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» All about ADSL
All about ADSL
ADSL (Asymmetric Digital Subscriber Line) uses an ordinary phone line to deliver a high-speed Internet connection. It does this by converting the data from your computer into high-frequency signals. These high-frequency signals can travel along a telephone cable at the same time as a voice call because the ADSL and voice signals use different frequency ranges:
However, although voice calls do not use frequencies above 3100HZ, the telephone equipment itself can generate spurious noise at higher frequencies which can lead to data corruption within the ADSL frequency band. To prevent this, phone equipment must be filtered to suppress this spurious noise and this can be achieved by fitting a micro-filter to each phone or by fitting a filtered faceplate to the phone socket.
There are two basic flavours of ADSL: ADSL1 and ADSL2, though there are a number of additional variations, the most significant of which are ADSL Max (an enhanced ADSL1) and ADSL2+ (an enhanced ADSL2).
The “correct” naming for ADSL uses a classification set by the International Telecommunications Union. So using this set of standards ADSL1 is actually ITU G.992.1 (G.DMT) and ADSL2+ is ITU G 992.5.
ADSL1 was the original and in the UK was launched with a fixed maximum downstream connection speed of 512k. This was subsequently boosted to an up to 2Mb speed. More recently this was boosted again to an up to speed of 8MB – known as ADSL Max and re-branded as Connect by Sky.
Sky also uses another ADSL1 standard for its LLU products. Details are not absolutely firm but this ADSL mode (which is reported as G.DMT by the Sky router) is a high revision which appears to offer similar performance characteristics to ADSL2.
ADSL2 builds upon ADSL1 to improve the rate and reach largely by achieving better performance on long lines in the presence of interference. It accomplishes this by improving modulation efficiency and providing enhanced signal processing algorithms.
ADSL2+ again builds on ADSL2 by doubling the downstream bandwidth, thereby increasing the data. This results in a significant increase in data rates on shorter phone lines. ADSL2+ can also double the upstream bandwidth which will also provide a doubling of the upstream data rate.
Sky ADSL products
Until recently all ISPs (internet service providers) were only able to resell existing BT products that were based 100% on the BT network infrastructure. However, with regulatory changes, BT is required by law to allow other organisations to fit their own equipment in BT exchanges (subject to space) and crucially also make its local network (the copper cables that run from customers premises to the telephone exchange) available to other companies. This is called LLU (local loop unbundling). A more detailed explanation of LLU is available here:
ISPs who have taken up this opportunity are therefore able to offer their own services and technologies not currently available from BT.
There are currently four Sky Broadband products:
Sky Connect. This is a re-badged BT ADSL Max product which is sold by Sky when they are unable to offer an LLU product. As noted above, it is ADSL1 technology but in its highest form. ADSL Max uses a complex, exchange based rate adaptation process. The most obvious consequence of this for the customer is that there is a 10 day “training period” once first connected. During this period connection speeds may vary considerably until the exchange equipment decides the maximum stable rate for the line conditions. Whilst this process should give a consistent and reliable connection it does mean that the line will not be used to its full potential.
Sky Connect offers connection speeds of up to 8Mb.
Sky Base. Sky Base utilises Sky exchange equipment, LLU and their G.DMT implementation capped to a maximum connection speed (router sync speed) of 2Mb.
Sky Mid. Sky Mid utilises Sky exchange equipment, LLU and their G.DMT implementation capped to a maximum connection speed of 8Mb. It is the most cost effective choice for customers who are unable to achieve speeds greater than this due to their line conditions.
Sky Max. Sky Max utilises Sky exchange equipment, LLU and ADSL2+ capped to a maximum connection speed of 16Mb, or, where line conditions are not suitable for ADSL2+, they use G.DMT, sometimes also capping this to 8Mb.
The ADSL2+ specification allows a potential connection speed of up to 24Mb. However, for this to be achieved the line conditions must be excellent and this is just not achievable for large numbers of UK households. No doubt taking this into account Sky caps the maximum connection speed and markets it as up to 16Mb.
Sky’s G.DMT and ADSL2+ use a “real time” rate adaptation process where the modem (router) and exchange equipment negotiate the best possible speed when the modem connects meaning there is no “training period”. In addition, ADSL2+ should monitor and adapt to changing line conditions continuously. Unfortunately, at least for the Sky router and exchange equipment, this does not appear to be implemented, leading to a common problem of disconnections due to falling noise margins, particularly in the evenings.
As with all ADSL technologies the actual sync speed achieved depends entirely on the line attenuation (in effect the line length) and the line condition (how “noisy” it is). As speeds get higher so the line condition becomes more important. Many customers have found that whilst their old 2Mb BT service gave no problems at all their new Sky 16Mb service is far less reliable. That’s the price of speed.
Guide courtesy of Saturday Last revised: 26/07/07
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