RJ48C vs RJ45

Unanswered Question
Jun 13th, 2002

I am setting up a T-1 circuit and the instructions for the WIC on the router request that I use a RJ48C cable. I do not have one. Can I get away using a normal RJ45 Cat5e cable?

I have this problem too.
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apf3885 Mon, 06/17/2002 - 10:27

The pinouts are different for the two types of cables, but does that matter? It is still 8 copper wires. Does it matter which color pair it uses? If I want it to work do I need to rearrange the color scheme of the cabling? I would just try it, but I do not want to damage any equipment.

If anybody has tried to do this your help would be great.

mbjohnson@thq.com Mon, 06/17/2002 - 10:36

The RJ48 is the actual connector. You can use your regular CAT 5 cable with RJ-45 connectors. Pin-out for a T-1 is 1-2-4-5

crgarcia3 Tue, 06/18/2002 - 05:11

Actually the pin out for a DS1 (T1) is 1&2 5&6 a DS0 (64k) 1&2 7&8. From an RJ48 jack, terminate 1 2 and 5 6, then if the CSU is internal, connect the cat 5 straight through to the DTE. The cross is done on the jack.

mfaust Tue, 06/18/2002 - 06:03

The pinout for a T1 is as follows: Pins 1&2 are used for your receive. Pins 4&5 are used for your transmit.

You are correct in stating that the color of the insulater does not matter. However, improper pinout and split pairs will drive you nuts. Your jack from your provider should be wired in such a manner that you can simply use a straight through cable. This means that the wires that you are using are are on the same pins at each end and there are no splits. An Ethernet straight through cable uses pins 1&2 and 3&6. The wires on 3&6 constitute a split because they are a pair and are not connected next to each other. This is correct for Ethernet but would not be correct for T1. You don't however use pins 3&6 for a T1. On the Ethernet cable pins 1&2 are a pair and pins 4&5 are a pair. This meets the requirement for no splits and should work.

This information is the same whether your CSU is internal or external. If you are connecting two CSUs back to back you will need to use a rolled (crossover) cable. Pins 1&2 at one end must be on 4&5 at the other end, and pins 4&5 at one end must be on 1&2 at the other. Your circuit will look like this: DTE - DCE - rolled cable - DCE - DTE.

If your T1 goes through a service providor such as a telco, your circuit will most likely look like this: DTE - DCE -straight through cable - jack - T1 line - telco repeater - rolled jumper - telco repeater - T1 line - jack - straight through cable - DCE - DTE.

If your DCE is external in the above examples, there will also be a cable between all of the DCE and DTE devices (likely V.35).

thirdp1@yahoo.com Tue, 09/03/2002 - 13:54


Great info. Thanks.

What is the pinout for E1 of 2651 router? It is RJ48 but I need to use a CAT5 cable with RJ45 connector on one end. I searched Cisco web but couldn't find it.

Thank you.



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Posted June 13, 2002 at 5:25 AM
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