Is your premises phone wiring impacting your broadband performance?

By Steve Biddle, in , posted: 2-Mar-2013 20:50

The vast majority of broadband users in New Zealand receive their broadband connection via copper cable owned by Chorus using ADSL, ADSL2+ or VDSL2 technology.  By far the most common technology in use is ADSL2+ which delivers speeds of up to approximately 20Mbps downstream and 1Mbps upstream. Uptake of VDSL2 is growing and this technology is currently capable of delivering speeds of up to 70Mbps downstream and 10Mbps upstream, averaging around 40Mbps down and 10Mbps up. Between 2008 and 2011 over $1 billion was spent by Telecom rolling out a Fibre To The Node (FTTN) network which consisted of over 3500 new roadside cabinets across the country, all connected by fibre optic cable. Because the speeds of all copper based technologies are limited by distance, bringing fibre fed cabinets with equipment to deliver internet closer to people’s homes meant approximately 85% of premises in New Zealand are capable of receiving an internet connection of at least 10Mbps using ADSL2+, and around 40% capable of receiving VDSL2.

(For the rest of this post I will use the term xDSL to refer to DSL technology as a whole, this includes ADSL, ADSL2+, VDSL, VDSL2 and SHDSL)

What is clear however is that the actual number of people who are receiving a 10Mbps connection is significantly below this figure. The reason for this happening is also pretty clear – apart from distance, xDSL performance is dependant on the quality of the copper cable that’s delivering the services. Statistically speaking poor quality phone wiring within the home or office is the single biggest contributing cause when it comes to poor internet speeds or performance. It’s also very clear that the vast majority of people are totally unaware that their home or office wiring is impacting their internet performance.

Before I start to explain how wiring affects your speed it’s important you understand some basics of how how xDSL broadband works.

In the old days the copper cable to your home (known technically as a MPF – Metallic Path Facility) only delivered phone services and used the frequency range from 300Hz up to 4kHz to deliver voice. In the 80’s ADSL was born, allowing both a phone and data connection to share the same copper line by using a frequency range above that used by voice for the data transmission.. ADSL, ADSL2+ and VDSL2 standards all allow broadband to co-exist with voice. The following image (courtesy of Wikipedia) displays a standard ADSL band plan.

File:ADSL frequency plan.svg

As you can see regular ADSL uses the section of spectrum from 25kHz up to 1.1MHz. Newer xDSL standards use even higher frequency ranges to increase speeds, ADSL2+ improves upon ADSL by using up to 2.2MHz, and VDSL2 goes even higher with profiles using up to 8Mhz, 12Mhz, 17Mhz and  30MHz. The 30MHz profile is able to deliver a data rate of up to 200Mbps over several hundred metres.

xDSL divides the available spectrum up into small sections known as carriers, tones or bins (all three mean the same thing and are simply different terminology). ADSL, ADSL2+ and VDSL2 (8MHz, 12MHz and 17MHz profiles) use a carrier width of 4.3125 KHz. Standard ADSL divides the available spectrum into 256 carriers, and ADSL2+ offers greater speeds by using 512 carriers as it doubles the available spectrum. VDSL2 uses anywhere from 2048 up to 4096 carriers depending on the profile type. As the frequency range increases however, limitations of copper cabling start to take effect. While standard ADSL can easily work over 4km or greater, VDSL2 using a 30a profile will only work up to around 300m as the higher frequency ranges are unable to travel as far over copper cable. In many ways this mirrors the radio world where an AM radio station transmitting on 1000kHz will travel a lot further than a FM station transmitting on 100MHz.

While both a phone call and your xDSL can be carried over the copper cable to your premises, to enable the simultaneous use of xDSL and phone at the same time a low pass filter is required to be used to split the signals so that they don’t interfere with each other. This low pass filter is commonly referred to as a line filter or splitter and is pictured below.

plug in dsl filter

Most people will have a  plug in filter on every device in their home that uses the phone line. Without a filter you will be able to hear the xDSL tones if you lift up the phone handset (assuming you’re not tone deaf!), and will typically see xDSL disconnections or high xDSL error rates if you try and use the phone while your modem is connected without filters.

So you’ve got your filters all plugged in and your internet is connected. You probably think everything is sweet.


While a plug in filter splits the voice and xDSL signals, it doesn’t do anything to eliminate other issues that exist in your home wiring that will have an impact on your xDSL connection. If you have a home alarm it will typically be configured as a “line grabber”, which means your incoming phone wiring will run through the alarm before it connects to the phone jacks in your home. This configuration allows the alarm to isolate the line with a relay before it dials out to a security company, meaning that even if you leave a phone off the hook in your home your alarm will still work. Because of this your xDSL connection will drop every time your alarm dials out. Many medical alarms are also wired in a similar fashion, once again to ensure that a phone off the hook doesn’t prevent the alarm from being able to call out. Alarms can also increase line attenuation and line resistance, which is the next thing you’ll learn about.

So.. Attenuation. What is it?

Copper cable has characteristics which are well known, one of these is attenuation. Simply speaking, the longer a piece of cable, the greater the signal loss will be – a figure known as attenuation. Attenuation is one of the reasons why the speed of xDSL technology decreases the further you are away from a roadside cabinet or exchange. A lower figure is better, so if you’re located right next to a roadside cabinet or exchange your line attenuation may be around 2 – 3dBm, if you’re located 2km away your line attenuation will typically be around 25dB – 30dB.

To confuse matter more, your distance from a roadside cabinet or exchange isn’t the only thing that can cause high attenuation figures. Joints in a cable can also increase attenuation, with the extent of this increase depending entirely on how well the cable is joined. The typical New Zealand home has it’s phone wiring connected in series, this means that your jack  points are all looped together, as the image below demonstrates.


no master filter


Phone wiring in series was recommended practice until the late 90’s when xDSL services started to make an appearance. Telecom started advising in 1998 that phone wiring in series should be avoided, and the current TCF home premises wiring regulations also advise against this. It is still common practice however for many incompetent electricians and data installers around the country to follow this method, and there are still new homes being built where home owners will suffer from a degraded broadband experience, purely because installers can’t be bothered in following industry guidelines that have now been in place for over 14 years. All premises wiring should consist of a structured solution where all jack points connect back to a single location. The TCF website  contains plenty of information regarding this.

So what’s wrong with series wiring?

Each jack point can increase the attenuation on your line by a small amount, and also introduces multiple locations where corrosion can occur, something that’s exceptionally common in damp New Zealand homes. Corrosion in a jack point can cause significant degradation of xDSL. Another common issue is the mixing of newer 2 wire and older master and secondary BT jack points. Until the mid 90’s many older style phones in New Zealand required a master jack point with capacitor to generate a ringing signal carried on a 3rd wire within the premises. The master jack point was fitted where the phone cable entered the premises, and secondary jack points were fitted elsewhere on the premises. These were signified with a M or S in the lower corner of the jack point face plate. These jack points were replaced in the mid 90’s with a single style known as a 2 wire jack  point (as only 2 wires are required to be used within the premises). Mixing older M or S jack points and 2 wire jack points will impact xDSL performance.

Last, but not least, an issue known as a bridged tap (also referred to as a line stub by some people) is a major issue in the xDSL world. A bridged tap is created when wiring is split and occurs if you have multiple jack points installed., regardless if they’re in use or not. If you’ve got 5 jack points on your premises for example any xDSL signals travelling along the copper cable to your home are transmitted not just to the jack point that has your modem connected, but to the other 4 as well. These signals then reflect back from these other jack points and cause  interference to the xDSL signal, ultimately degrading the performance of your broadband connection. Higher frequencies are affected to a far greater degree by a bridged tap, so they’re not so noticeable if you’re using ADSL but can have a large impact on ADSL2+ and VDSL2. Bridged taps can also exist in Chorus wiring external to your home where wiring may be looped down your street, but the instances of this are now very low.

So now that I’ve told you everything that’s wrong with your connection, I’ll tell you how to fix it. The solution is simple – it’s called a master filter.


A master filter does exactly the same job as a plug in filter – it’s a low pass filter that splits voice and xDSL signals. What it does differently is eliminating all of the internal wiring issues discussed above, essentially isolating your xDSL from your home wiring. A master filter is installed at the point where your phone cabling enters your premises, with the xDSL output of the master filter connected to a dedicated xDSL only jack point somewhere on your premises. The voice output of the master filter is then connected to your existing phone wiring. You’ll still be able to use your phones anywhere on the premises, but will only be able to plug in your xDSL modem to the dedicated xDSL jack point.

master filter

Because the master filter isolates the premises wiring issues from the xDSL signal it means all of the issues I’ve discussed above can no longer impact your broadband performance.

It’s a common myth that having a naked xDSL connection (a xDSL connection without a phone) removes the need to use any filters. This is incorrect. While plug in filters are not needed as there is no need to split voice and xDSL signals, a naked xDSL service will still continue to be impacted by premises wiring issues that can only be eliminated by the installation of a master filter or disconnecting all other internal wiring so that only a single jack point remains connected for the xDSL modem.

So how much faster will my broadband be with a master filter? That’s a tough question to answer, because the results are going to depend entirely on the condition of your existing wiring. It’s very rare for a master filter to not increase broadband speeds by a minimum of 5 to 10%. It’s entirely realistic in circumstances where wiring is very poor to see speed double, or even triple. While certainly not the norm, I’ve installed master filters that have resulted in speeds going from under 5Mbps to in excess of 15Mbps. Based upon the vast number of threads on Geekzone where internal wiring is discussed and on statistics gathered from a recent project undertaken by a major ISP looking at the internal wiring issue, it’s safe to pick a modest median speed increase in the vicinity of 20% to 50%.

Most people would have never heard of a master filter, but they’re actually nothing new. When ADSL was first deployed by Telecom in the late 90’s a master filter was a mandatory requirement for an installation. To transform ADSL into a mass market product this requirement were lifted in favour of plug in filters. In many ways this made sense from a marketing view – the cost of a technician having to visit before broadband could be connected was something that would have seriously impacted take up of broadband services. This practice has still continued today and most new connections are DIY modem installs within the home.

All ISP’s as part of the sign up process for broadband can request that Chorus visit the premises and install either a master filter or install a new connection to a new jack point solely for the xDSL modem. In the vast majority of cases this doesn’t happen, primarily because the average home or business owner is going to baulk at the cost of doing this, which ranges from $199 right up to $400 depending on the work required and the connection type. This cost is too high for ISP’s to absorb on the average low margin residential broadband plan, and the average homeowner sees it as an unnecessary cost, in part because they have no idea what it’s actually achieving.

With VDSL2 services the installation of a master filter or dedicated jack point isn’t just a nice to have, it is essential. The greater frequency ranges used by VDSL2 (8Mhz for a standard 8b profile or 17MHz for a 17a profile) result in significant degradation of performance even with minor wiring issues. ISP Snap! made the decision to deploy VDSL2 as a mass market offering with no requirement for this, and anecdotal evidence shows that many users are receiving connection speeds well below what they should be receiving. A downside of this is that users with poor connections are also theoretically able to impact on the performance of other xDSL users. VDSL2 uses UBPO (upstream back-off power) to regulate the modem power output to minimise cross talk. Cross talk is essentially interference that occurs within a multi pair MPF (which can easily be several hundred lines connecting your premises back to the exchange or cabinet) where one cable pair causes interference to other nearby cable pairs. A poor VDSL2 connection results in the modem increasing it’s transmit power to attempt to compensate, which can cause cross talk, and ultimately affect other xDSL connections in the same multi pair MPF closer to the cabinet or exchange.

While I’ve explained how a master filter makes a difference I also want to show some technical data showing the differences between good and poor connections. All ISP’s have access to a tool known as SPM within the Chorus system that displays a myriad of graphs and statistics that are helpful in diagnosing xDSL connection issues. 

The image below shows a SNR capture of a good VDSL2 connection running on a standard 8b 8MHz profile off a Chorus ISAM several hundred metres from the cabinet. This line is synced at around 45Mbps downstream and 10Mbps upstream. You can see the performance of the individual carriers across the bottom of the table, referred to as tones in the following images.

vdsl2 snr

Now lets show a SNR graph of a poor quality VDSL2 connection, also off a Chorus ISAM, but being subjected to significant degradation due to wiring issues. You can see the SNR levels across the entire frequency range are lower due to the distance, and between tones 1100 and 1200 there is a large gap that is rendering this section of spectrum unusable, hence the poor upstream performance. The 2nd downstream block also has very low SNR, which ultimately means  a limited ability to carry data, which limits overall downstream performance.

vdsl2 snr poor

I’ll compare these two connections further with bit loading graphs of both connections. First off the picture below shows the 45Mbps / 10Mbps VDSL2 connection. As you can see there is good performance across the board.

vdsl2 bitloading


And below the 20Mbps / 5Mbps connection. You can very clearly see the gap in the upstream band, and also the very poor performance in the 2nd downstream band.

vdsl2 bitloading poor


I’ll now show an example of a before and after bit loading and SNR graphs where a master filter has been installed on an existing ADSL2+ connections to improve the performance. This connection is located approximately 2km from an exchange and had an ADSL2+ sync rate of 696kbps upstream and 5360kbps downstream prior to a master filter being installed.





As you can see from these images the connection is very poor, with the section of spectrum from tones 350 upward being unusable which is significantly impacting downstream performance.

What follows below are results from this same line after the installation of a master filter. The result was an increase of the ADSL sync speeds to 989kbps upstream and 10444kbps downstream. This has meant the customer’s downstream speed has close to doubled, and their upstream speed has increased by nearly 50%.





Now that I’ve explained the impact of internal wiring, how can you tell if it’s  impacting your connection? Chorus have a brilliant web based tool that will show ADSL, ADSL2+ and VDSL2 availability to any address in New Zealand. It also shows those areas that are within a 10Mbps zone – in effect around 85% of the premises in the country.

If you’re in a 10Mbps zone you will typically have a modem sync speed of at least 10Mbps. Testing to sites such as isn’t a smart way of diagnosing your connection, the best approach is to log into your modem and view the connection statistics. These are easily viewable within most modems and will look something like the following


So how do I go about getting a master filter installed?

Your ISP can arrange a master filter installation which will be performed by Chorus at a fixed cost of approximately $199. Installation can also be performed by anybody competent with installing phone wiring, with the cost varying depending on the complexity of your home wiring.

My personal view is that every home in New Zealand that has a xDSL connection should have a master filter fitted. As harsh as it sounds, If I ran Chorus I would also refuse to investigate any speed or connection related issues that end users lodge with their ISP until the end user committed to the installation of a master filter. The simple reality is that premises with multiple jack points and a xDSL connection that doesn’t have a master filter has degraded performance – while this degradation could only be minor, it could also be very significant. The issues caused by series wiring are well known, and these issues need to be eliminated to ensure the best possible connection.

Other related posts:
Is the TCF mobile blacklist fuelling New Zealand’s latest crime fad?
Spark Paging network shutdown – the event nobody cares about? Not quite.
UFB voice, power cuts, copper invincibility and mainstream media FUD.

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sbiddle's profile

Steve Biddle
New Zealand

I'm an engineer who loves building solutions to solve problems. I'll also a co-founder of the travel site. 

I also love sharing my views and analysis of the tech world on this blog, along with the odd story about aviation and the travel industry.

My interests and skillset include:

*VoIP (Voice over IP). I work with various brands of hardware and PBX's on a daily basis
  -Asterisk (incl PiaF, FreePBX, Elastix)

  -xDSL deployments

*Structured cabling
  -Home/office cabling
  -Phone & Data

*Computer networking
  -Mikrotik hardware
  -WAN/LAN solutions

*Wireless solutions
  -Motel/Hotel hotspot deployments
  -Outdoor wireless deployments, both small and large scale
  -Temporary wireless deployments
*CCTV solutions
  -Analogue and IP

I'm an #avgeek who loves to travel the world (preferably in seat 1A) and stay in nice hotels.

+My views do no represent my employer. I'm sure they'll be happy to give their own if you ask them.

You can contact me here or by email at