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Jacks, Modular connectors

We cover here Jacks and Modular Connectors, Telephone wiring standards and LAN wiring standards. If you have suggestions, corrections, comments please e-mail 

Jacks are called by various names RJ11, RJ45 etc. These terms are incorrect (though widely used by all of us -OK, OK we use them as well), they should be called Modular Connectors according to the following table:

Common Name Correct Name No of connectors Notes
RJ11 4 Position Modular Connector 4 mostly used in analog (telephone) wiring
RJ11 or RJ12 6 Position Modular Connector 6 more commonly used these days in analog (telephone) wiring.
RJ45 8 Position Modular Connector 8 Used in telephone wiring (ISDN), LAN (10baseT and 100BaseTX) and RS232 (RS232D) wiring

Male Modular connectors are numbered LEFT to RIGHT when viewed from the TOP (TOP is when the plastic lever is on the bottom). Female connectors are numbered from LEFT to RIGHT when viewed from the FRONT. An RJ45 (or 8 Position Modular Connector) example for both Male and Female connectors is shown below (the same principle applies to all modular connectors)

Male Connector Numbering Female Connector Numbering

Telephone Wiring is defined by USOC (Universal Service Order Code - now handled and maintained by the TIA) and uses the following conventions for cable pairing.

NOTE: All numbering is viewed from the TOP (TOP on a Modular connector is when viewed with the lever on the bottom). The diagrams show all possible types that will satisfy the Wiring Code.

USOC Code No of Pairs


USOC RJ11 or RJ11C


USOC RJ14C or RJ14


USOC RJ25 or RJ25C


USOC RJ48 or RJ48C


LAN Wiring is defined by EIA/TIA and uses the following two conventions for RJ45 pairing on UTP cable.

EIA/TIA wiring Code Diagram Notes
568A (color diagram)
568B (color diagram)

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Last modified: July 11 2011.

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