VDSL faces interference

Broadband VDSL might support video and fast network and Internet connections for consumers and businesses. But the technology is not yet proven, says Bill Pechey

IT Week readers know about the problems with the rollout of broadband ADSL services. What you might not yet know is that the next steps towards Very high-rate Digital Subscriber Line (VDSL) could be even more problematic.

VDSL offers enough bandwidth for high-quality digital TV to the home and, perhaps, a tenfold improvement in Internet speeds to the office. Wonderful stuff, but when will it be available?

BT is likely to run trials very soon and could offer a pilot service later this year. Unfortunately, VDSL won't be available everywhere as it relies on a fibre-optic connection from the local phone exchange to a wiring cabinet on the street near the customer.

VDSL is not simply a high-speed version of ADSL. VDSL modems only have to work on the short length of wire between the cabinet and a subscriber's house or office. This means that the modems can run faster and don't suffer so much from interference caused by other systems on the same cable. Expect services offering up to 24Mbit/s downstream and up to 4Mbit/s upstream, although more symmetrical services may be also be offered.

At least three standards bodies have been specifying VDSL: the International Telecommunication Union (ITU); The European Telecommunication Standards Institute (ETSI); and the American National Standards Institute (ANSI). ETSI and ANSI have compromised and produced standards that include multiple schemes, while the ITU is still trying to write a single-mode standard. However, as long as the equipment at each end of the wire is compatible, multiple standards are not necessarily a major problem.

What could be a bigger headache is interference. VDSL uses signals that operate in the 1MHz to 30MHz frequency band, which are to be carried over a pair of wires that are situated overhead for part of their length and are not too well balanced. Consequently, they act as an antenna and will transmit and receive radio waves.

Short-wave broadcast stations, radio amateurs, and marine, aeronautical and military communications all use this frequency band, and face the possibility that VDSL may cause interference with their services and degrade signal quality. Conversely, the phone companies are wondering whether the radio transmitters will cause interference to their VDSL services.

Many calculations have been done by standards bodies, but no one can be sure of the interference effects until many field tests have been done. The DTI's Radio Communication Agency has begun a project to get some answers.

Radio amateurs may have the worst problems because they are usually located in residential areas, where VDSL will be most commonly deployed.

The armed forces have been very quiet so far but they will be monitoring things closely. If they see danger to their communications, VDSL may have a really hard time getting going.

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