Ethernet RJ45 Connector Wiring: T568A vs T568B Standard

Ethernet cable wiring is one of the most misunderstood topics in networking. Walk into any office building or computer lab in India and you will find a mix of T568A and T568B terminated cables — often in the same installation. While both standards work perfectly well for data transmission, mixing them incorrectly on a single cable creates a crossover cable that does not connect standard devices as expected. Understanding the difference prevents costly troubleshooting and failed network installations.

This guide explains the T568A and T568B wiring standards, how to terminate RJ45 connectors, when to use straight-through vs crossover cables, and how to test your work.

RJ45 Connector Basics

RJ45 (Registered Jack 45) is the 8-position 8-contact (8P8C) connector used for Ethernet networking. Despite being called “RJ45,” the correct technical name for the Ethernet version is 8P8C — true RJ45 is a single-pair telephone standard. However, “RJ45” is universally used in the networking industry to refer to the Ethernet connector.

The connector has 8 pins numbered 1 through 8 from left to right when looking at the front face of the plug (with the tab at the bottom).

Pin Numbering

RJ45 Front Face View (tab facing down):
|  1  2  3  4  5  6  7  8  |
[          tab              ]

Pin 1 = leftmost
Pin 8 = rightmost

T568A vs T568B: The Difference

Both standards use the same 4 twisted pairs of wire and deliver identical electrical performance. The only difference is which color goes to which pin number. The two standards swap the positions of the green and orange pairs.

T568A Wiring Order (Pin 1 to Pin 8)

T568B Wiring Order (Pin 1 to Pin 8)

Summary: What Changes

Between T568A and T568B, only pairs 2 and 3 swap positions:

• T568A: Orange pair on pins 1-2, Green pair on pins 3-6
• T568B: Green pair on pins 1-2, Orange pair on pins 3-6
• Blue pair (pins 4-5) and Brown pair (pins 7-8) stay the same

Which Standard to Use in India

T568B is the dominant standard in India and most of the world for commercial installations. Most structured cabling contractors default to T568B. Use T568B for new installations unless you are extending an existing T568A infrastructure.

T568A is required for US government installations per ANSI/TIA-568 standards. International: ISO/IEC 11801 recommends T568A.

The most important rule: use the same standard on both ends of every patch cable.

Straight-Through vs Crossover Cable

Straight-Through Cable

Both ends use the same wiring standard (both T568A or both T568B). Pin 1 on one end connects to Pin 1 on the other end.

Use for: Computer to switch/router, switch uplink port to router, most modern connections. 99% of Ethernet cables you encounter are straight-through.

Crossover Cable

One end is T568A, the other is T568B. This crosses the transmit and receive pairs, allowing two similar devices to communicate directly without a switch.

Use for: Direct computer-to-computer connections, daisy-chaining older switches without uplink ports, connecting two routers back-to-back.

Modern note: Most network equipment made after 2005 supports Auto-MDI/MDI-X — it automatically detects and compensates for crossover, so straight-through cables work between any two modern devices. Crossover cables are rarely needed today.

Quick Identification

Hold both RJ45 connectors side by side with tabs facing the same direction:

• If the wire colors match on both sides → Straight-through cable
• If pins 1-2 on one end match pins 3-6 on the other end → Crossover cable

How to Terminate an RJ45 Connector

Tools Required

• RJ45 crimp tool (crimper)
• Cable stripper or utility knife
• RJ45 connectors (buy extra — termination takes practice)
• Cable tester (optional but recommended)

Step-by-Step Termination (T568B)

1. Strip the cable: Remove 25-30mm of outer jacket. Use cable stripper at 25mm mark to avoid nicking inner wires.
2. Untwist pairs: Separate all 4 pairs. Untwist each pair only as much as needed — keep twists as close to the termination point as possible (maximum 13mm untwisted for Cat6).
3. Order the wires (T568B): Left to right: White/Orange, Orange, White/Green, Blue, White/Blue, Green, White/Brown, Brown
4. Align and trim: Hold wires in order between thumb and forefinger. Trim to approximately 13mm from the jacket end — all wires at exactly the same length.
5. Insert into RJ45: Slide all 8 wires into the connector simultaneously. Each wire must reach the front metal contacts. The jacket should enter the connector and be gripped by the strain relief tab.
6. Verify positions: Before crimping, view from the front of the connector. Verify the wire order matches T568B from left to right (pin 1 = leftmost = White/Orange).
7. Crimp: Insert connector into crimper, squeeze firmly until it stops. The crimper simultaneously drives the metal IDC contacts into each wire and secures the strain relief.
8. Repeat for other end
9. Test with cable tester

Common Termination Mistakes

• Wires not all reaching the contacts (too much untwisting, cable too short)
• Wire order wrong (common: confusing White/Green and White/Orange)
• Jacket not gripped by strain relief (cable will fail under tension)
• Too much untwisting near the connector (degrades high-frequency performance)

Keystone Jack Wiring (Wall Plates)

Keystone jacks (110 punch-down type) are used for wall outlets and patch panels. They use IDC (Insulation Displacement Contact) termination instead of crimping.

Punch-Down Steps

1. Most keystone jacks are labeled with both T568A and T568B color codes
2. Route each wire into the labeled slot (choose T568A or T568B — consistent with your installation)
3. Use a 110 punch-down tool (impact-style) to seat and trim each wire
4. The blade side of the punch-down tool faces the outside (the wire you are trimming)
5. Punch down firmly — the tool cuts the wire flush with the IDC contact

PoE Wiring Considerations

Power over Ethernet (PoE) transmits DC power over Ethernet cable pairs alongside data. Standard PoE (IEEE 802.3af, up to 15.4W) and PoE+ (802.3at, up to 30W) use the same pins as data. PoE++ (802.3bt, up to 90W) uses all 4 pairs.

For PoE installations:

• Use Cat5e minimum (Cat6 strongly recommended for long runs)
• Maximum cable length: 100m (same as regular Ethernet)
• Wire gauge: 24 AWG minimum. 23 AWG preferred for PoE to reduce voltage drop over distance.
• Voltage drop calculation: at 100m of 24 AWG, expect ~5V drop at 1A. Use larger wire gauge or shorter runs for high-power PoE devices.
• T568A and T568B both work identically with PoE

Cable Testing

Basic Cable Tester (Continuity Tester)

A simple cable tester checks that all 8 pins are connected and not crossed. Shows pass/fail for each pin pair. Essential for every termination — available in India from Rs 300-800 at Robu.in, Amazon, or electronics shops.

Fluke/Advanced Cable Tester

For professional installations, a certification tester (Fluke DSX, Softing, etc.) verifies cable performance to Cat6/Cat6A specifications. Measures insertion loss, return loss, NEXT (Near-End Crosstalk), and propagation delay. Required for infrastructure installations with performance guarantees.

Reading Cable Tester Results

• All LEDs 1-8 light in sequence = straight-through cable, correct pinout
• Lights 1 and 3 swap, 2 and 6 swap = crossover cable
• Any LED does not light = open circuit (broken wire or bad contact)
• Multiple LEDs light for one = short circuit (two wires touching)

Cat5e vs Cat6 vs Cat6A in India

Recommendation for India: Use Cat6 for new installations — it supports 1 Gbps at full 100m and is future-ready for 10G at shorter distances. Cat5e is acceptable for retrofits or budget projects. Cat6A is warranted for data centers or when 10G runs exceed 55m.

Common RJ45 Wiring Mistakes

• Wrong standard on one end: Accidentally terminating one end T568A and the other T568B creates a crossover cable. Label your cables before and after termination to track which standard you used.
• Exceeding maximum untwist: Cat6 allows maximum 13mm of untwisted wire near the connector. More untwisting reduces NEXT performance. Keep pairs twisted until the last possible moment.
• Using cheap connectors: Low-quality RJ45 connectors have soft contacts that do not fully penetrate the wire insulation. Use reputable brands (Belden, Panduit, AMP). In India, Honeywell/Genesis and D-Link branded connectors are reliable.
• Not testing: A cable that looks correct may have an intermittent connection due to a wire that did not quite seat in the contact. Always test with a cable tester.
• Exceeding 100m segment length: Ethernet segments are rated for maximum 100m including patch cables. A 90m in-wall run plus two 5m patch cables = 100m total. Keep track of total segment length.

Frequently Asked Questions