What is ADSL?
ADSL, or Asymmetric Digital Subscriber Line technology
is a means of transmitting digital information at high
bandwidth down existing copper telephone cable. It is
permanently 'switched on', unlike normal dial up access
and is asymmetric in that it uses most of the available
bandwidth downstream. This makes it particularly suitable
for applications where customers expect to receive more
data than they transmit such as for Web browsing, use
of corporate intranets, and reception of digital audio-visual
material. The technology also supports analogue connection
on the same line, which enables voice or fax calls to
be made simultaneously.
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What are the key
benefits of ADSL for me?
Firstly, the Speed. ADSL provides high-speed digital access
- e.g. potentially 10 to 40 times faster than today's
typical modems.
It’s Always On. You will no longer have to waste
valuable time obtaining a dial-up connection to your
Internet Service Provider or Corporate Intranet
It’s Flexible. It does not affect the normal
telephone, so you can make and receive telephone calls
or faxes whilst you’re online.
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What are the
key benefits of ADSL based services for Business?
ADSL based services will allow SME's and Corporates
to:
* Take full advantage of broadband capabilities.
* Offer innovative services to your end user base.
* Extend the reach of your Intranet to encompass your
remote teleworkers and satellite offices.
* Manage and predict your costs with the flat rate charging
scheme
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How fast is it?
Typical rates for ADSL are shown in the table below.
Downstream speed Upstream speed
Up to 500 kbit/s Up to 250 kbit/s
Up to 1 mbit/s Up to 250 kbit/s
Up to 2 mbit/s Up to 250 kbit/s
This is between 10 to 40 times faster than today's typical
modems.
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Who are our ADSL
based services aimed at?
Our services are aimed at a wide range of consumers.
Corporate business users will be able to provide their
teleworkers or satellite offices with remote access
to the corporate network, whilst home users will be
able to take advantage of the incredible download speeds
to view digital media on the Web, such as movies, music
and similar bandwidth hungry applications.
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What can ADSL
be used for?
ADSL's higher potential bandwidth has the potential
to enhance access to existing services and applications
and stimulate the development of new ones:
Internet access
The access speeds available with ADSL available make
it practical for web sites to carry more multi-media
rich information such as embedded video clips, animations
and good quality audio - significantly enhancing the
online experience
Intranet access
Fast access to the Intranet makes teleworking an attractive
option - which in turn can bring benefits to both the
organisation and the individual
TV based services
ADSL can be used to deliver interactive applications
to the TV, such as real time video and audio, home shopping
and home banking
Future services
ADSL will be the catalyst for a wealth of new information,
education and entertainment services
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When will I be
able to have ADSL installed?
This is dependent upon BT's roll-out schedule for ADSL
serviceability of your geographical area.
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How have BT chosen
which exchanges get ADSL enabled first?
BT have chosen a number of criteria to establish which
exchanges should be provisioned for ADSL first. These
included demand for bandwidth and where the early adopters
of these services are most likely to be located.
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What do I need
in order to be ADSL serviceable?
There are a number of factors that will determine whether
you will be able to take advantage of ADSL.
These include:
That you are a BT subscriber. You must have a valid
contract for the use of a BT analogue direct exchange
line forming part of the BT network over which the ADSL
service will be provided. Note that if you rent your
premises, you must have a valid contract for a BT or
Calls & Access provided PSTN line. It is not suitable
that the contract be held with your landlord rather
than yourself.
Whether BT has upgraded your telephone exchange to ADSL
technology, and whether you are located within a specified
distance from the exchange.
That you are aware of the installation procedure and
requirements in terms of equipment as well as location.
That there are no incompatible services on your PSTN
line.
That you have sufficient mains power sockets for the
BT NTE (Network Terminating Equipment)
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Are there any
Service Constraints I should be aware of?
The transmission performance of some local loops will
mean that it is technically impractical to deliver ADSL
service to all end users with a particular serviceable
area. BT reserves the right to refuse ADSL installation
even if you are situated in a serviceable area.
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What services
on my line are incompatible with ADSL?
ADSL service is not compatible with these BT services:
Subscriber private metering
30k loop
Private circuits
Red ABC
RedCare
Meterpulse facility
PBX lines
It is only possible to provide a single broadband service
over a single BT or Calls & Access PSTN exchange
line.
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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 replace
cable and install it. The ADSL signal is carried by
two ADSL modems - one in the end user's premises and
one in the local exchange. These ADSL modems are designed
to exploit the physical transmission capabilities of
the copper line to the maximum extent. The resultant
‘broadband’ medium is used to achieve much
higher data rates than were previously possible. Two
'splitters' (filters), one in your premises and one
in the BT exchange, separate the telephony signal from
the ADSL signal. This means that telephone calls can
be made at the same time that data is being sent or
received (i.e. a customer can surf the Internet and
still make telephone calls concurrently).
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Is my fax compatible
with an ADSL set-up?
It is not guaranteed that your existing fax machine
will work after you have installed ADSL. Please check
with the fax machine manufacturer/vendor to check ADSL
compatibility.
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Will ADSL affect
my normal telephone connection?
You may notice a slight change to how your telephone
sounds after you have ADSL installed. This is normal
for ADSL lines and is not a fault. Any standard modems
operating on your PSTN line (standard telephone, not
the ADSL line) may operate at a reduced speed after
ADSL installation.
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Will telephone
calls be charged separately?
Yes. Normal telephone rental and call charges will still
apply.
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Can customers
keep an existing telephone number?
Existing BT PSTN numbers will be unaffected; most end
users will be able to keep their existing telephone
number.
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Who is responsible
for billing?
You be billed by us for your ADSL service. If you have
any queries about your ADSL account you should contact
us and not BT.
The price, billing and payment for your standard BT
telephony service will remain unchanged and BT’s
responsibility.
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Will all BT
Select services be supported?
Yes, all Select Services can be supported on the PSTN
line, including Call Waiting, Call Return and Caller
Display.
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Will ADSL affect
my burglar alarm or security system?
Installation of ADSL may adversely affect security or
burglar alarm systems that use your PSTN telephone line
and fail to operate correctly. You should check with
your suppliers for compatibility of your system before
arranging for ADSL installation.
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Which IP services
are compatible with ADSL?
The service supports most protocols including Telnet,
SMTP, HTTP, FTP, TFTP, VDO, RealAudio, Cu-SeeMe, QUAKE
(2.8.0). ADSL will also support instant messaging and
communication programs such as NetMeeting, ICQ or Microsoft
Messenger. Some of the advanced functions of these programs,
such as video conferencing or file sharing, will not
operate behind a firewall or Network Address Translation
(NAT) without some technical configuration, or possibly
not at all. If you have any questions, consult the user
Help for the specific program.
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What computer
equipment do I need?
The equipment required and minimum specification of
PC depends upon which installation you are having, USB
or Ethernet version.
For the ADSL Home and ADSL NetStart USB installation,
you will need the following:
PC with the minimum specification
of:
Windows 98 or Windows 2000 operating system
USB port (if you are not sure about this, please see
your supplier. The ADSL modem uses USB to connect to
your PC).
4-speed CD ROM drive
200Mhz Pentium or above
32 Mb RAM or higher
Video card/display capable of 800x600, 256 colours
SVGA monitor
16 bit sound card
150 MB free hard drive space
For the Ethernet multi-user modem/router installation,
you will need the following:
PC with the minimum specification
of:
Network Interface Card (10BaseT Ethernet port)
TCP/IP communications stack
Netscape 4 or IE 4 or above must be loaded onto at least
one of the PCs connected to the service. The browser
must be JavaScript enabled.
Other specifications will depend upon the applications
you want to use with your ADSL service and the operating
system you are using. For basic Internet browsing this
should be at least:
Pentium 166Mhz, MMX or above
32Mb RAM or above
150MB available hard disk space
SVGA or higher rated monitor
Video card/display capable of 800x600, 256 colours
Windows 3.11, Win 95 or Win 98 - or, Windows NT or Windows
2000
4-speed CD ROM player
16 bit sound card
Mouse or other pointing device
If you wish to use Apple MacIntosh on your network,
the machine should conform to the following specification:
Or a Apple Macintosh with the following is recommended:
PowerPC 601 or equivalent
32MB System Memory
2-speed CD ROM player
Video driver and display capable of 800 x 600, 256 colours
100MB free space on hard drive
OS8.1 or higher
Open Transport 1.3 Networking Software
10baseT Network Adapter with an RJ45 connector
You will also need appropriate lengths of Ethernet cable
to connect your PCs to the ADSL router/hub. Note that
a length of 100 meters must not be exceeded between
the PC and hub.
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Can you tell
me more about the line testing and validation?
After initial ordering, BT engineers will make preliminary
tests on your line to make sure that you are ADSL serviceable.
If the tests conducted on your line show that BT are
unable to install ADSL at your location, you will be
told the reason for this. Some examples of situations
may be line quality, distance from the exchange, incorrect
information being supplied upon application etc.
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What is the
minimum contract period?
The minimum contract period is one month. Your contract
is with us and you should contact us rather than BT
if you have any queries.
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If I move premises
what will happen to my ADSL connection?
Your contract with BT and ourselves is for your specific
location and subject to a minimum period. This minimum
subscription period would remain payable if you move
location whether or not your new location is ADSL serviceable.
If you require ADSL at the new premises then you should
make arrangements for a new installation and your old
contract will be replaced by a new one.
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Who do I report
faults to?
You should report faults to us and not BT in the first
instance. The only exception to this is if the fault
affects only telephony services.
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Is it dangerous
to be permanently attached to the Internet?
Any computer that is connected using ADSL to the Internet
is exposed to the same security risks as a dial-up user,
however since ADSL users are typically connected to
the network for longer periods of time (always on) this
risk is potentially that much greater. You should be
aware of the potential of hacking attacks and the precautions
that you should take. Please read our security section.
Some examples of good security
practice are:
Turn off your PCs when not in use.
Turn off Windows File and Printer Sharing
Install a virus checking software product and ensure
that it is maintained to the latest version.
You should provide a Firewall in the case of a sensitive
installation.
You should provide encryption of data transmission for
sensitive data.
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How do I set
up my USB modem?
The details, together with diagrams, explaining how
you should set up your USB modem with your PC are available
here.
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How do I set
up my Ethernet equipment?
The details, together with diagrams, explaining how
you should set up your Ethernet equipment with your
local area network or single PC are available in here.
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Can you explain
how the contention ratios work?
The USB modem and the Ethernet router solutions have
different contention ratios set up by BT, with these
values set at 50:1 and 20:1 respectively. This means
that for a Home user with a USB modem, the BT connection
at the exchange is shared with another 50 users. Whilst
some degree of performance loss will be experienced
during peak periods, it is generally accepted that few
people will be downloading or uploading data all at
the exact same time. For the business user it makes
sense to go for the 20:1 business version, which will
not be subject to so much slowdown at peak periods.
The two systems will be able to co-exist on the same
'local loop' due to BT's network tunnelling protocols,
each connection being given its own port on the exchange
terminating equipment (DSLAM).
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Do ADSL services
work with non-BT PSTN lines?
Currently the Broadband services are only available
over a BT PSTN line. A non-BT PSTN variant is being
investigated.
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What standard
does BT's ADSL modem adhere to?
Various forms of ADSL signal modulation are possible
but the two more prevalent are: Discrete Multi-Tone
(DMT), and Carrierless Amplitude modulation Phase modulation
(CAP). DMT uses interleaving to achieve good error performance
and has become the international standard.
BT evaluated both DMT and CAP modems during their extensive
trials throughout 1998 and 1999. However, their commercial
ADSL based services will use DMT. Alcatel and Fujitsu
will supply these modems.
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Will BT be blocking
any TCP port numbers?
There has been some speculation in the press regarding
the supposed prospect of port blocking on the ADSL service.
The huge popularity of distributed networking programs
such as Napster, Gnutella and iMesh has led to fears
of excessive bandwidth consumption. BT have stated that
they have no current plans to restrict bandwidth use
or block particular ports, although they may review
this in the future if the service is adversely affected
by flagrant bandwidth abuse.
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What about other
technologies?
What impact will UMTS have on ADSL?
Universal Mobile Telecoms Service - UMTS is the called
the 3rd generation mobile service. It is a mobile network
whereas ADSL runs over a fixed network. So, whilst UMTS
will deliver broadband access too, the two technologies
are more likely to be complementary rather than head-on
competitors.
What is DSL Lite technology compared
to ADSL?
DSL Lite/G.Lite is a lower speed, splitterless version
of ADSL. It is designed to offer around 1Mbps of capacity
downstream, and 120kbps upstream. Because there is no
Splitter, it is easier to install but does mean that
the end user may experience significant interference
between the telephony and ADSL services when they are
used at the same time.
BT's ADSL based services are based on the full ADSL
standard, rather than DSL Lite. BT is involved in the
evolution of the standard for DSL Lite but has no immediate
plans to launch services based on it.
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Disclaimer:
Please note that although we endeavour to make sure
that the information included in this FAQ is up-to-date
and accurate we cannot guarantee that it is exhaustive,
nor that changes won’t be made at any time without
prior notice.
ADSL Glossary
of terms
ADSL – asymmetric
digital subscriber line
BROADBAND –
the technology of using the whole available bandwidth
of the transmission medium, in this case, copper telephone
cable.
CONTENTION –
refers to the number of people who share the same local
loop as yourself. This means that the higher the contention
rate, the slower the ADSL network will become at peak
periods. This is one reason why we regard the Business
ADSL variant as being the superior product because of
its low contention ratio of 20:1
CROSSOVER CABLE –
type of Ethernet cable required to connect a PCs NIC
directly to a router rather than a hub. This was required
during BT’s ADSL trials but is no longer required
because the NTE equipment (for the Ethernet variant)
now has a built-in hub.
DHCP - Dynamic Host
Configuration Protocol. A process by which a machine
assigns a single IP address to a PC on a network. This
IP address is taken from a 'pool' of addresses. The
PC would be set up to receive its IP address 'dynamically'
via its networking settings. This system allows easy
administration of a network since individual addresses
are not 'tied' to a particular machine. Together with
the concept of a 'lease' whereby a time is specified
for the duration of that IP address, it becomes possible
to administer a network where there are more machines
than available IPs in the pool.
DSLAM – Digital
Subscriber Loop Access Multiplexer. Located at the BT
exchanges, these are what your ADSL modem connects to.
ETHERNET –
the technology commonly used to network PCs (also referred
to as 10BaseT or 100BaseT)
IP - Internet Protocol.
All machines on a single network require a unique IP
address in order to communicate with one another. An
IP address is divided into a network part, which must
be the same on any one network, and a host portion,
which must differ.
LOCAL LOOP –
refers to the telephone network between your location
and the nearest BT exchange.
NAT - Network Address
Translation. The name given to the process whereby one
IP (Internet Protocol) address is mapped (or translated)
to another. This generally occurs when a globally valid
IP address is mapped to an IP on a private (internal)
network. The benefits of this are several. Firstly it
becomes possible to connect many machines to the Internet
with only a single Static IP address. It also 'hides'
the actual addresses of the internal machines, since
to the outside world, all the machines share the same
IP address - the network looks like a single machine.
NIC – Network
Interface Card (Ethernet card, 10BaseT card) that is
installed in your PC in order to connect to a local
area network.
NTE – Network Terminating Equipment –
the BT equipment installed at you location. Refers to
the USB modem in the case of a single USB installation
or combined modem/router/hub in the case of the Ethernet
installation.
PSTN – Public
Switched Telephone Network. The telephone network we
use for telephony services.
SPLITTER - device
that splits the available bandwidth of the copper cable
into two or more sections, on for analogue voice transmission,
the other for digital data communication.
NIC – Network
Interface Card – a PC expansion card that allows
a PC to communicate with an Ethernet network via appropriate
cabling.
USB – Universal
Serial Bus - Universal Serial Bus technology means that
your computer will be able to connect to the very high
speeds of ADSL without slowing down the data (USB can
potentially support speeds of 12megabits/per second).
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