m asymmetric digital subscriber line. The advantage of asymmetry was first observed by Joseph Leichleder, a scientist at the Bell Core laboratories in 1989 when working on options for delivering TV over the local loop 19 A full treatment of these topics can be found in Newnes Telecommunications Pocket Book, by EA Edis and JE Varrall, ISBN 0-7506-0307-0, pages 29 to 31, 57, 58, 85 and 158. Making Telecoms Work Table 2.8 Wire size and distance Wire size Data rate Distance 0.5 mm 1.5-2 Mbps 5.5 km 0.4 mm 1.5-2 Mbps 4.6 km 0.5 mm 6.1 Mbps 3.7 km 0.4 mm 6.1 Mbps 2.7 km to compete with cable TV. HFC networks proved to be technically and commercially more efficient than ADSL and quickly became dominant in this application domain. In retrospect this hardly mattered, because ADSL proved to be just what was needed for low-cost internet access. Trials in 1996 led on to implementation from 1998 onwards. The ADSL 1standard G992.1 ratified by the ITU was finalised in 1999 with the option of deploying on an ISDN line from 120 kHz upwards or a POTS 20 line from 25 kHz to allow for a microfilter to protect existing voice services between 300 Hz and 4 kHz (If implemented with a legacy voice service). An ISDN 2B+D line supports two 64-k 'bearer voice channels and a 16-k data channel. The majority of ADSL implementation in a majority of countries is on the legacy POTS network. The ADSL upstream bandwidth on a shared POTS line with legacy voice extends from 25.875 to 138 kHz, which is divided into discrete multitones (frequency subcarriers) set at a channel spacing of 4.3125 kHz. Each tone carries information bits from a bin. A bin, as the name implies, is a destination for data bits that are then mapped onto available subcarriers. The mapping can be selective in order to avoid impaired subcarriers or otherwise unavailable bandwidth. Upstream ADSLI has up to 32 bins (32 times 4.3125 kHz = 138 kHz). Downstream bandwidth is from 138 kHz to 1104 kHz supporting 256 tones at 4.3125 kHz spacing mapped to 256 bins. Headline data rates are 1