General overview
Details & benefits
Existing ADSL Users
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.
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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 pay a monthly fee and can be online all day,
every day for no extra charge.
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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.
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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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Can I use my own ADSL Modem?
If you have a BT Router/Modem it will work with Crosbynet
ADSL Broadband. You will simply need to re-configure it
with your Crosbynet ADSL username and password etc.
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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 over 71% 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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Existing ADSL Users
Would the details like IP addresses change or be the
same?
The IP address would change and be Crosbynet ADSL IP
range
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What about the DNS settings?
DNS Settings would also change. DNS Settings are ...
84.12.110.60
84.12.110.61
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Would the log in names we have at present stay the
same?
No. Config of present routers will change i.e
usernames, passwords, realm etc
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What would my outgoing email SMTP settings be?
You existing email accounts would need to change their
outgoing settings to mail.crosbynet.co.uk
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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.
Top
I have a cable telephone. Can I receive ADSL?
No. Our ADSL broadband services are only available over
a BT exchange line.
Top
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 |
|
VC-MUX |
|
