RJ45 and RJ11 Connectors Explained: Crimping for the Network+ Exam

Crimping puts a modular plug onto the end of a cable. It is the counterpart to punching down, and the exam tests which technique belongs where as much as it tests the mechanics.

RJ45 versus RJ11

	RJ45	RJ11
Positions / conductors	8P8C	6P2C or 6P4C
Width	Wider	Narrower
Used for	Ethernet	Analogue telephone, DSL

The notation is worth knowing: 8P8C means eight positions with eight conductors. RJ11 is typically 6P2C — six positions, two wired.

An RJ11 plug physically fits into an RJ45 jack because it is narrower, and doing so can damage the jack's outer contacts. An RJ45 plug will not fit an RJ11 jack. If a scenario describes a phone cable plugged into a network port, that is the situation being described.

Solid versus stranded

The distinction that decides crimping versus punching down, and a reliable exam point.

Solid-core cable has one solid conductor per wire. Better performance over distance, and it fractures if flexed repeatedly. Used for permanent runs in walls and ceilings, terminated by punching down into patch panels and keystone jacks.

Stranded cable has many thin strands per conductor. Flexible and durable under movement, with slightly higher attenuation. Used for patch leads, terminated by crimping RJ45 plugs.

Plugs are manufactured for one or the other. A stranded plug's contacts pierce between strands; a solid plug's contacts straddle the single conductor. Using the wrong plug produces a connection that tests fine initially and fails intermittently later, which is among the most frustrating faults to chase.

So: solid to punchdown, stranded to crimp, and match the plug type to the cable.

T568A and T568B

Two wiring standards, both valid. What matters is consistency.

T568B: orange/white, orange, green/white, blue, blue/white, green, brown/white, brown.
T568A: swaps the orange and green pairs — green/white, green, orange/white, blue, blue/white, orange, brown/white, brown.

Same standard both ends gives a straight-through cable. Different standards at each end gives a crossover. Modern equipment with Auto-MDIX handles either, so crossover cables are rarely needed now — but in structured cabling a mismatch is still a fault, and it will confuse whoever troubleshoots it next.

Pins 1, 2, 3 and 6 carry data on 10/100 Ethernet. Gigabit and above use all four pairs, which is why a cable with a fault on the blue or brown pair may work at 100 Mbps and fail at gigabit — a genuinely common and confusing symptom.

Crimping properly

Strip about 25 mm of jacket without nicking the insulation of the conductors. Untwist only as much as necessary and keep it under 13 mm, because the twist is what cancels crosstalk. Arrange the wires in order, flatten them, and trim them square so they all reach the end of the plug.

Insert so that the jacket goes inside the plug and is captured by the strain relief — this is the step most often done badly, and a plug crimped onto bare conductors will fail as soon as the cable is moved. Confirm all eight wires are visible at the front of the plug before crimping.

Then crimp firmly and fully. A partial crimp leaves the contacts not fully seated into the conductors.

Testing

A cable tester checks continuity and pin mapping. A certifier measures against the category standard — crosstalk, attenuation, return loss — and continuity alone does not prove a Cat6 link will run at 10 Gbps.

The faults a tester reveals: an open, where a wire does not connect; a short between conductors; reversed pair, where two wires in a pair are swapped; crossed pairs, where two pairs are transposed; and a split pair.

The split pair deserves special mention because it is the one that catches people. Each pin connects to the correct pin at the far end, so a simple continuity tester shows a perfect cable — but the wires have been taken from different twisted pairs, destroying crosstalk cancellation. It passes continuity and fails certification, and it produces errors at gigabit speeds while appearing fine at 100 Mbps.

When to crimp at all

Worth noting for practical questions: in structured cabling, field-terminated plugs are generally discouraged. The professional approach is punching down to patch panels and jacks and using factory-made, tested patch leads. Crimping is for making a lead when you need one, and for exam questions about the technique.

What to remember

RJ45 is 8P8C for Ethernet, RJ11 is 6P2C for telephone and fits into an RJ45 jack. Solid cable is punched down, stranded is crimped, and plugs are specific to each. T568A and T568B differ by swapping the orange and green pairs; same at both ends is straight-through. Keep untwist under half an inch and capture the jacket in the strain relief. A split pair passes continuity and still fails.

Related reading: Punchdown tools and 110 blocks · Toner and probe · Network types