Existing twisted pair copper telephone wires could provide broadband speeds of up to 50Mbps, while half the homes in the United Kingdom could receive nearly 500Mbps. That is the theoretical limit of the “last mile” of the telephone network according to the results of research commissioned by the communications regulator Ofcom.

The objective was to look beyond the limitations of existing equipment to the fundamental theoretical limits of the copper twisted pair network according to communications theory. The speeds indicated are the theoretical total of both upstream and downstream capacity.

Over 99% of households could achieve over 50Mbps while over 95% could reach 100Mbps if modems were placed in street cabinets rather than at the exchange, according to the theoretical model.

Installing modems in just a quarter of the street cabinets could still yield significant improvements in speeds for many homes, enabling up to 40% of households to receive 50Mbps.

The overall implication of this research is that the capacities currently achieved are vastly lower than theoretically possible. In practice, current ADSL and VDSL technologies can only deliver a fraction of these speeds. The degree to which it may be practically possible to reach the results indicated by this theoretical model is not considered.

The research was carried out by the Sagentia consultancy, based in Cambridge, in the United Kingdom.

Ofcom cautions that it is not aware of any practical implementations of the technology approaches implied in the report that would lead to substantial increases in speed being achieved on the existing BT copper network.

However, the research suggests that a fibre to the cabinet approach could theoretically deliver speeds comparable to the average office network, which could be sufficient for many applications, even high-definition video services.

Virgin Media, which operates a hybrid fibre co-ax cable network, is planning to roll out a 50Mbps service to its customers. Three quarters of them currently only receive a 2Mbps service.

In some countries where fibre to the premises is deployed, speeds of up to 1Gbps are possible, which is the top speed available with most current home and office network equipment.

BT has made a conditional offer to cover 40% of households in the United Kingdom with an upgraded fibre network by 2012. The proposed cost of £1.5 billion works out at just £150 per household.

Investment in is essential for BT, argues Tim Johnson of broadband research company Point Topic. “If BT doesn’t renew its local loop infrastructure its existing copper network will be worth only scrap value within 10 years.”

The roll-out of next-generation access will be much like any other cycle of renewal in a utility network, he argues, saying that BT shareholders should be able to finance the investment, carry the risk and reap a good profit in return.