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T1 and T3 RJ-48 Cable

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56K CSU/DSU†† RJ-48S Connector

To connect a Bridge or Router to a 56K, CSU/DSU Network, T3, T1 or Fractional T1 line, use a cable with an RJ-48S jack connector.The RJ-48 connector may look very similar to an RJ-45 connector found on a typical CAT-5 cable, but donít be fooled, they are very different in many ways.A RJ-48 connecter is fastened on to a STP (Shielded Twisted Pair) cable, not the standard UTP (Unshielded Twisted Pair) CAT-(1-5) cable.A RJ-48 also uses a different pin out arrangement, voltage level, and line capacitance then a RJ-45.See below for details on the RJ-48C, and RJ-48X connectors.

Pin Assignments for a RJ-48S Jack Connector.

CSU/DSU RJ-48S Cable Pin outs

Pin Number



R1, transmit ring to network


T1, transmit tip to network


(not used)


(not used)


(not used)


(not used)


T, receive tip from network


R, receive ring from network


Pin Assignments of a cable with both ends terminated with RJ-48S connectors.


RJ-48S for DDS Telco on RJ-45


Mechanical Arrangement: Miniature 8-position keyed jack. Typical Usage: Local area Data Channels or Sub Rate Digital Services.Electrical Network Connection: One or two line T&R or T&R T1 R1.

Rj48s.gif (8276 bytes)



Mechanical Arrangement: Miniature 8-position jack. Typical Usage: 1.544 Mbps digital services.Electrical Network Connection: Single Line T&R, T1 R1, conductors 7 and 8 provide cable shield integrity.Conductors 3 and 6 are reserved for future use.

Rj-48c.gif (12039 bytes)



Understanding the USOC and RJ-xx Codes

Then following will help break down what those USOC and RJ-xx codes really mean. The Universal Service Ordering Code (USOC) system was developed by the Bell System and introduced by AT&T in the 1970s to connect customer premises equipment to the public network. These codes, adopted in part by the FCC, Part 68, Subpart F, Section 68.502, are a series of Registered Jack (RJ) wiring configurations for telephone jacks that remain in use today.

Registered Jack numbers end with a letter indicating the wiring or mounting method being used.

  • "C" - Identifies a surface or flush mounted jack.
  • "W" - Identifies a wall mounted jack.
If you (the telphone subscriber) want to have a standard jack other than the RJ-xxW or RJ-xxC installation, you should specify the appropriate USOC when requesting the installation.
  • "S" - Identifies a Single-line jack.
  • "M" - Indentifies a Multi-line jack.
  • "X" - Identifies a Complex Multi-line or Series Type jack.

You may also see these terms associated with the USOC code system.

  • "T/R" - Denotes connections to the Tip and Ring wires of a Telephone Communictions Line, Trunk Channel, or Facility.
  • "A/A1" - Signifies connections to the Hold Functions of key telephone systems. The "A" lead corresponds to a particular line shorted to the "A!" lead when that line is Off-hook in order for that line's "Hold" function to operate correctly.
  • "Bridged" - Signifies a Parallel Connection.
  • "Data" - Data configurations use jacks that incorporate components to Limit Signal Power Levels of Data Equipment.


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