ADSL

ADSL is an asymmetrical DSL service (see below), meaning that it allows faster download speeds than uploads. Typical line speeds are 512kb/s downloads, with 256kb/s uploads.

Cable modems

Cable companies can put Internet traffic down the same coaxial cables that they use for their digital services. Cable modems are the devices they install to allow subscribers to access this data channel for Internet use. Cable modems share the bandwidth available with all the other users in each neighbourhood, which means that it is difficult to guarantee the bandwidth each customer really gets.

DSL

Digital Subscriber Line is the term used to describe the use of high-speed modems over copper telephone wires. The technology is similar to normal modems used on PCs for the last 30 years, but uses a different frequency range, and operates at very high speed. The advantage of DSL is that it uses existing copper telephone wires, and so does not require ISPs or telcos to dig up the streets or put in place new infrastructure. The disadvantage of DSL, as with any modem, is that the line speeds possible are dependent on the distance of the customer from the central exchange, and the quality of the actual line (e.g. line noise).

FTTH

Fibre To The Home requires a telco to dig the road, and lay fibre optics to each home. The bandwidth possible on fibre optics is huge (gigabits and even terabits per second of data) and means that the local loop would no longer be a bottleneck. Producing a business case for building this infrastructure is difficult, and depends on what demand there is for building this kind of infrastructure in Scotland.

ISP

Internet Service Provider.

SDSL

Symmetrical DSL offers the same line speeds in both directions, allowing both uploads and downloads at up to 2mb/s. BT started offering this service in October 2003, which was previously only available from a handful of independent service providers such as edNET. The ITU (International Telecommunications Union) agreed standard that BT will allow operators to use on their networks is known as either ITU G991.2 or gSHDSL.

VDSL

Very high speed DSL is a faster variant of xDSL, which provides speeds in both direction of up to 50mb/s on very short distances (e.g. 300m). Given the data rates, it is highly line-quality dependent, and will require significant amounts of work to see it take off in the future. When these levels of bandwidth are required, it may be necessary to move on straight to FTTH (see above).

VoIP

Voice over IP is the technology used to allow voice communications over the Internet. Many carriers already use this with voice compression to fit more calls down their lines, and hence reduce the price of long distance phone calls. It is now being rolled out for use to end customers, for new services such as roaming, IP centrex/virtualPBXs, and unified messaging.

VOD

Video on Demand allows the subscriber to request a particular video stream (e.g. a film or documentary) at any time from a database held by the service provider.

xDSL

This is a term used when describing all the variants of DSL (e.g. ADSL, SDSL, and VDSL). See DSL above.