General overview
Details & benefits
Availability
Technical
What are DSL, xDSL,
ADSL, etc?
Digital Subscriber Line, or DSL, is a broadband technology. It
transforms a normal telephone line into a high speed broadband connection.
xDSL is a generic term used to refer to the family of DSL technologies.
There are several different forms of DSL with different capabilities
suited to different markets including:
ADSL (Asymmetric DSL)
Provided over an existing telephone line (one copper pair), ADSL
converts it to a high speed digital line up to 40 times faster
than a traditional modem. It is asymmetric because it provides
more bandwidth in one direction than the other.
SDSL (Symmetric DSL)
Similar to ADSL, but it offers the same bandwidth in both directions.
It is suited to situations where the user needs to send and receive
large amounts of data.
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 an exchange,
which can be difficult and expensive to install.
Our first commercial DSL services are based on ADSL technology,
but we are exploring both SDSL and VDSL.
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How does ADSL work?
ADSL operates over a normal 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 the home or office premises and the other in the
telephone exchange. These modems exploit the physical transmission
capabilities of the copper line to achieve much higher data rates.
A 'splitter' separates the telephony signal from the ADSL signal so phone calls
can be made at the same time that data is being sent or received,
without interference.
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What is broadband?
Broadband refers to services that provide high-speed communications,
usually to access the Internet. There is no common definition of broadband,
but typically, when people talk about 'broadband', they mean a service
that provides:
Faster speed
e.g. A peak bandwidth of at least 500Kbps, which is 10 times the
speed of a 'normal' connection.
Always on connection
i.e. You do not need to 'dial up'. Instead, the broadband line
is always connected, like your water supply or electricity.
Flat rate charge
i.e. You pays a monthly fee and can be online all day, every day
for no extra charge.
For home users and small/medium businesses
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What is ADSL?
ADSL stands for Asymmetric Digital Subscriber Line. It transforms
the standard telephone line between a local telephone exchange and
a customer's telephone socket into a high-speed digital line. It is
called 'asymmetric' because it moves data more quickly from exchange
to customer than from customer to exchange. This makes it particularly
suitable for applications where customers expect to receive more data
than they transmit such as use of internet surfing, receiving digital
audio-visual material and working from home over corporate intranets.
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How fast is it?
These speeds are between about 10 to 40 times faster than today's
typical modems. Top
How does it work?
ADSL enables your normal BT phone line to operate as though there were two cables. You can receive phonecalls whilst surfing the net at the same time. Your BT line is converted to ADSL by sending two different frequencies down the line. One frequency is for voice calls the other for the computer.
A splitter/microfilter is added to the phone socket of the ADSL enabled phone line. The splitter/microfilter has two sockets, one for the phone cable connection and one for the cable that connects to the ADSL modem or router that you would need to use for this service.
If you have more than one phone socket connected to the ADSL enabled phone line then you will need a microfilter for each one. The extra phones can be plugged into the correct socket of the microfilter, the other socket of the extra microfilters, where you would normally attach the modem/router cable, can remain empty. If you run an extension cable from the microfilter that is attached to the master socket to another phone then you do not need a microfilter at the end of this extension.
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What can ADSL be used
for?
ADSL's higher bandwidth has the potential to improve access to existing
online services and applications and to stimulate the development
of new ones:
Internet access
The access speeds available with ADSL make it practical for web sites
to carry more feature-rich information such as embedded video clips,
animations and good quality audio. The virtually instant downloading
of internet pages significantly enhances the online experience.
Intranet access
Fast access to a corporate Intranet (internal communications network)
makes teleworking a realistic proposition for businesses as well as
their employees, bringing benefits to both the organisation and the
individual.
TV based services
ADSL can be used to deliver interactive applications to the TV, such
as email and net surfing, movies and TV on demand, music and internet
radio and home shopping and banking.
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What if my exchange
has been upgraded for ADSL, but I live too far away?
Broadband ADSL is a distance dependant technology which means that
you need to be located within 5.5 kilometres (about 3 miles) of your local exchange in order to be able to subscribe. About 95% of homes in the UK are located within this distance from their
exchange - in cities, the percentage is higher, and in the rural areas, the percentage of people that live within this distance
from the exchange is lower.
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Will telephone calls
be charged separately?
Yes. Normal telephone rental and call charges will still apply.
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What quality of service
is provided?
We aim to provide excellent service quality. Contractual
terms and conditions govern the broadband services that we
offer.
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What recommended specification of computer is required for ADSL?
- Windows 95, 98, 2000, NT, ME, XP, Apple Mac and Linux
- 32Mb RAM (Depending on OS)
- CD-ROM drive (Depending on modem type)
- USB port (Depending on modem type)
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What standard does
our ADSL modem adhere to?
Discrete Multi-Tone (DMT), which uses interleaving to achieve
good error performance and has become the international standard.
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How does ADSL compare
to cable modems?
Cable modems attach to the cable TV network connection in the home.
Cable networks are broadcast oriented, with each subscriber in an
area receiving the same signal as all others in the area. Cable
modems have a shared router port at the head end. As the bandwidth
is shared, a high number of users can cause congestion on the network,
reducing available bandwidth and speeds for all end user customers.
An ADSL line is dedicated up to the local exchange and we have
one ADSL modem per end user in the local exchange. ADSL is circuit
oriented rather than broadcast, so each connection is independent
of all others. Commentators and technical analysts tend to feel
that ADSL is a more secure technology because of this.
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Can we provide
ADSL over ISDN?
No, because ADSL cannot be provided using BT's ISDN lines. To make
ADSL work over ISDN, the ADSL would need to be shifted to a higher
signalling frequency in the cable, reducing its transmission reach,
and resulting in fewer people being able to benefit from the technology.
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Should I select ADSL
or ISDN?
This depends on what you want to do. The key benefits of ADSL are:
- Higher bandwidth
- Always on capability
- Flat rate charging
- ADSL is currently available in 66% of the UK.
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How does ADSL speed
compare to other access technologies?
Speed comparisons are very hard to make since there are a number of
influencing factors - the design of the network, host server speed,
international bandwidth constraints etc.
Technology |
Limits |
Dial-up modem |
28 kbps |
56 kbps |
ISDN |
64 kbps |
128 kbps |
ADSL |
0.5 Mbps |
2 Mbps |
Cable modems |
0.5 Mbps |
15 Mbps |
Leased lines |
Huge, only limited by cost |
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What are the key
benefits of ADSL for end users?
A faster connection to the Internet
- at least 10 times the speed of a normal connection. Web sites with
feature rich pages will download virtually instantly, compared to
a 'normal' dial up connection.
An always on connection - so no need to waste time 'dialing
up'.
Flat rate tariff - no call charges for the time spent online,
so you have a predictable price for unlimited access.
Simultaneous phone usage - you can surf and talk 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.
(Note that normal telephone rental and call charges will still apply
though)
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What are 'Upstream'
and 'Downstream'?
'Upstream' describes the flow of information (data) from
your equipment to the BT exchange. 'Downstream' describes
the flow of information from the BT exchange to your equipment.
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What are microfilters?
To ensure the quality of both the telephony and the ADSL, it is imperative
that every piece of telephony equipment must be filtered including telephones,
fax machines and even simple tone callers.
This does not mean that every piece of equipment must have a filter, only
that every telephone socket that has telephony equipment attached to it must
have a Microfilter fitted first. The diagrams below show you how to connect
your telephony equipment and your BT Voyager USB.
If more than the two Microfilters supplied are to be used on one line as in
the above diagram then you should be certain that any additional Microfilters bought
are designed to operate in this configuration. Incorrect filters may cause problems
with either your telephony products or impede the ADSL speeds.
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Why is my postcode not recognised?
The Post Office issues an update to the postcodes quarterly. We use
this information to update our checker with the postcodes of ADSL
enabled areas. If your telephone number is recognised but your postcode
is not, it is possible that your postcode has yet to be added to the
system. If neither is recognised it is likely that your exchange has
not been enabled for service.
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I have a cable telephone. Can I receive ADSL?
No. Our ADSL broadband services are only available over a
BT exchange line.
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If the availability checker says
that Broadband is in my area, does this mean I can
definitely get ADSL?
The checker, which interrogates a BT database, can only provide you with an estimation of the likelihood
of receiving service. It cannot take into account all the factors
that may prevent service. It is an indication that your location
is within the distance limitations regarded as having a good chance
of receiving service at the maximum upstream speed. In all cases
a physical survey is required by BT engineers to confirm that service
can be received.
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The availability checker
says that my exchange is not currently planned to have ADSL, what
does this mean?
It means that BT have no plans to upgrade the exchange for ADSL working
yet. They are reviewing the opportunity for upgrading more exchanges
and will announce further rollout details in due course.
Because of the technical limitations of ADSL and lack of customer
demand in some areas, some areas are not commercially viable for ADSL.
BT is exploring a number of options for these areas.
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Reasons why ADSL may not be available to you
Some users may not be able to access ADSL services because their local telephone exchange has not yet been enabled to provide this service.
Sometimes, although you have an ADSL enabled exchange, your phone line may be of too low quality to enable ADSL to run effectively. This is either due to the length of your cable from the exchange (which must be 5.5km for 512k connections and 3.5km for the 1024k and 2048k connections) or it may be that there is interference from surrounding cables that lay with your cable underground. Sometimes your cable will be part fibre-optic and ADSL cannot run on a fibre-optic cable.
The line quality of your BT cable that runs from your house to the exchange is paramount. A line test is carried out and a reading is taken in decibels. The first test must read under 55 decibels. Once this test has been passed a test must be carried out directly from your property. The reading here must be under 60 decibels. This second test is carried out once you have connected your equipment - an engineer at the exchange will send signals down the line to take a reading.
These are just some of the reasons ADSL may not be able to run on your phone line. All these reasons will give high decibel readings that inhibit the ADSL service from working at the optimum level.
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Data for router setup
VPI (virtual path identifier) |
|
0 |
VCI (virtual circuit identifier) |
|
38 |
ATM encapsulation |
|
aal5mux |
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