The first VDSL demonstrator
Written on 3:09 PM by ooe
The VDSL story begins around 1994 when it was realised that broadcast quality video could be delivered to a very high proportion of the UK population using the existing telephone network, without the disruption then being caused by cable digging. However, restrictions in BT's operating licence prevented it from offering broadcast entertainment using its network, and it was decided that an impressive demonstration of broadband capability was required to stimulate public debate.
Unfortunately, VDSL technology was not then commercially available and it was decided in the summer of 1995, to pull together a small highly skilled technical team at Adastral Park (formerly BT Laboratories) tasked with building an end-to-end VDSL demonstrator by the end of that year. An informal collaboration was established with DigiMedia Vision Ltd. (now a division of Tandberg), who loaned state-of-the-art MPEG-2 video coding and multiplexing equipment.
In less than four months a VDSL transceiver was designed and constructed from the ground up with general purpose components and using novel design techniques and training algorithms (now patented). In its very first test the modem operated at 23 Mbit/s on 500 m of cable for over 48 hours without a single error – an impressive achievement.
This early VDSL demonstrator delivered two simultaneous high quality digital TV programmes, plus high speed Internet over 600 m of typical telephony cable, and was sufficiently impressive for it to be shown to members of a House of Commons Select Committee early in 1996, and many other important influencers in the following months.
The prototype BT VDSL modem was based on the single-carrier modulation technique (SCM). The SCM variant selected was carrierless amplitude and phase (CAP-16) noted for its simplicity. A close collaboration with Nortel Networks' DSL team in Harlow was established who implemented a similar demonstrator using the competing, but more complex, discrete multi-tone modulation technique (DMT). This co-operation with Nortel provided valuable implementation insights for both SCM and DMT technologies, enabling BT to play an informed role in progressing VDSL standards.
This ground breaking work also provided the initial stimulus for the comprehensive programme of technical work carried forward since 1995 which underpins BT's acknowledged leadership in VDSL network technology and standardisation.
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