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64 posts

Master Geek


Topic # 24061 14-Jul-2008 12:20
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Good Afternoon,

I have moved to a new place and the telephone connection is not in the right place.. What is the maximum length extension cord I
can use to connect my Broadband Router?

Secondly - What is the difference between Straight and Crossed Wire for Telephone Cables? 5-6 years ago I know you needed
cross wired cables for modems but this is not so important anymore? Can anybody please explain this for me?

Thank you,

Brad Wright

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  Reply # 147712 14-Jul-2008 12:34
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The use of long (>1m) flat non twisted line cords is a common source of interfenece issues with DSL, so best avoided. Another option is to leave the modem where it is and run a cat5e ethernet cable to where you want it.

Obviously the very best solution is to have a central filter installed, with a dedicated line (or pair in current inwall wiring) to the DSL point that you want.

Cyril



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Master Geek


  Reply # 147719 14-Jul-2008 12:59
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Thanks Cyril, It looks like I will be running a couple of Cat5e patch leads.

Are you able to explain the difference between straight and crossed telephone cable??  Does it make a difference anymore?

Thanks again,

Brad Wright

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  Reply # 147739 14-Jul-2008 13:37
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Do you mean
The plug ended line cord? Modems and telephones both used cords with a BT plug at one end and a US RJ style plug at the other. The modem needed BT 5&6 to appear in the middle of the RJ plug (3&4) whereas the telephone sometimes didn't. That meant a known "modem cord" saved some grief.

The internal cable? Old cable had a lazy twist at best with the wires not paired. Later cable used the same internal wire twisted pairs used in Cat 5 and had better noise rejection.

Ethernet? Think that is auto detect now.

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  Reply # 147750 14-Jul-2008 13:45
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As Bung said, most devices autodetect these days. Other than that stick with non cross over, ie straight pair to pair.

Ahhh, bung, the BT5&2 (not 5&6) need to go to RJ11 centre pair.

Cyril



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Master Geek


  Reply # 147755 14-Jul-2008 13:50
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I have a couple of old RJ11 - RJ 11 cables which have been crossed and same with BT - RJ11 which have been crossed.

Where all new cables seem to be straight.

I guess my first questions is - Are Broadband line cords meant to be straight?

Secondly - This doesn't really relate to the forum but why were crossed telephone cables ever used and why are they
still sold today?


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Master Geek

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  Reply # 147767 14-Jul-2008 14:03
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They are still sold, as an ex DSE employee they caused no end of confusion because literally the only difference is the wiring.




------------------------------------------------------
David Elsbury
Freelance Lighting, Sound and AV Technician
"Technician like ninja... live in shadow, move in silence"


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  Reply # 147772 14-Jul-2008 14:11
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Just to clarify a phone line either for POTS (voice) or DSL does not care it the wires of the pair are crossed or not. What does happen are BT to RJ11 line cords that preserve the positions of pin 2&5 in both, these common on older phone gear, avoid if you can. A modem line cord connects the BT2&5 to the centre pair of the RJ11. By convention and pretty much universal wiring standard the centre pins of an RJ11 should be the line, the next pair out are line2. Its only older phones that in contray to the US standard wired the RJ11 with the same straight through pinning of the BT.

Ethernet on the other hand once again does not care if the wires of data pairs are crossed or not, as the modulation scheme is phase tolerant, however the actual forward and reverse pairs can be crossed over or straight through. As noted pretty much all ethernet PHY's automatically negotiate the forward and reverse channels these days, so if its crossed or not it does not matter.

Cyril



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Master Geek


  Reply # 147778 14-Jul-2008 14:21
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I know the wiring is different, I just want someone to explain why?

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  Reply # 147784 14-Jul-2008 14:38
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brad.wright: I know the wiring is different, I just want someone to explain why?



What cross over are you referring to? The BT style cords had more plastic on one side to accept the plug strain relief spur so a straight through cord would have a centre line flip ie 1 on plug 1 would be 6 on plug 2. That would occur whenever plugs are crimped into the same side of a flat cable.

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  Reply # 147785 14-Jul-2008 14:38
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Is the above enougth.

Cyril



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Master Geek


  Reply # 147793 14-Jul-2008 14:45
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I am probably really confusing things here:

I have a RJ-11 to RJ-11 with me at the moment:

RJ11:

Pin 1: Yellow
Pin 2: Green
Pin 3: Red
Pin 4: Black

RJ11:

Pin 1: Black
Pin 2: Red
Pin 3: Green
Pin 4: Yellow

I also have a BT - RJ11

BT:
Pin 2: Green
Pin 5: Red

RJ11
Pin 2: Red
Pin 3: Green

Both have been crossed? What function does this serve?

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  Reply # 147797 14-Jul-2008 14:56
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In the 1st case it is a normal outcome of crimping the plug into the same side of the cable, swapping sides in a pair doesn't matter.

In the 2nd case it is the above plus getting from the US line position to the BT line position.

Older cables where the BT 2/5 came out at the US 5/2 just saved some money before they got clever enough to direct the wires into the plugs.

EDIT can't remember us numbering convention whether RJ11 centre pair 3/4 ??



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Master Geek


  Reply # 147823 14-Jul-2008 16:40
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So to put it simply, it doesn't matter which order pin 2 & 3 (green & red) are??

cisconz
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  Reply # 147846 14-Jul-2008 18:12
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brad.wright: So to put it simply, it doesn't matter which order pin 2 & 3 (green & red) are??


Correct




Hmmmm


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  Reply # 147853 14-Jul-2008 18:27
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Just to requote my earlier post.

Just to clarify a phone line either for POTS (voice) or DSL does not care it the wires of the pair are crossed or not.


Cyril

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