The man who put the “asymmetric” in front of DSL has died aged 82.
As an engineer at Bellcore – Bell Communications Research, later Telecordia Technologies – Joseph Lechleider worked in a research team trying to solve the problems of digital telecommunications.
In those days, analogue transmissions were limited by line filtering, which meant the only way to get better speeds – for example, on dial-up modems – was to cram in more bits per hertz.
It was already known that removing the filter would let the carrier signals move up into the radio-frequency range, but there was a problem: crosstalk (near-end crosstalk, or NEXT) between the upstream and downstream signals severely limited the bandwidth available.
The crosstalk problem had already been identified in ISDN, and was one reason why its performance on copper lines was an unsatisfactory 128 Kbps.
Lechleider's insight was that the crosstalk problem could be solved by an asymmetric scheme with the carrier end transmitting at higher power than the customer end.
Since bandwidth is a function of transmitter power, asymmetric power meant downstream bandwidth would be higher than upstream.
It was left to others – most notably John Cioffi, now at Stanford University but in the early 1990s with Amati Communications – to turn Lechleider's insight into hardware by way of his important contribution, DMT (discrete multitone) modulation.
Cioffi told The New York Times Lechleider's idea was “a simple, elegant solution to the problem. His contribution was essential to the development and spread of the Internet.”
In recognition of his contribution, Lechleider was inducted into the National Inventor's Hall of Fame in 2013.
His son, Dr Robert Lechleider, said cancer of the oesophagus was the cause of death. Lechleider is survived by his wife, Marie, Robert, a daughter Pamela, and four grandchildren. ®