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.
* Remember, not everyone can receive broadband. To
see if you can, use our checker...
|
