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Details & benefits
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.
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. 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:
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. These speeds are between about 10 to 40 times faster than today's typical modems. ADSL enables your normal BT phone line to operate as though there were two cables. You can receive phone calls 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.
ADSL's higher
bandwidth has the potential to
improve access to existing online
services and applications and to
stimulate the development of new
ones: 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. Will telephone calls be charged separately? Yes. Normal telephone rental and call charges will still apply. What quality of service is provided? We aim to provide excellent service quality. Contractual terms and conditions govern the broadband services that we offer. What recommended specification of computer is required for ADSL?
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. 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. 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. This depends on what you want to do. The key benefits of ADSL are:
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.
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. 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.
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. 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. I have a cable telephone. Can I receive ADSL? No. Our ADSL broadband services are only available over a BT exchange line. 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. 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. 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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