Monday, May 07, 2007


New BT faceplate - BFFP

I previously speculated that BT might abandon the 3rd wire (bell wire) but it appears they are taking a different tack and have come up with a new type of faceplate for the NTE5 master socket. This is the BFFP (Bellwire Filter Front Plate) also marked in some cases as "Induction Face Plate". I believe this may become standard issue shortly.

The BFFP inserts a single component - an inductor or choke - between the ring circuit capacitor in the NTE5 backplane and terminal 3 on the faceplate socket and extension wiring connectors. A simple and elegant solution to reducing interference pickup from the bell wire. An inductor has a low resistance to DC current but an increasing resistance (strictly speaking increasing reactance ) to higher frequencies. This allows it to pass the low frequency ringing voltage but to attenuate high frequency ADSL-type signals or noise. The DC resistance of the inductor measures at about 35 ohms, presumably because it consists of a long length of fine wire wound round a ferrite core.

A nice man sent me some of these early BFFP's to try. I installed two on domestic installations where the ADSL may have been affected by extension wiring. Case A was a new ADSL customer with a wired extension that was not in use at the time. Case B was a customer with problems getting ADSL to work reliably, suffering low speeds and unable to connect their two Sky boxes to the phone line. Case B had several extension legs connected to the NTE5 master and a further extension plugged in, the ADSL modem was at the end of one of the extensions.

Case A.

In this case the ADSL router was plugged into the faceplate, so the BFFP was only needed for the extension wire. Removing the extension would have been better, or to preserve it for future use I could have fitted a full filtered faceplate.

Using a BT Voyager 105 USB modem the test socket in the NTE5 gave a downstream line rate of 4544 kbits/s with an attenuation of 48.5 dB. Restoring the faceplate with its extension wiring gave a line rate of 3520 kbits/s, a loss of 1 Mbits/s. Switching the standard faceplate to the BFFP with the extension connected gave a line rate of 4256 kbits/s - 288k less than the test socket but 736k faster than the standard faceplate - a useful improvement from a single electronic component !

From the bit plots below we can see that the ring wire reduces the number of bits carried in higher frequencies and prevented use of the "scatter" of higher channels that were capable of carrying 2 bits. The BFFP restored the use of many, but not all, of the high frequency channels out to the right of the plot.

Clearly the BFFP makes a substantial improvement, reducing the impact of the extension wiring by over 70%. This may not be enough for a perfectionist, but its a very quick and low cost fix.

Case B.

This was a more complex case, an existing broadband user with their "Home Hub" on an extension but only connecting at 288k downstream, 448k upstream. A reboot lifted this to 1152 kbits/s downstream. The Voyager 105 test modem managed a line rate of 960 kbits/s with a fairly poor looking bin plot (see below). Connecting to the test socket behind the NTE5 upstairs gave a big increase to 2688 kbits/s which was more like the right speed for the location (over 2 miles from the exchange).

With a BFFP fitted and back downstairs on the extension the Voyager 105 gave a downstream line rate of 2560 kbits/s which is only 128k less than the test socket. This was a good outcome as it avoided wiring a long unfiltered extension or moving the customer's router upstairs. After hooking up one of the Sky boxes and retraining the Voyager managed the same 2688k as it had in the test socket.

Connecting the Home Hub back up gave a surprisingly good connection at 448 / 3,392 so the BFFP accounted for a threefold increase in downstream sync speed !

Finally, a quick look inside the BFFP to see how it works. Note that this may be a prototype and the final product may look different, which would be handy as currently they are nearly identical to look at apart from the "bulge" covering the inductor.



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