ADSL and copper lines
This page is only relevant to services still connected to the analogue PSTN, i.e. our traditional dial telephone network. Australian telecommunications systems are migrating across to the NBN digital transmission network, so at some time in the future (2021?) the PSTN will be shut down forever. After that time this article will simply become an historic curiosity.
Due to the obsolescent nature of this advice, this page has not been updated with internal links. You will need to scroll down to the desired topic.
Due to the obsolescent nature of this advice, this page has not been updated with internal links. You will need to scroll down to the desired topic.
Contents
- ADSL
- ADSL filters
- Why you should be wary of naked DSL
- Corrosion in copper cables and contact whetting
- Cable network quality
- Telstra's report
ADSL
ADSL (Asymmetric Digital Subscriber Line) enables high speed computer/data signals to be transmitted between the provider's equipment and customer's premises over existing, copper, (PSTN) telephone lines.
The term "asymmetric" means that the data speed leaving your terminal (a.k.a. upstream bandwidth) is less than that incoming (downstream). The reasoning is that, for most internet applications, you only need limited bandwidth to search or set up communications but a wide bandwidth (i.e. high data speed) to view/download high quality images, video etc. As I understand it, this is the main reason why uploading large files is comparatively slow and video communications programs such as "Skype" have relatively poor video quality. What you transmit is limited by the upstream data speed of your ADSL service.
An ADSL service is generally "bundled" with a (PSTN) telephone service (although not necessarily - see comments on "naked DSL" below).
In simple terms, the digital data is converted to high frequency, electrical signals and transmitted in the frequency band above telephony. Electronic "filters" separate the telephony and data signals. In this way telephony and data can share the same line simultaneously. The following image from Telstra documentation describes the principles of ADSL.
Various versions of ADSL exist e.g. ADSL1, ADSL2 etc. In general terms they refer to the data speed available which in turn will be largely dependent on the telephone line quality and length. A short line in good condition will have higher data speed capability than a long or poor quality line. Note the bandwidth required for VDSL2 in the diagram below. VDSL2 is the platform for NBN FTTN.
ADSL (Asymmetric Digital Subscriber Line) enables high speed computer/data signals to be transmitted between the provider's equipment and customer's premises over existing, copper, (PSTN) telephone lines.
The term "asymmetric" means that the data speed leaving your terminal (a.k.a. upstream bandwidth) is less than that incoming (downstream). The reasoning is that, for most internet applications, you only need limited bandwidth to search or set up communications but a wide bandwidth (i.e. high data speed) to view/download high quality images, video etc. As I understand it, this is the main reason why uploading large files is comparatively slow and video communications programs such as "Skype" have relatively poor video quality. What you transmit is limited by the upstream data speed of your ADSL service.
An ADSL service is generally "bundled" with a (PSTN) telephone service (although not necessarily - see comments on "naked DSL" below).
In simple terms, the digital data is converted to high frequency, electrical signals and transmitted in the frequency band above telephony. Electronic "filters" separate the telephony and data signals. In this way telephony and data can share the same line simultaneously. The following image from Telstra documentation describes the principles of ADSL.
Various versions of ADSL exist e.g. ADSL1, ADSL2 etc. In general terms they refer to the data speed available which in turn will be largely dependent on the telephone line quality and length. A short line in good condition will have higher data speed capability than a long or poor quality line. Note the bandwidth required for VDSL2 in the diagram below. VDSL2 is the platform for NBN FTTN.
Image source: Telstra Specification 012882 “Alteration of Telstra Facilities in Homes and Small Businesses Information for Cabling Providers”
ADSL filters
Filters are required to ensure telephony devices (telephones, faxes etc.) don’t cause interference to the ADSL Internet signals and vice versa.
If a phone and computer are co-located at a chosen socket, an ADSL splitter is required to separate the telephony and ADSL broadband; If no phone is required adjacent to the computer, the ADSL modem can be plugged directly into the socket. At all other sockets, each telephony device must be connected via an ADSL filter to ensure no interference occurs.
Other configurations may be installed (e.g. central filter system) but the above splitter/filter arrangement will be effective in most cases.
A wiring diagram below of a typical home ADSL setup from http://www.tpg.com.au/support/filter_guide.php
ADSL filters
Filters are required to ensure telephony devices (telephones, faxes etc.) don’t cause interference to the ADSL Internet signals and vice versa.
If a phone and computer are co-located at a chosen socket, an ADSL splitter is required to separate the telephony and ADSL broadband; If no phone is required adjacent to the computer, the ADSL modem can be plugged directly into the socket. At all other sockets, each telephony device must be connected via an ADSL filter to ensure no interference occurs.
Other configurations may be installed (e.g. central filter system) but the above splitter/filter arrangement will be effective in most cases.
A wiring diagram below of a typical home ADSL setup from http://www.tpg.com.au/support/filter_guide.php
_
Why you should be wary of naked DSL
Naked DSL is essentially the above configuration minus the "bundled" (PSTN) telephone service. All the customer sees is a pair of wires and an ADSL modem. It is generally cheaper than a bundled service and, on the face of it, is a good option but some technical issues arise.
Corrosion in copper cables
Wire joints and switch contacts can develop a film of corrosion on the mating metallic surfaces. This is made worse if the connections are exposed to dampness or high humidity (e.g. a typical Telstra cable joint or box). Left unchecked, these corrosion products will ultimately result in high resistance and possible open circuited connections.
Contact wetting (more correctly spelt "whetting" as in whet - to sharpen) is a well known phenomenon. Occasional application of a small direct current through switching contacts and wire joints effectively "blows away" this film of corrosion that can develop on the mating surfaces.
In a conventional PSTN telephone service, voltage is present at all times on the line and every time the handset is lifted a small DC current flows, whetting switch contacts and wire joints and preserving the connections. Naked DSL services don't have a DC voltage source and the copper wire portion of a FTTN NBN connection won't have it either, so these types of services will have greater fault liability than ones bundled with traditional PSTN lines.
Anecdotes abound on the internet about naked DSL and line faults and these are supported by Telstra's own observations in a report to the ACCC in 2012 (below). Another issue with naked DSL lines is that, because no DC voltage or dial tone are present on the line and the high frequency data signals may not be obvious, there is a risk of the line being accidentally disconnected by an inexperienced technician who perceives it to be "dead".
Cable network quality
There is a lot of discussion around on the general condition of the copper cable customer access network and its suitability for NBN. Prior to deregulation of the industry in the early 1990s, the cable network was fairly robust. This was mostly because its maintenance was wholly controlled by Telecom Australia and its huge team of lines and technical staff. Since 1993, the company has morphed into Telstra and staff numbers reduced dramatically. If my memory is correct, in the decades prior to deregulation the PMG/Telecom Australia had something like 90000 employees. Since then, dramatic corporate remodelling combined with mass redundancies means that Telstra now (2015) has around 32000 employees and cable install/maintenance is largely done by contractors.
Other comments on the copper cable network can be found at the following sites:
2012 article where Telstra refutes stories about serious degradation of the copper cable network
https://delimiter.com.au/2012/11/27/copper-network-not-rooted-says-telstra/
2013 ABC article; Employee’s and customer’s anecdotes on the state of Telstra’s copper network:
http://www.abc.net.au/technology/articles/2013/09/18/3851118.htm
Sydney Morning Herald (2014) reporting on the CEPU WA branch president’s comments on corrosion in cable joints: http://www.smh.com.au/technology/technology-news/gel-casing-degrading-telstras-copper-union-20140209-32aqk.html
Telstra's report
Telstra's full report can be found at:
http://www.accc.gov.au/system/files/Telstra%20WDSL%20Non-Price%20Submission%20August%202012.pdf
An extract of the relevant points appears below.
EXTRACT
TELSTRA CORPORATION LIMITED: Response to the Commission’s Issues Paper (a second discussion paper) into the public inquiry to make a final access determination for the wholesale ADSL service – Non-Price Terms
Public version 24 August 2012
“Telstra does not require wholesale customers to bundle PSTN and ADSL services. Currently WDSL acquirers are free to choose to offer their end users a bundle of PSTN voice and ADSL services by acquiring a WLR service in conjunction with WDSL. Only a minority of WDSL services are bundled with the underlying PSTN service in this way.
Telstra does not supply Naked ADSL to either itself or to wholesale customers. A requirement to provide a Naked ADSL service would require Telstra to develop and provide a new service.
Currently all ADSL services supplied by Telstra are ordered, provisioned and managed by reference to an underlying active PSTN service. A requirement to provide Naked WDSL would require significant system and process changes.
Naked WDSL would increase the costs of service assurance. All of Telstra’s network management protocols, network management experience and tools and assets are built upon the CAN being primarily utilised for the provision of PSTN services. The presence of the PSTN service enables Telstra to utilise its automated line testing and assurance infrastructure.
Without the ability to carry out metallic line test (“MLT”) on end user access lines, Telstra’s ability to detect, manage and restore network faults is significantly compromised.
Over time, the removal of the PSTN service is also likely to lead to a greater number of line faults. The presence of wetting current provided by PSTN equipment mitigates the effects of oxidisation build up that can lead to line faults. Telstra has observed that in the absence of an active PSTN service, the number of line faults increases”
Why you should be wary of naked DSL
Naked DSL is essentially the above configuration minus the "bundled" (PSTN) telephone service. All the customer sees is a pair of wires and an ADSL modem. It is generally cheaper than a bundled service and, on the face of it, is a good option but some technical issues arise.
Corrosion in copper cables
Wire joints and switch contacts can develop a film of corrosion on the mating metallic surfaces. This is made worse if the connections are exposed to dampness or high humidity (e.g. a typical Telstra cable joint or box). Left unchecked, these corrosion products will ultimately result in high resistance and possible open circuited connections.
Contact wetting (more correctly spelt "whetting" as in whet - to sharpen) is a well known phenomenon. Occasional application of a small direct current through switching contacts and wire joints effectively "blows away" this film of corrosion that can develop on the mating surfaces.
In a conventional PSTN telephone service, voltage is present at all times on the line and every time the handset is lifted a small DC current flows, whetting switch contacts and wire joints and preserving the connections. Naked DSL services don't have a DC voltage source and the copper wire portion of a FTTN NBN connection won't have it either, so these types of services will have greater fault liability than ones bundled with traditional PSTN lines.
Anecdotes abound on the internet about naked DSL and line faults and these are supported by Telstra's own observations in a report to the ACCC in 2012 (below). Another issue with naked DSL lines is that, because no DC voltage or dial tone are present on the line and the high frequency data signals may not be obvious, there is a risk of the line being accidentally disconnected by an inexperienced technician who perceives it to be "dead".
Cable network quality
There is a lot of discussion around on the general condition of the copper cable customer access network and its suitability for NBN. Prior to deregulation of the industry in the early 1990s, the cable network was fairly robust. This was mostly because its maintenance was wholly controlled by Telecom Australia and its huge team of lines and technical staff. Since 1993, the company has morphed into Telstra and staff numbers reduced dramatically. If my memory is correct, in the decades prior to deregulation the PMG/Telecom Australia had something like 90000 employees. Since then, dramatic corporate remodelling combined with mass redundancies means that Telstra now (2015) has around 32000 employees and cable install/maintenance is largely done by contractors.
Other comments on the copper cable network can be found at the following sites:
2012 article where Telstra refutes stories about serious degradation of the copper cable network
https://delimiter.com.au/2012/11/27/copper-network-not-rooted-says-telstra/
2013 ABC article; Employee’s and customer’s anecdotes on the state of Telstra’s copper network:
http://www.abc.net.au/technology/articles/2013/09/18/3851118.htm
Sydney Morning Herald (2014) reporting on the CEPU WA branch president’s comments on corrosion in cable joints: http://www.smh.com.au/technology/technology-news/gel-casing-degrading-telstras-copper-union-20140209-32aqk.html
Telstra's report
Telstra's full report can be found at:
http://www.accc.gov.au/system/files/Telstra%20WDSL%20Non-Price%20Submission%20August%202012.pdf
An extract of the relevant points appears below.
EXTRACT
TELSTRA CORPORATION LIMITED: Response to the Commission’s Issues Paper (a second discussion paper) into the public inquiry to make a final access determination for the wholesale ADSL service – Non-Price Terms
Public version 24 August 2012
“Telstra does not require wholesale customers to bundle PSTN and ADSL services. Currently WDSL acquirers are free to choose to offer their end users a bundle of PSTN voice and ADSL services by acquiring a WLR service in conjunction with WDSL. Only a minority of WDSL services are bundled with the underlying PSTN service in this way.
Telstra does not supply Naked ADSL to either itself or to wholesale customers. A requirement to provide a Naked ADSL service would require Telstra to develop and provide a new service.
Currently all ADSL services supplied by Telstra are ordered, provisioned and managed by reference to an underlying active PSTN service. A requirement to provide Naked WDSL would require significant system and process changes.
Naked WDSL would increase the costs of service assurance. All of Telstra’s network management protocols, network management experience and tools and assets are built upon the CAN being primarily utilised for the provision of PSTN services. The presence of the PSTN service enables Telstra to utilise its automated line testing and assurance infrastructure.
Without the ability to carry out metallic line test (“MLT”) on end user access lines, Telstra’s ability to detect, manage and restore network faults is significantly compromised.
Over time, the removal of the PSTN service is also likely to lead to a greater number of line faults. The presence of wetting current provided by PSTN equipment mitigates the effects of oxidisation build up that can lead to line faults. Telstra has observed that in the absence of an active PSTN service, the number of line faults increases”