Cable factors

Written on 3:10 PM by ooe

High capacity transmission systems such as ADSL and VDSL operate at much higher frequencies than the voice telephony which the cables were designed to support. But when BT embarked on development of broadband capability in the early 1990s it was discovered that little research had been carried out anywhere in the world to quantify telephony cable performance at frequencies above 1 MHz. And yet to commit to huge broadband investment without being able to accurately predict the transmission performance of the modems was judged to be unacceptable. BT therefore initiated the most comprehensive programme of technical investigations ever carried out for DSL transmission systems by any network operator in the world.

These technical investigations included:

• A comprehensive network survey involving high frequency transmission measurements and recording the physical condition of over 7000 telephone pairs throughout the UK selected to be representative of the network as a whole.

• High frequency crosstalk measurements on 70 live 100-pair cable infrastructures throughout the UK – a task previously thought impossible.

• High frequency impulsive noise measurements and analysis.

• Investigations to assess unwanted emissions from ADSL and VDSL systems

Finally, there has been an extensive and ongoing programme of laboratory and field testing with the assistance of modem vendors, to relate knowledge of real network characteristics to modem performance to predict ADSL and now VDSL service capability.

The BT measurement work to verify DSL service capability has to date not been mirrored anywhere else in the world. For example, it was discovered that far-end crosstalk (FEXT) is actually significantly higher in the UK network than had been predicted by extrapolating previous data. This vital information is built into UK planning rules for DSL systems. Moreover, BT's measurement data underpins the UK's Access Network Frequency Plan (ANFP) – the regulatory framework which all UK operators must obey in their DSL deployments to avoid chaos.

The practical effect if the vital information on network transmission performance and crosstalk had not been discovered, would be that high rate DSL systems deployed in the UK could have become progressively unreliable, and even fail, as more and more systems were deployed, with disastrous consequences for operators and customers alike.

In recognition of the significance for the UK of this work, BT was awarded the IFE/National Physical Laboratory Measurement Prize in 1998.

The BT work has been selectively published to influence key international standards. Not surprisingly, the findings have stimulated urgent measurement work in other countries to verify that their broadband deployments are in fact sustainable in the long term.