information about adsl

What is ADSL? (Asymmetric Digital Subscriber Line)

ADSL is a broadband technology (just like Cable). It transforms a normal BT telephone line into a high speed broadband line. ADSL converts the line into a high speed connection. It is "asymmetric" because it provides faster downloads than uploads.

What is SDSL? (Symmetric Digital Subscriber Line)

Similar to ADSL, but offers the same speed in both directions. It is suited to users who need to upload large amounts of data.


What is VDSL? (Very High Speed Digital Subscriber Line)

VDSL can operate in symmetric or asymmetric mode, providing speeds of between 6 Mbps and 25 Mbps. VDSL is typically provided from a street cabinet which is directly linked by fibre to your local exchange, which can be difficult and expensive to install. It is not currently offered in the UK.
What is ADSL Max?

ADSL Max is the latest offering which provides a downstream speed of up to 8 Mbps. At the same time, it provides an upstream speed of up to 832 Kbps (448 Kbps for home packages). This technology was rolled out to the majority of exchanges in April 2006. Whereas with standard ADSL connections, where you usually sign up to a particular speed, eg 512k, 1Mb or 2Mb, ADSL Max will be a variable connection speed. This means that you sign up to an "up to 8 Mbps" service - and the length / quality of your phone line will determine how much speed you actually receive.


What is ADSL2+?

ADSL2+ is another type of ADSL, this time providing speeds of up to 24 Mbps. ADSL Max can only provide speeds of up to 8Mbps, thus to obtain anything faster, ADSL2+ is the only way to go. The ADSL2+ service is not currently available in the UK through BT's network, although some ISPs are starting to offer ADSL2+ speeds of up to 24 Mbps over their own network (Be, UK Online etc).


Can I receive ADSL Max or ADSL2+?

All ADSL connections rely on two main "figures", that are unique to your phone line. These are known as SNR and Attenuation. SNR is how much signal you are receiving as opposed to noise on the line, and attenuation is how much loss of signal you have on the line. The SNR margin will often fluctuate at different times of the day, so it is difficult to monitor, whereas attenuation is fairly static, and depends on the actual length of your phone line between your premises and the BT exchange. For SNR margin, higher is better (meaning not much noise on the line), but for attenuation, lower is better (meaning not much loss of signal on the line).

The SNR margin will reduce as you connect at higher speeds. For a reliable service, the SNR margin has to be above 6, and ideally above 10.

For example, if you have an SNR margin of 10, and you are on a 2 Mbps ADSL service, you would not gain very high speeds from ADSL Max /ADSL2+, as if you where to connect any faster, the SNR margin would drop to about 6, causing disconnections and download speed problems. But if you had an SNR of 30 on a 2 Mbps ADSL service, you could probably receive the full 8 Mbps on ADSL Max, which will cause the SNR to drop to about 8-10, just enough to operate reliably. Lines that are quite close to the exchange, eg <2km, should be able to receive speeds of "up to 24 Mbps", once ADSL2+ is launched. Once again, there is no sure fire way to find out what your phone line will support, until a service is installed on the line.

One thing worth noting is that for the ADSL Max and ADSL2+ services, there will not be a "limit" on whether you can sign up to it or not. You will simply be provided with the service, and the equipment at the exchange will automatically determine what speed your line is capable of providing each time you connect, based on your SNR margin. This is welcomed, as while it may connect at a slower than expected speed, it will help stop your connection from disconnecting due to a low SNR margin. I am sure most people would rather have a stable, always on (slightly slower) connection, than a (slightly faster), intermittent one.


How does ADSL work?

ADSL operates over your normal BT telephone line. This means that there is no need to dig up any roads to install it. The ADSL signal is carried by two modems - one in your premises, and one at your local BT telephone exchange (called a DSLAM). These modems use the existing copper telephone line to your premises, to achieve much higher speeds. A 'splitter' or micro-'filter' separates the telephone signal from the ADSL signal, so phone calls can be made at the same time as you are online without interference.


How fast is ADSL?

The speed of ADSL is typically between 256 Kbps and 8Mbps.


What are the key benefits of ADSL?

- A faster connection to the Internet - at least 5x the speed of a 56K dial-up modem. Web sites with feature rich pages will download virtually instantly, and downloads will take seconds/minutes not hours/days.

- An always on connection - so no need to waste time 'dialing up' every time you want to go online.

- Flat monthly fee - no call charges for the time spent online, so you have unlimited access.

- Simultaneous phone usage - you can use the Internet and your phone at the same time. The ADSL service runs over your existing BT telephone line, so there is no need to pay for an extra phone line to be put in.