pins 4‑5 (+) and 7‑8 (-) or alternative pairs.
6. How to Crimp an RJ45 Connector (Brief Steps)
- Strip about 2 cm of the cable jacket.
- Untwist the pairs and arrange wires according to T568A or B.
- Flatten the wires, trim them to 1.5 cm, and carefully insert into the RJ45 plug.
- Ensure all 8 wires reach the front pins and the jacket enters the strain relief.
- Crimp firmly with an RJ45 crimping tool.
- Test with a cable tester for continuity and correct pairing.
7. Quick Troubleshooting
- No link light: Check if both ends follow the same wiring standard.
- Intermittent connectivity: May indicate a poorly crimped pin; re‑crimp or replace.
- Flattened or broken locking tab: Replace the connector—a loose plug causes random disconnects.
—— End of Note ——