Is a DSL splitter required, if only a Modem is attached to the line?

Question

Annex B is the current standard for digital subscriber line (DSL) in Germany. A DSL-splitter is an analog L-C-filter to separate the POTS (plain old telephone service, like voice and ISDN, FAX) signals from the DSL signal.

Is the DSL-splitter required, if no devices but a DSL-Modem is connected to the line?

If yes, one could test if a splitter is faulty by connecting the Modem direct to the line. The bandwidth should be similar in both situations.

Solution 1

A DSL filter performs 2 functions:

1. To prevent DSL frequencies from reaching non-DSL devices

2. To prevent spurious noise from non-DSL devices from interfering with DSL frequencies

If there are no non-DSL devices on the line then no filter is required.

Solution 2

Reference ADSL Filters & Splitters

You should use a filter anyway. The filter performs two main functions:

1. Prevents the ADSL signal interfering with the voice part of your phone line (not required in your case as you have no phone connected)

2. Ensures that the ADSL line is terminated correctly. An unmatched termination may cause reflections back down the line. This can cause errors on the ADSL connection.

Comments

[cde]

No splitter or filter needed.

[Jonas Stein]

The answers are in contradiction. Can you please provide sources?

[Jonas Stein]

Is the situation different, if the line provides Annex A instead of Annex B?

[DavidPostill]

From community.linksys.com/t5/Cable-and-DSL/… "Annex A is ADSL used in most of North America and Europe where the telephone line carries voice and ADSL. As far as I know Annex B is only used in Germany where they have have voice, ISDN and ADSL on the same line."

[Arjan]

My provider (XS4ALL, The Netherlands, FRITZ!Box 7360) explicitly asked me to remove the splitter for a situation where only (V)DSL is used, without any analogue telephony. It might be different for other modems?

[Arjan]

Indeed, my provider (XS4ALL, The Netherlands, FRITZ!Box 7360) explicitly asked me to remove the splitter for a situation where only (V)DSL is used, without any analogue telephony. I'm not sure if the type of modem matters.

[Jonas Stein]

The reference is very informative. It seems that the DSL hardware used in UK is very different from that in DE.

[Daniel B]

@JonasStein *DSL splitters are basically ultra low tech devices with very few components. They can be designed pretty much any way.

[Jonas Stein]

@DanielB For example: I have not seen a DSL Y-connector for two ADSL MoDeMs on one line as shown on this UK website in germany before. And in UK only 4 kHz seem to be reserved. Perhaps Annex B?

[Daniel B]

@JonasStein I don’t see any Y-connector on that page. Except in Germany, everyone is using Annex A (“ADSL over POTS”) whenever possible, because it’s superior in range and speed. Since ISDN isn’t all that popular world-wide, Annex B (“ADSL over ISDN”) is rather uncommon.

[Jonas Stein]

@DanielB I think the BT Doubler on kitz.co.uk/adsl/splitter_install.htm could be a kind of Y-connector.

[DavidPostill]

@JonasStein BT Doubler - "This instant plug in adaptor from BT, allows you the freedom to convert a single BT socket to a double (2 way) outlet." so it converts 1 telephone socket into 2 (shop.bt.com/products/bt-socket-doubler-4KQT.html)

[Jonas Stein]

Can you provide a good source for this statement? Some devices have a splitter/filter included. Hence a simple report of personal experience is not so helpful.