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Can I use CAT 5 cable instead of RJ-48?

nmxsupport used Ask the Experts™
Hi, I need to connect a Cisco UC560 to a BT ISDN 30 circuit.  I am having problems getting it to work and althrough a RJ45 cable works fine, others seem to infer that I need a RJ58 cable however there seems confusion from others as to whether the RJ45 wiring will work fine in place of the RJ58.
Any ideas??
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RJ-45 only one has 1 data line where RJ-48 has 4, so it may work but may not work at its full capacity. RJ-48 is used for T1 and ISDN termination. It uses the same 8 position connector as RJ-45 but is wired differently. Here's a wiki article that shows the wiring so you can make your own rj-48s cable.


Thanks so you agree CAT 5 should work then it is fault elsewhere I need to investigate?
From what I can find on the UC560 it's recommended to use RJ-48 for T1 and ISDN. RJ-48 use the same cable(cat5, cat5e, or cat6) and ends(8p8c modular plug) as RJ-45 but RJ-48 is wired differently. Since you are having problems getting it to work properly you should try the recommended cable.
Top Expert 2012
Cat5 has nothing to do with the jack - it's a wire specification, and it will work with RJ-45 or RJ-48.  Here's the difference between RJ-45 and RJ-48: http://www.differencebetween.net/technology/difference-between-rj45-and-rj48/:

1.RJ45 and RJ48 use the same connector
2.RJ45 differs from RJ48 in the way they are wired
3.RJ45 is predominantly used in LAN while RJ48 is more commonly seen on T1 lines
4.RJ45 is connected with UTP cable while RJ48 uses STP

The most important thing is the longer distance involved and the shielded twisted pair wire, which is important for long runs, so you should use cat5e shielded twisted pair.


Correct and proven, phone system all working fine using CAT 5 cable, that's all I wanted to know.