How broadband works
The term broadband simply means a broad band of
frequencies has been used. It is a radio term and normally means that multiple
frequency carriers are used to carry one signal. It describes the way ADSL
(Asymmetric Digital Subscriber Line) services work so has become a term to
describe a fast internet connection. The term is then being rather
confusingly used to describe fibre connections which are not broadband at all,
but use a single frequency laser.
Normal telephone service is an analogue of sound waves. It
carries sound using electrical signals that work in the same way as they do
though air. In terms of frequency this means that telephone calls use very low
frequency signals on the telephone line.
The line itself can carry signals at a much higher
frequency. The limiting factor is the quality and length of the cable used.
Normal telephone lines are a copper pair (two insulated copper conductors
twisted around each other) that can go for several miles from the local
telephone exchange to your premises.
ADSL makes use of the rest of the capabilities of the
cable. It carries data using a wide range of frequencies much higher than that
used for telephone calls. The fact that it uses this broad band of frequencies
is why it is called broadband.
Adapting to the line characteristics
One of the key features of ADSL is that it can adapt to
work on a variety of line quality and lengths. A long line cannot carry the
same range of signals as a short line. Lines can also suffer from specific
interference which only affects some frequency bands. So the line will
sync to start with.
Syncing means that each end sends a range of different
signals and the other end reports back what it can hear. This allows the modems
at both ends to work out what the line can handle, and what frequencies work.
For each of hundreds of different frequency bands the modems agree how many
bits of data can be carried at a time in that band.
What this means is that the line can work at different
speeds depending on the length and quality of the line. If, over time, the line
characteristics change the line will have to re-sync which usually means a
short outage in service.
Re-syncs are a pain as they normally mean a broadband
service stops for several seconds to re-establish the line characteristics.
This is not generally a good thing. To help avoid re-syncs the initial
measurements of the line characteristics are adjusted to allow a margin.
This means not all of the line's capabilities are used. If the line degrades
over time, and that degradation is within this margin, then no re-sync is
Some lines are very stable, having the same
characteristics all of the time. These lines need very low margins. However
some lines vary a lot, and this can be over a daily cycle depending on
temperature or weather conditions which affect the performance of the line and
levels of interference affecting the line. Such lines need higher margins.
There is an automatic system to establish that a line is
re-syncing a lot and adjust the margin to be higher in future. This is called
Dynamic Line Management (DLM) and runs all of the time. It can take a few days
for a new line to get the right margins for stable operation when first
install, but in practice it is rarely more than the first few hours.
The margin on the line sync helps accommodate general
changes in the performance of a line over time. However there can be
interference that is more like "pops and clicks". Impulse interference that
causes corruption of data. This causes packets to be lost and resent and so has
the effect of making the overall performance of the line slower.
To accommodate this type of interference a system of
error correction is used. This means extra parity data is sent, and if
data is corrupted then this can be identified and corrected. It uses a small
amount of the available bandwidth to provide this extra parity data but makes
the line much less prone to errors. At the same time the data is interleaved.
This means overlapping each block of data with the next and is the same trick
used on CDs to make them resist the effects of scratches on the surface. The
effect is higher latency (the time taken for data to get through the ADSL).
ADSL has been defined for use along side normal phone
service as well as along side ISDN. The different frequency for ISDN means it
is a different standard. In Canada we use ADSL over POTS (Plain Ordinary
Telephone Service) following ITU G.922.1 Annex A which allows in theory up to
12Mb/s downstream and 1.3Mb/s upstream. However BT offer only 8.128Mb/s
downstream and 832Kb/s upstream maximum using ADSL1.
A new standard for ADSL called ADSL2+ provides extended
bandwidth. This follows ITU G.992.5 and provides up to 24Mb/s downstream and
A variation of the ADSL2+ specification called Annex M
allows up to 24Mb/s downstream and up to 3.5Mb/s upstream. In Canada we cannot
achieve the full 3.5Mb/s upstream as there is a frequency plan that must be
followed on all phone lines to avoid interference. Therefore, in Canada, Annex
M allows around 2Mb/s uplink.
Upgrade to 21CN
Existing lines on 20CN will be upgraded to 21CN and so
allow ADSL2+. This is part of an ongoing programme. The upgrade may be some
months after ADSL2+ is available on the exchange, and so customers can pay a
small fee to upgrade sooner if the exchange is ready. Upgrades as part of our
upgrade programme are free of charge and we email you several weeks in advance
with details. When we upgrade we initially keep your line on ADSL1 on the new
21CN kit, and then change to ADSL2+ a few days later.
Beyond the exchange
The ADSL/broadband bit connects between your premises and
the local telephone exchange, at which point the line is split between voice
(for telephone calls) and ADSL. The local exchange is connected via fibre to
one of 20 main interconnect points where a large fibre back-haul network
connects back via gigabit fibre links to our rack in a data centre in
Docklands. We are then connected by fibre so that we can communicate directly
with Canadian based ISPs as well as international (transit) links to the rest
of the world.