Course contents

Unit 4: Ethernet and CablingLesson 4.9 (9 of 11 in this unit)38 of 104 in the Network Fundamentals course

Network Connectors

Pick up any network cable and the first clue to what it does is the plug on the end. This lesson shows the connectors you will meet, how to tell them apart, and how to choose between them.

Beginner · 10 min read · Before this: Copper cabling, Fibre-optic cabling

Covers RJ45 and RJ11, BNC and F-type, LC, SC, ST and MPO, SFP and QSFP modules and Choosing a connector.

Technical definition

A network connector is the fitting on the end of a cable or port that locks it to a device and lines up the conductors or fibres so a signal can pass. The connector type tells you the cable family: twisted pair, coaxial or fibre.

Connectors you will meet, front view
Connectors you will meet, front viewSeven common network connectors. RJ45: Ethernet over twisted pair. RJ11: telephone and some DSL lines. BNC: twist-lock coaxial, legacy Ethernet and video. F-type: screw-on coaxial for cable broadband. LC: small fibre connector used on transceivers. SC: larger push-pull fibre connector. SFP: a pluggable transceiver module that takes fibre or copper cables.RJ45Ethernet over twisted pair8 contacts, latch tabRJ11telephone, some DSL linesnarrower, 4 or 6 contactsBNCtwist-lock coaxlegacy Ethernet, video, CCTVF-typescrew-on coaxcable TV and cable modemsLCsmall fibre connectorcommon on transceiversSCsquare push-pull fibreolder, larger than LCSFP modulepluggable transceivertakes LC fibre or RJ45Rule of thumbround, screws or twists:coaxialsmall and clicks:fibre or twisted pair

On small screens, swipe sideways or focus the diagram and use the arrow keys.

The shape tells you the cable family. Connectors are drawn front-on and simplified; sizes are not to scale.

Twisted-pair connectors: RJ45 and RJ11

The RJ45 (strictly an 8P8C modular plug) is the connector on almost every Ethernet cable. It holds eight contacts for four twisted pairs, and a plastic latch tab clicks it into the port. The wiring order, T568A or T568B, is covered in Copper cabling.

The RJ11 looks like a smaller sibling. It is built for telephone lines and for the line side of some DSL modems, and it carries fewer pairs. Because an RJ11 plug can slide into an RJ45 socket, a phone cable in a network port is a classic way to bend pins.

Coaxial connectors: BNC and F-type

Coaxial cable ends in a round connector that grips the shield. The BNC connector twists and locks on a quarter turn; you will see it on legacy Ethernet, test equipment, CCTV and video gear. The F-typeconnector screws on and is the standard on cable TV and cable broadband. More on the cable itself is in Coaxial cable.

Fibre connectors: LC, SC, ST and MPO

A fibre connector holds the glass fibre in a precise ceramic ferrule so that two fibres meet face to face. The common shapes differ in size and how they lock.

Common fibre connectors
Common fibre connectorsConnector; How it locks; Typical use. LC: Small push-pull latch, 1.25 mm ferrule Transceivers and dense patch panels; the most common today SC: Square push-pull body, 2.5 mm ferrule Older and some enterprise patching ST: Round bayonet twist Older multimode installations MPO / MTP: Rectangular multi-fibre push-pull (12 or 24 fibres) 40G and 100G parallel optics, high-density trunksConnectorHow it locksTypical useLCSmall push-pull latch, 1.25 mmferruleTransceivers and dense patch panels;the most common todaySCSquare push-pull body, 2.5 mm ferruleOlder and some enterprise patchingSTRound bayonet twistOlder multimode installationsMPO / MTPRectangular multi-fibre push-pull (12or 24 fibres)40G and 100G parallel optics,high-density trunks

On small screens, swipe sideways or focus the diagram and use the arrow keys.

Read each row across. A duplex LC or SC pair carries one fibre each way.

Transceivers: SFP and QSFP

A transceiver is a small module that plugs into a switch or router port and provides the actual optical or electrical interface. SFP and SFP+ modules carry 1 and 10 Gb/s, and QSFP variants carry 40 and 100 Gb/s and higher. The same cage can take a fibre module (with LC connectors) or a copper module (with an RJ45), which is why one port can serve different links.

Choosing a connector

Start from the cable you have and the port you need to reach. Twisted pair to a desktop is RJ45. Cable broadband in a home is coaxial with F-type. A switch uplink between floors is usually fibre, with LC connectors on SFP modules. Match the fibre type and the optics before you buy patch cables, because a connector that fits is not the same as a link that works.

Test yourself

Predict · scenario 1

A cable ends in a screw-on round connector and connects your cable modem to the wall. Which connector is it most likely?

Predict · scenario 2

A switch uplink uses SFP modules with small push-pull duplex fibre connectors. Which connector is it?

Predict · scenario 3

Which connector has eight contacts and is used on Ethernet twisted-pair cable?

FAQ

Is an RJ11 connector the same as an RJ45?
No. RJ11 is narrower and carries two or three wire pairs, mainly for telephone lines and some DSL. RJ45 is wider with eight contacts for four pairs, which Ethernet needs. An RJ11 plug can physically fit partway into an RJ45 socket, so check the shape before you push.
Why do fibre connectors come in pairs?
Most fibre links use one fibre to send and another to receive, so a duplex LC or SC connector carries both. Single-fibre bidirectional optics exist but are less common.
Can I connect LC fibre to an SC port?
Not directly. You need an LC to SC patch cable, which has a different connector on each end, or an adapter. The fibre type must match too: single-mode to single-mode, multimode to multimode.