There are many different types of DSL, but the one most readily available to Australian consumers and small businesses is Asymmetric Digital Subscriber Line, or ADSL. The asymmetric term means that the service has a much higher downstream bandwidth speed than upstream bandwidth (downstream refers to a transmission from the network to the user-- also known as download speed--, and "upstream" is in the other direction -- also known as upload speed).

ADSL is added to an existing analogue phone line in the same way services such as call waiting are added to existing phone services, in conjunction with a special modem. The modem plugs into your computer through either the Ethernet or USB connection (depending on the type of modem), and in the case of connecting through the Ethernet, a network card will be required.

For an ADSL connection to work, your phone needs to be connected directly to the phone exchange by a copper wire connection, and you need to live within a 5km radius to the exchange. The distance isn't a problem for most city residents, but is a great deal more of a problem for rural residents. Plus, if any part of the connection is digital, for example through a switchboard, ADSL won't work. For people in businesses with PABXes, remember that most fax lines are not connected through the switch, and may be suitable for ADSL.

The best way to find out if you're eligible for ADSL is to check with the service provider. Several ISPs now offer services on their Web sites which can tell you if the copper wires around your area are suitable for DSL services.

ADSL is usually available to consumers in the following range of speeds:

• 256Kbps downstream/64Kbps upstream
• 512Kbps downstream/128Kbps upstream
• 1500Kbps downstream/256Kbps upstream

While ADSL can reach speeds of up to eight to 10Mbps over short copper lengths, it is almost never run at this speed. Most carriers cap the service at a rate of 1.5Mbps. This is because only a few customers in a close proximity to the exchange will be able to get these higher data rates. So, to avoid favouring users who are geographically close to the exchange, carriers elect to offer the same speed to all users.

Other DSL services available in Australia which are targeted at the SME and larger business market include SDSL (symmetrical DSL, which basically provides users with equal upstream and downstream speeds of up to 2.3Mbps), HDSL or High bit-rate DSL. HDSL, one of the earliest forms of broadband, is predominantly a business grade service used for wideband digital transmission within a corporate site and between the telephone company and a customer. Like SDSL, HDSL is a symmetrical service and offers speeds of up to 1.5Mbps. Both symmetrical services are targeted at businesses that require a higher level of upstream speed, to perform applications such as Web hosting and teleconferencing (more on these later).

Another DSL standard gaining widespread use is ADSL2+ or ADSL2Plus. ADSL2+ extends the capability of basic ADSL by doubling the number of downstream bits, and has a maximum theoretical speed of 24Mbps. However, to take advantage of these increased speeds, you will need an ADSL2/2+-compatible DSL modem and live in close proximity to the telephone exchange. Your chosen ISP will also need to have appropriate hardware (called a DSLAM) installed in your telephone exchange to enable ADSL2+ speeds.

Naked DSL is a term that has only recently surfaced in the ADSL industry. It refers to an ADSL service that doesn't require a phone line to be connected to a phone service. For example, once you inform the ISP that you want a Naked DSL plan, the ISP will enable the service and remove your landline phone. This service is only available in certain areas and its primary advantage is that you don't have to pay line rental. While the service initially requires a working phone line to be present so that the ISP can check for availability in your area, it may also be connected even if no phone service currently exists on the phone line, which is perfect if you've just moved house.

Another broadband technology is Very high data rate DSL, or VDSL. VDSL promises to offer speeds of up to 50Mbps for lines up to 300 metres in length and runs on existing single copper pairs. There are two competing standards of VDSL: one which uses the same technology as ADSL and essentially adds extra frequencies to the standard ADSL frequencies; and a second which employs a completely different coding scheme.

Unfortunately, VDSL is a very expensive technology to deliver and is unlikely to be deployed on a wide scale because most customers won't be close enough to the exchange, according to Alcatel's principal engineer Tim Barrett.

Barret says while VDSL has a much higher possible throughput than ADSL, it doesn't really help in extending reach over ADSL. Where it is likely to be used is in an environment where customers are close to the termination point and a service such as video requiring more than 7Mbps is being delivered, he said. An example of VDSL currently in use is the Transact video-on-demand service in Canberra. (The supplier also uses VDSL for Internet and WAN applications over their Fibre-To-The-Curb network.)