Monday, 28 January 2013

When is a filter not a filter

When you don't have any phones!

The typical ADSL/broadband filter contains a number of components to filter the telephone line from the broadband line parts (which use different frequencies).

The usual arrangement is a plug-in filter, which has a phone plug, and then has a phone socket and a modem socket on it. There are also faceplate filters which do the same, but also filter the extension wiring (which connects to the back of the face plate).

This picture gives you some idea, with the normal passive faceplate (left) and a plug-in filter (right).


What surprises many people is that the broadband part of the filter is pass through. The two wires in the phone line go straight through with no change at all from the plug to the modem socket. The filtering is all on the phone side. It stops any unexpected sounds from broadband getting to the phone, and stops any unintended high frequency from the phone affecting the broadband.

If you have a broadband only phone line, with no extensions and no phones, as increasingly common, you do not need a filter. All you need is a means to connect the modem lead (RJ11) to the phone line.

The answer is a non-filter modem-only faceplate like this (below). These have RJ45 sockets (8 pin) which means they can connect directly to structured cabling but they are designed to take an RJ11 modem lead directly as well (as RJ45 is normally).


This has much less in it - and so less to go wrong. It has no phone socket, but also specifically does not have the extension wiring connection at the back either, which is important. Even with no phones in use, normal filters can be a cause of ADSL/Broadband issues if they develop a fault. This faceplate, however, is so simple there really is nothing that can go wrong that is not a direct short or open-circuit.


There is one downside, which we are bracing ourselves for now that we are sending these as standard for any broadband-only phone lines, and that is that BT engineers expect a filter. They have been known to go to a site that has no filters (modem to phone plug leads) and fit a filter, leave, and say that this was the cause of the fault (not having a filter). We expect they will be confused by these face plates, but we may be surprised. We'll see.

15 comments:

  1. The Plug in filters (not sure about the face-plates, would have to look at one) Often have capacitors present as well

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  2. A BT engineer fitted a "Service Specific Faceplate" today after a fault diagnosis.

    EU says (and I'm not sure I believe them) that there is only "one hole, not two"

    It would be interesting if they were already ahead of you ;)

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  3. These are a good idea, but potentially confusing, and when dealing with a company that are confused about their own identity often enough, could be a real problem.

    An Irish broadband filter will fit, as it happens, but is not necessary for this scenario.

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  4. Provide a "This is a broadband only socket and doesn't need a filter" sticker?

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    Replies
    1. Labelling is one of the things on my list, honest :-)

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    2. Even better if you slide an official-looking BT logo onto the sticker ;)

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    3. Even better if you provide an official-looking BT logo on the sticker... and perhaps "pre-yellowed" and "half-peeling" :)

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  5. On the ADSL only lines, are BTw applying a wetting current anywhere or are they leaving it to its own devices?

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    1. We can order lines in several ways, and initially the lines had current but no dial tone, which caused no end of confusion (and pairs going AWOL, allegedly) so these days we ensure we order with a dial tone. They can in fact call 17070, and 999/112, and freephone numbers which is handy for testing (remove faceplate and plug in phone for that).

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    2. Pair misappropriation is something I've dealt with several times before. Testing to see if a pair is free is often, it seems, done by listening for a dial tone and nothing else. It's about time that was updated, then lines with no dial tone would be OK and not likely to be misappropriated.

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  6. Current issue lineman's handsets indicate when DSL is present on the line. The older types aren't supposed to be used. How often does stealing of "quiet" pairs happen nowadays, I'd presumed it was a thing of the past?

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  7. So when can we buy these interesting devices from you? (not that I need one...)

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    Replies
    1. Err, now, £5+VAT plus 1st class post. Ask sales. I'll add to price list.

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  8. While these are in theory a great idea, they aren't really necessary. For years we've been using modem (yes dial up) cables with pins 2/5 on the BT LJU to the centre pair on the RJ11 - available cheap as chips and means you don't loose the quick ability to stick a phone in for a quiet line test. Of course you can pay a fortune for gold plated uber screened things with some pretentious brand name on, but there really is no point when we are talking about the 1-2 metres of internal cable as clearly as a percentage of the hundreds or thousands of meteres of external cable, it is insignificant.

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    Replies
    1. Oh, I quite agree - we do have modem leads with BT plugs, and they are fine. This, however, solves the issue of extension wiring too, which is why we are using them now.

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