How can extension wiring affect SNR?

First a quick description of the wiring... The BT drop cable enters the loft and into a junction box - 3 pair CW1308 cable exits with one pair connected to A/B in the NTE5 master upstairs routed internally. A spare pair in that cable is connected to 2/5 on the faceplate for the extension wiring, and the orange wire from the remaining pair is connected to 3 for the ring wire. Back in the loft the 2/3/5 extension wires from the NTE5 are then connected to the corresponding wires in 2 other cables that each lead to a standard extension socket. Simple, right?

Ok. I am using an XTE-2005 filtered faceplate so that the extension sockets are pre-filtered and therefore do not carry the ADSL signal. With everything connected normally my router syncs at 9Mbps with a 6db SNR. Also, if I remove the faceplate and connect the router directly to the test socket via a microfilter, I still get 9Mbps with 6db SNR. All sounds good.

What I am trying to understand is if I disconnect the cables in the loft for both extension sockets, my router then syncs at 10Mbps with 6db SNR. Compared to 9Mbps when they are connected, but not connected to the NTE5 (faceplate off). So, the extension cables are causing a 1Mbps drop even though they are not connected at the NTE5 end. Anybody know the reason?

FYI, the wiring for both extensions runs back out the loft and down outside walls and back through the walls downstairs. One directly to an extension socket, and one via a BT77A junction box. The BT77A is brand new (replaced an old corroded BT52A junction box - it was full of dead ants which I'm sure didn't help ). The extension wiring is around 20 years old fitted by BT when the house was built. Incidentally, the extension wiring was laying across the insulation in the loft across many mains/lighting cables - yesterday, I clipped these cables to the rafters so they are now well away from any electricity cables. The CW1308 cable leading to the NTE5 and the NTE5 itself are only a year or so old.

Any ideas appreciated

the extension cables acting as aerials, picking up interference and inductively coupling it into the A/B pair in the CW1308 ?

try just disconnecting the ring wire in the loft JB as that will be unbalanced in the extension cables, to see if that has the same impact as disconnecting them altogether.

Yes - disconnecting the ring wire was the first thing I did, and does make a bit of difference (I always used to have the ring wire disconnected, before I had a filtered faceplate). But only physically disconnecting 2/5 at the JB made the speed shoot up, which surprised me. Interference between the incoming and outgoing pairs in the CW1308, as you suggest, was something I feared may be the case.

If I were to only use the single A/B pair in the existing CW1308 and instead route a seperate new cable (another CW1308?) to handle the 2/3/5 extension wiring, would that help given that the 2 cables would be in close proximity for approx 2 metres (i.e. they would both be hangling vertically down the internal space behind the wall to reach the wall mounted NTE5).

Or would replacing the extension cabling altogether be a better choice? The existing extension cabling looks to be regular 3 pair CW1308 also, and looks in good condition. That said, if I were to replace the extension cabling, it would still have to take the same route down the outside walls which is presumably where the interference creeps in (we do generally get more than average interference here in Norwich, being located relavatively close to an AM radio transmitter, which I'm sure affects the drop cable outside too - DMT Tool shows it knocks out certain ADSL2+ frequencies where radio stations are broadcasting).

I guess having the NTE5 moved to the loft is another option with a filtered faceplate up there, and A/B run down to where the NTE5 is now and connected to the router. Obviously, it will be more convenient to leave the NTE5 where it is though

A solution to your problem would be to put the router in the loft, and only have one extension cable for the phones. Then run a Cat5 cable down from the loft to feed your PC's. Keeping unfiltered wiring as short as possible is the key to good SNR. If you have to have two extensions, use two cables - I've been warned off using the same cable for filtered and unfiltered feeds.

I guess having the NTE5 moved to the loft is another option with a filtered faceplate up there

that's what I did, but I was starting with a star wired non-NTE setup. That way you have unfiltered ADSL going down to the current NTE5 location and filtered extensions splitting off in the loft.

Separate cables should have less crosstalk than the intimately close pairs within one cable.

You could try filtering the extensions in the loft so there's no interference going back down to the NTE5. There are various hard wired filters on sale eg for burglar alarms that would do. That would also allow you to miss off the ring wire on the leg to/from the NTE5.

Just a thought - there isn't another master socket in use ie ring capacitor elsewhere than in the NTE5 ?

Definitely only one master socket. Originally my wiring was star wired too - came into the loft, into a JB and from there one wire ran back outside to the downstairs old style master and another ran back outside at the other side of the house to an upstairs bedroom extension socket where the router ran off via a long flat RJ11 cable. A while ago I (I mean BT ) added another cable to JB to create a convenient extension upstairs where the router could be plugged into - star wired again. And before that I'd already removed the ring wire, thanks to your webpage on the topic, which helped a lot with SNR.

A year ago I decided to address the star wiring issue... the extension upstairs where the router was plugged into was replaced with a NTE5 and the old style master downstairs by a regular extension socket. Then the wiring/JB in the loft was adjusted as per my original post. I guess I was trying to be clever using 2 pairs in the same CW1308 for the A/B and the filtered extension 2/3/5 wiring - seemed like a elegant solution and meant I only had to fumble around feeding one cable down the inside of a wall cavity . I didn't think of it as a problem at the time as my sync speed was still higher than it was with the original star-wired configuration.

Anyway, to get to the point, yesterday I did run some more CW1308 down the wall cavity to the NTE5 to use as the extension wiring and I'm glad to say I'm now syncing at over 10Mbps with XTE-2005 attached and all extensions attached. So, the problem was using 2 pairs in the same cable instead of using 2 seperate cables - a valuable lesson learnt.

Thanks for your advice! Now I'm off to buy an NTE-2005 faceplate as I found that with my revised setup the XTE-2005 with no extension wires attached syncs 0.5Mbps slower than using the test socket.

Edited by Futaura (Sun 06-Sep-09 10:46:18)

Yes, from a theoretical point of view, put the NTE5 and router in the loft would be the optimized solution when it comes to SNR, although I don't have any power up there. That said, I did try this to establish a baseline, and found I got the same speed up in the loft as I did with the master in its current location - only an extra 8 metre cable run. So, as it didn't make any difference it was easier to leave the NTE5 where it is.

I wish I'd read about not using the same cable for filtered and unfiltered feeds a year ago - would have saved me a lot of agro . You're right though, it was that which was causing the SNR hit. Having now run a seperate cable for the filtered feeds, the problem has been solved.

I wish I'd read about not using the same cable for filtered and unfiltered feeds a year ago

it doesn't cause me any problems or measurable speed differences, I have CW1308 3 pair with the A/B of Line 1 and filtered and unfiltered 2/5 of Line 2 in it coming from the loft. The difference between us is that in my case there's no ring wire in the bundle and the upstairs extensions are filtered off in the loft so the signals are all going in the same direction, as it were.

In reply to a post by Futaura:

Yes, from a theoretical point of view, put the NTE5 and router in the loft would be the optimized solution when it comes to SNR, although I don't have any power up there.

Of possible help to others- if you have access to the lighting circuit you can use the old 2 Amp round pin plugs and sockets for low-power stuff like routers, although I prefer to have my router where I can see it .

I used one when I wanted to put a TV distribution amplifier up there. You probably won't find them in the usual DIY sheds, but a decent electrical supplier should have them.

Usual proviso- make sure the circuit is dead before fiddling with mains supplies!