While ISPs often advertise ADSL2+ and ADSL services as having maximum downlink connection speeds of 24 Mbps and 8 Mbps, respectively, few broadband users will achieve such speeds. Broadband signals from the exchange suffer attenuation as they travels along the cable from the exchange to your broadband modem, reducing the speeds that can be delivered.
ADSL2+ and ADSL signals pass along the cable connecting your modem to the BT mastersocket and then along the cable between your premises and your local BT street cabinet, which is connected to the BT exchange using another cable. With ADSL2+ and ADSL services, the broadband signal has to pass along all these cables. In general, the longer the total distance between your modem and the exchange, the more that signals are attenuated. In general, the longer the total cable length between your modem and the BT exchange, the greater the so-called line loss (also known as line attenuation). This is measured in dB.
Most modems can display the measured line loss. Using this figure, you can estimate your connection speeds using the graph below, which we have derived from a number of sources, including our own measurements. Note that the graph assumes an SNR target margin of about 6 dB - which is the default initial setting for BT exchange equipment.
Chart of ADSL2/2+ and ADSL connection speed and line loss
Visit our article Chart of ADSL and ADSL2+ speed versus distance for a corresponding chart of downlink connection speed against distance from the exchange.
As can be seen, ADSL2+ can provide substantially higher downlink connection speeds compared with ADSL for those lucky enough to live relatively close to the BT exchange. For line losses between 40 dB and 50 dB, ADSL2+ provides relatively modest performance gains over basic ADSL, although these are certainly worth having! ADSL2+ may not provide noticeable gains for line losses above 55 dB.
The table below provides the chart data in more granular form. Also included is an estimate for line length. Note that the length of the cables between your location and the BT exchange may be considerably more than the direct distance between your location and the exchange 'as the crow flies'. This is because the cable first travels to a street cabinet, and there may be a number of possible routes.
|Line loss (dB)||Line length (between premises and exchange) (km)||ADSL2/2+ speed (Mbps)||ADSL speed (Mbps)|
Relationship between line loss, line length and downlink connection speeds for ADSL2+ and ADSL
While the cables between your premises and the street cabinet and between the street cabinet and the exchange are beyond your direct control, you can ensure that line loss is minimised through the use of a filtered faceplate and connecting your modem directly to the BT mastersocket, to maximise connection speeds.
As discussed in our Target SNR margin tweaking page, reducing the SNR target margin can be an effective way to significantly increase broadband speeds, particularly for those 'stuck' with a high target margin. The impact on downlink connection speed from a reduction in SNR target margin can be easily determined from the table above. Reducing SNR target margin by a given amount produces the same effect as reducing the line loss by the same amount. For example, if your current SNR target margin is 12 dB and you decrease this by 9 dB to 3 dB, this will provide a boost in downlink connection speed equivalent to a 9 dB reduction in line loss.
Don't forget to read our Increase Broadband Speed Guide to get the most out of your broadband service.