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.
Top
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.
Top
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
Top
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.
Top
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.
Top
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.
Top
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.
Top
Will telephone calls be charged separately?Yes. Normal telephone rental and call charges will still apply.
Top
What quality of service is provided?We aim to provide excellent service quality. Contractual terms and conditions govern the broadband services that we offer.
Top
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)
Top
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.
Top
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.
Top
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.
Top
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.
Top
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 |
Top
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)
Top
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.
Top
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.
Top
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.
Top
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.
Top
Reasons why ADSL may not be available to youSome 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.
Top
Data for router setup
| VPI (virtual path identifier) | | 0 |
| VCI (virtual circuit identifier) | | 38 |
| ATM encapsulation | | aal5mux |
