the digital inquisition

Fritz!Box Fon and differences in ADSL standards (Annex A & Annex B)

By  , in , posted: 26-May-2007 14:43

In order to run a regular telephone line (also called PSTN) and ADSL simultaneously without interferences allthough using the same pair of copper wires, these two services must operate on different frequencies. Therefore ADSL-filters must be installed on all phone jacks after subscribing to ADSL - these tiny boxes separate the frequencies used by the telephone signal from those used by the ADSL signal.

In the scheme below the spectrum from 0-4 kHz would be used for the telephone service, whereas 25.875 kHz to 1104 kHz is reserved for ADSL (divided in Up- and Downstream).

This sharing of frequencies for regular phone lines is specified by the ITU according to ITU G.992.1 Annex A.

However besides regular analogue telephone lines there is a digital telephone system called ISDN, which is mainly used for business purposes. ISDN requires more bandwidth than assigned by the named ITU standard for regular telephony. So ITU G.992.1 Annex B was passed, which defines another frequency usage for ISDN + ADSL.

When purchasing a FBF, like with an ADSL-modem or router, you need to know which Annex you need: Annex A for a regular phone line + ADSL or Annex B for ISDN + ADSL.

Luckily some of the FBF Annex B models, especially the older ones ("AVM Fritz!Box Fon (WLAN)", "FBF 5050" and "FBF 7050") can easily be switched to Annex A by a small patch.
However switching the opposite way - from an original Annex A FBF to Annex B - won't work. Anyway the Fritz!Box FON also supports ATA-mode, so it can be connected to an external modem (ADSL or cable) or to an ethernet network.

Switching to Annex A may be interesting for those, who prefer buying a used FBF from Germany (which are all Annex B versions) for less than NZD 100 (+ NZD 70 shipping) from eBay instead of spending more than NZD 500 at the official NZ distributor

As you can see from the screenshot below the frequency spectrum still looks fine after an Annex change from B to A on a German "Fritz!Box Fon WLAN", which runs flawlessly on an ADSL line:

Other related posts:
How to connect a Fritz!Box Fon in New Zealand
What's (so special about) the Fritz!Box Fon?

Comment by Sally Simpson, on 26-Mar-2008 15:32

If you buy for "more than NZD 500 at the official NZ distributor" then you deserve to be ripped off by them.

There are way cheaper alternative distributors.

Comment by Colin, on 7-Aug-2011 12:47

Hi there. Have got hold of a fritzbox 7270 V2 from Germany but they did not send the cables. Are you able to tell me the pinouts of the 7270 cable so I can make my own?

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