This FAQ discusses xDSL technologies - ADSL, HDSL, VDSL and DSL-Lite - terms relating to higher performance telecommunications networks and services that are being introduced or planned. For completeness where some older terms are used they are also explained. Other FAQs are available addressing other telecommunications terminology.

Deployment of xDSL technologies is accelerating after a very slow and reluctant start by most telecom operators. An accompanying page shows ADSL roll out status in European Union countries as at January 2000, according to the European Commission.

xDSL

• Digital - means a line able to carry data traffic in its original form, as opposed to analogue (see below)
• Subscriber Line* - the line connecting the individual subscriber (eg a household) to the local exchange
• Analogue - the phone lines we have used for voice phone calls until today have been analogue lines and we have used MoDems (Modulator-Demodulator) to convert the digital output of the computer to analogue form for transmission and back into digital form at the other end for use by the computer. The performance of conventional modems and analogue lines has restricted bandwidth - the speeds at which computers can be connected across the phone network and the capacity of the networks to handle traffic.

* Sometimes the L in DSL is expanded to mean loop instead of line (see below).

What's special about xDSL?

DSL technology enables much higher speeds across the twisted pair lines from the exchange to the home. Speeds up to 2 Megabits per second are readily achievable - 30 or more times faster than today's fastest modems. This means that consumers and teleworkers can use applications that need these higher speeds even if their town or village doesn't have new, high performance cable networks. DSL has its own kind of "modems". Also, the technology allows the network to manage traffic rather than allocating complete end-to-end circuits, so that from the user perspective it can appear that the data connection is "always on". xDSL deployment to homes will make it easier for Internet connections can be sold on a fixed rate "per month" basis rather than per minute, encouraging wider and more intensive use of Internet, e-commerce, teleworking etc.

Sounds great, is there a catch?

Are there any constraints or can xDSL be deployed everywhere?

What are ADSL, HDSL and VDSL?

ADSL - Asymmetric Digital Subscriber Line

HDSL - High speed Digital Subscriber Line

VDSL - Very high speed Digital Subscriber Line

DSL-Lite

Are there any technical issues?

First, there are two competing and mutually incompatible technologies for implementing of ADSL: CAP (Carrierless Amplitude Phase Modulation) and DMT (Discrete Multi-Tone). CAP was first on the scene but standards organisations appear to favour DMT which provides more effective use of the bandwidth but is more complex.

Second, concern has been expressed about possible interference between the various xDSL technologies, or interference between xDSL technologies and existing services. There is no real evidence of this but some feel that is perhaps due to lack of detailed exploration.

Some people believe that doubts about xDSL have been encouraged by telecom operators who have been uncertain about the impact of DSL deployment on their revenues! In reality xDSL technology has been widely used for years in the provision of leased lines for business use.

When can I expect any of these?

What are the alternatives?

Line or Loop?

Implementation announcements

Bell South announced that, beginning August 1998, they would join GTE, US West, and others in rolling out "FastAccess" ADSL service to 30 of local markets over the following year or so. Speeds initially would be 1.5 megabits/second downstream and 256 kilobits/second upstream (later climbing to downstream speeds of up to 7 megabits/second). The service cost would be $200 for the equipment, $100 for installation, and $50-$60/month for unlimited, "always-on" Internet access and conventional telephone service. Several regions were deployed by Autumn 1999.

Pacific Bell also announced plans to roll out ADSL service to half of all Californians(!). Downstream speeds will range from 384 Kbits/second through 1.5 Mbits/second, and monthly package prices (ADSL service and ISP fees) will range from $89 through $339.

ADSL service is also widely available in Canada.

September 1999, BT announced plans to deploy ADSL across several hundred UK exchanges, in response to proposals by the UK regulator, OFTEL, to allow competitors to install ADSL technology in BT exchanges.

February 2000, the European Commission published a working document with proposals for "Unbundling the local loop", which would help accelerate deployment of xDSL technologies in Europe generally. This is linked from the Resources Database, search individual resources under "Unbundling". The document includes an annex setting out the ADSL roll out status in European Union countries as at January 2000, according to the European Commission.

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