While trying to improve our Adsl connection, I found that we use a splitter that is from an unknown ventor and have no clue whether is for AnnexA (PSTN) or AnnexB (ISDN) line.

Is it possible that somebody used the wrong splitter and just because it connected nobody ever cared?

We have an ISDN line but the splitter used is blank. I am not really sure if it is AnnexB. Would it be possible to use an AnnexA filter over an ISDN line (even with that much signal loss)?

(I am posting here because this is about our connection to the rack, I'm not sure if there is a more proper SExchange site)


If you take a look at the frequency plan (eg on wikipedia), you will see that the frequency bands for Annex A and B differ significantly.

If you require Annex B, an Annex A splitter would cause Adsl signals to overlap with the band for voice data, which obviously won't work (especially if the ISDN line is effectively used for voice)

| improve this answer | |

I would have thought brain99 is spot on hence +1, but I have actually run my home ADSL connection without any filters for a day just as a test - I found speeds dropped, and loads of crc errors and connection restarts but it did work, even when I used the phone!

Filters are dirt cheap (you may even have unused spares lurking somewhere). Replace the suspect one with a known good one that's doing it's job on another line and put a brand new one where you took the replacement from and observe any effects on both lines which should help you reach a conclusion.

| improve this answer | |
  • 1
    The reason is that your modem will send signals on a different band than the voice frequencies (as per the image on wikipedia). The reason we use filters is to reduce any interference: your phone may cause noise on the line on non-voice frequencies, and your adsl modem may cause noise on voice frequencies. Filtering out that noise will thus improve signal quality. So, yes, adsl without filters will still work. The problem however is that when you use a wrong filter, it may actually block your adsl or voice signal from the line... – brain99 Aug 29 '12 at 19:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.