ADSL and Filters

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What Is ADSL ?         

The copper wires that are installed for your telephone have lots of room for carrying more than your phone conversations -- they are capable of handling a much greater  bandwidth or range of frequencies, than that demanded for voice. ADSL exploits this extra capacity to carry information on the wire without disturbing the line's ability to carry conversations. The entire plan is based on matching particular frequencies to specific tasks.

To understand ADSL, it helps to know a few things about a normal telephone line -- the kind that telephone professionals call POTS for Plain Old Telephone Service. One of the ways that POTS makes the most of the Telstra wires and equipment is by limiting the frequencies that the switches, telephones and other equipment will carry. Human voices, speaking in normal conversational tones, can be carried  in a frequency range of 0 to 3,400 Hertz , this leaves a lot of bandwidth to be used for other purposes.

ADSL (Asymmetric Digital Subscriber Line) exploits this by sending your digital internet traffic in a different frequency band, but more  of this later.

ADSL offers a much higher transmission rate than the old Dial up Service and the various ISPs providing ADSL service offer many plans, however there are other considerations of which you should be aware.

Precisely how much benefit you see will greatly depend on how far you are from the  telephone exchange providing the ADSL service. ADSL is a distance-sensitive  technology: As the connection's length increases, the signal quality decreases and the connection speed goes down. The design limit for ADSL service is 18,000 feet (5,460 meters), though for speed and quality of service reasons many ADSL providers  place a lower limit on the distances for the service. It is possible to have a reliable connection at distances greater than this, but the speed and reliability will be downgraded.

At the extremes of the distance limits, ADSL customers may see speeds far below the promised maximums, while customers nearer the exchange have faster connections and may see extremely high speeds in the future. ADSL technology can provide maximum downstream (Internet to customer) speeds of up to 8 megabits per second (Mbps) at a distance of about 6,000 feet (1,820 meters),  and upstream speeds of up to 1024 kilobits per second (Kbps).

The new ADSL 2 and 2+ services being offered provide speeds far in excess of this, up to 24 Mbps download, but be aware that distance is very significant. For example the ADSL2+ speeds advertised are correct at distances of 10,000 feet , but reduce dramatically above this to be about the same as normal ADSL at 18,000 feet.

Why do I need an ADSL Filter?
As mentioned before , the ADSL signals are in a different frequency band to the normal voice traffic on your telephone lines. This creates two problems.

Because there are two different signals,  the two have to be separated at the receiving end, and this is done using high and low pass filters.

A high pass filter is installed within your ADSL Modem, this stops the voice traffic from interfering with the ADSL signals, but you must install your own filter on telephone devices, including fax Machines etc, to isolate the devices at ADSL frequencies.

The ADSL filters that  you have to install are low-pass filters -- simple filters that block all signals above a certain frequency.

Since all voice conversations take place below 4 KHz, the low-pass (LP) filters will block everything above 4 KHz, thus isolating these devices at ADSL frequencies.

It is commonly claimed that the main purpose of the filter is to prevent the ADSL data signal from interfering with the voice traffic, however the most important function of ADSL filters is impedance matching of the telephone line. This becomes very important at higher Data speeds and I discuss it in greater depth in "The New ADSL2+" guide.

It is also often claimed that the filter is to prevent noise from the ADSL signal  from being heard on the telephone. This is also incorrect. The ADSL frequencies are in the range of 25 KHz to 1.1 MHz for ADSL and ADSL2 or 25KHx to 2.2 MHz for ADSL2+. This is way beyond the range of human hearing (maybe a dog could hear some of it?) and also beyond the capability of telephone equipment.

What Filters are available?
There are numerous types of filter available but the basic ones are:

  • Single In-Line Filter 

These are the simplest and most common. They come in various shapes and sizes, but as long as they meet the standards of  RCIT.0004, your choice may be simply cosmetic or manufacturer preference. They generally have an RJ11 plug and an approximately 6cm input lead and an RJ11 socket to connect to your phone or other device. Some models have the old 600 Series plug on the input lead, but the standard wall connection for telephones is now the RJ11. Some manufacturers will quote an RJ12 connection. Don't worry about this, an RJ12 is exactly the same plug (or Socket) but it has 4 wires connected, rather than 2, see the explanation below. Some single in line filters have a short lead(pigtail) and socket on the input side. This can be helpful in some installations, since you can replace the 6cm input lead with a longer lead, or you can replace it if you have an older 600 style wall socket .

If you have a number of  telephone devices at the same location a most quality line filters are designed to handle up to five devices using a double or triple adapter, although Telstra regulations limit this to three. 

     • Dual Line Filter
These are almost identical in appearance to single line filters. They are really two separate filters in the one package.
If you have an ordinary single line telephone they work exactly the same as a single line filter. If however you have a two line telephone, they will filter both lines independently.

They do this because thay actually contain two filters and the first telephone line is connected to a filter on pins 3 and 4 of the RJ11/RJ12 connector and exits, filtered, on the same pin numbers.
The second telephone line is connected to the second filter on pins 2 and 5 of the RJ11/RJ12 connector and exits, filtered, on the same pin numberss.
This makes them quite versatile since
• If you have a normal single line phone this filter will work exactly the same as any single line filter
• If you have a two line phone this filter will filter both lines independently.
• If you have a single line phone on line 2 this filter will filter that line without changing any connections. 

  • ADSL Filter/Splitter

This is simply a combination device which contains a normal ADSL filter and  has two output sockets. One of the sockets is filtered and connects to you phone and the other socket is direct and connects to your ADSL Modem. This saves you from having to install a double adapter at the wall socket to connect both  the telephone and modem. Again, some Splitters have a detachable input lead to enable neat installations.

Bear in mind that if you purchase a Filter/Splitter, you will only ever need one since you only ever have to have one direct connection from your ADSL modem to your telephone line. With you modem connected directly through the filter/splitter all the other telephone devices can be connected using in-line filters.

  • ADSL Wall Filter

Where you have a wall phone installed, there is generally not sufficient room to mount a normal in line filter or splitter. For these installations you can use a wall filter.

With the newer Telstra phones, the phone just clips onto a hanging wall plate, this is a wall plate with two studs at the top and the bottom. The phone is unclipped from these studs and the filter is clipped on to the studs. The phone is then clipped back on to the studs on the filter

Most Wall filters also combine a splitter and have an RJ11 output socket to connect to your ADSL Modem

Some Wall filters also have an extra RJ11 output for you to connect to another device, e.g. a Fax Machine, or cordless phone base station or answering machine. 

  • ADSL Central Filter

Central Filters are a little different, they are a very effective method of isolating in-house wiring problems, especially in older installations.

A Central Filter is a modified version of a Filter/Splitter. It contains a low pass filter and a direct output for the ADSL Modem. The difference is that  a central filter (or splitter) is installed at the telephone line entry point (the NBD) to the premises and is installed by a Registered Cabling Technician. A special telephone line is then installed for the ADSL Modem.  

All of the existing telephone outlets will be filtered and there will be a special (new) outlet just for your computer or ADSL Modem.

One difficulty with a Central Filter is that it lacks versatility - you cannot move your computer to another location without having a new telephone line installed from the central filter.

Another problem is that under ACMA Regulations, a central filter must have terminal connections, it cannot have simple RJ11 socket conneections and so it must be installed by a Registered Cabling Technician.

Most central filters advertised as such cannot qualify to this standard. One which does conform is the C10 Central filter



These appear on eBay occasionally and are quite expensive. The costs involved for the purchase and installation  make the use of a central filter expensive.

  • Alarm Filter

Common "back to base" alarm systems use the Telephone lines to dial out and send a message to either a pre-defined nunber, or to a monitoring company.

The alarm panel is a "telephone device" and so must be filtered. A normal in-line filter, or even a Central filter cannot do this without compromising the security of the system.

A special Alarm Filter has been manufactured for this application and I discuss this in another guide.


Most filters are connected through the new modular type RJxx connectors, although some have an input lead for the older 600 Series Telsra wall sockets. If you have the older type of wall connection, it is not difficult to replace it with a modular one or to use a 600 to modular adapter.

The exception is the central filter which must, By ACMA Regulation, have terminal connections.

There are actually 4 types, RJ11, RJ12, RJ14, and RJ25. The plugs are the same at each end, for all of them, and  they have up to six connecting pins, the difference is simply the number of connecting pins or wires, so it should not matter which you use.

The RJ11 has 1 Pair (2 wires), with two pins (electrical connections) installed at positions 3 and 4

The RJ14 has a second pair (4 wires) for an additional telephone line on pins 2 and 5

The RJ12, also known as RJ-25,  has 3 pairs (6 wires) with the third pair on pins 1 and 6

Most people don't differentiate and just call them all RJ11.

There is also another connector, the RJ31X, which is a special purpose connector of 8 pins used for connections of Mode 3 Alarm systems and Filters. This is discussed in another Guide.

RJ11 type connector:

Please Note.  The colours shown above are the international standard.

Unfortunately Telstra has not always conformed to this and your incoming wiring may have the following


The most common problems associated with ADSL are slow speed, noise and dropouts.

It must be remembered that any noise created on an ADSL connection which falls within the Voice band - that is 300 HZ to 3.3kHz cannot be reduced by a filter. Since the filter is designed to pass these frequencies so that you can hear voice conversations, noise in this range will also be passed. This sort of noise is beyond the scope of a short article, but most commonly is due to a faulty telephone line

Speed problems may be due to many causes, however incorrect use of filters is very common..

    • Make sure that the filter is connected the right way round, the "line" connection of a filter goes to the wall socket.
    • Most importantly, ensure that a quality filter is used. I have personally tested over 30 different filters and some will just not perform, and slow down the connection, especially at the higher attempted speeds,
    • There is a difference between ADSL and ADSL2+ filters.

A common test is an "Isolation Test". To do this, disconnect all of your telephone devices, including all extension phones etc, and plug the modem/router directly into the line from the exchange with no filter or splitter. If speed or noise improves, your problem is most likely in house.

You should always use as short a lead as possible (less than 2 metres) from the wall socket to the modem/router.

If your connection speed increases significantly, you almost certainly have a problem.
This can be anything from a bad line, bad filters, bad telephone line etc.

You can narrow your problems down by first checking your telephone line for noise.
Plug in a normal telephone instead of the modem, dial one to remove the dial-tone and listen.
If you hear noise, contact your voice provider (not you ISP) and complain, do not mention ADSL or they will try to fob you off.

Other noise or speed problems can be identified by progressively adding your filters and telephone devices and checking for noise and slow speed.

These are just a couple of items, if you have problems, feel free to contact me and I'll try to help.



The Australian Communications and Media Authority (ACMA) is responsible for the regulation of standards . The ACMA has allowed the industry to be predominantly self regulating, The relevant code and standards are contained  in RCIT.0004. The specification is available here:

The specification is based upon the European ETSI specification, which in turn is based on the American Specification. Although not all filters available are approved under RCIT.0004, the main reason that many are not is that the manufacturers have not submitted them for approval in Australia due to the high cost. As far as performance is concerned it is safe to say that if the filter has been accredited under  European or US Authorities it will more than meet Australian Standards.



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