Assessment of the theoretical limits of copper in the last mile

15 August 2008

In the UK, broadband penetration has increased dramatically over the last six years; most of these broadband connections are delivered across the copper local loop. Whilst there are no definitive indications of whether consumers will want significantly higher speeds, we are seeing evidence of increasing use of IPTV and other bandwidth hungry audio/visual applications. This begs the question of when the current copper network would be unlikely to meet the expectations of the majority of UK consumers.

In practice, the answer to this question depends not only on the types of services consumers require but how technologies evolve: it is difficult to predict either accurately as there are many factors which effect both. To give some insight, we commissioned a study based on an idealised environment that does not reflect all the complexities of the current underlying network. This abstraction enabled us consider the theoretical capacity limits of copper networks and set an upper bound for broadband data rates that could be achievable across copper. This work was commissioned as part of our technical research programme and is subject to the following caveats:

  • The project was commissioned in order to try to establish a theoretical, rather than practical, limit;
  • The degree of which practical implementation can get close to the theoretical limit is not investigated within the project;
  • At the time of writing, Ofcom is not aware of any practical implementations of the technology approaches implied in this report that would lead to substantial increases in speed being achieved on BT’s copper network;
  • Neither is there any indication that the changes in standards implied in this study are being considered by the relevant bodies;
  • Ofcom is publishing the report to gain views from interested parties on the possibility that future technologies could exploit, in practice, the theoretical potential described in this report.

Given the important relationship of distance to data rate, we based our model on information on cable lengths from a real network. We concluded that, in our idealised environment, capacities can further improve, compared to today’s deployments. We found that if the DSL transmission system  is hosted in the exchange, households within 2km of the exchange (approximately 18% of the total number of households) could, in theory, receive data rates above 50Mbit/s. If the DSL transmission system  is moved closer to the customer premises and into the street cabinet, then almost 100% of households are within 2km of the street cabinet and could, theoretically, expect a data rate of 50Mbit/s.

These results are theoretical and do not reflect what could be achieved in practice. Data rates experienced by end users depend not only on the distance between the customer premises and the exchange but also on home wiring and interference at the exchange, cabinet and in the home. In the real world there are different providers with different equipment sharing the exchange, and perhaps the cabinet, and therefore impacting performance. Nevertheless the real value of this study is to suggest an upper limit, given all technical progress possible, of 100Mbit/s, with fibre to the cabinet.

Ofcom's technical research programme enables us to keep up to date with technologies and trends, so that we can be in the best possible position to execute our regulatory duties. We do not conduct research in-house but make use of external resources, such as private commercial organisations, university departments and government funded research institutions. These reports present the findings of technical work conducted on Ofcom’s behalf. The opinions and conclusions stated within these reports are those of the organisations who conducted the research and may not reflect the view of Ofcom or imply any future policy work in related areas.

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