DSL and ADSL broadband Internet access technology


ADSL (Digital subscriber line) is a popular technology for delivering broadband (broadband) information to homes and small businesses through standard (copper) telephone lines. DSL comes in many different flavors, such as SDSL and ADSL2. The most common forms of DSL in the UK, ADSL (Asymmetric DSL) and ADSL2 +, divide a single phone line into separate voice and data channels, allowing you to make a phone call while surfing the Internet at the same time. time.

ADSL offers speeds of up to 8 Mbps (Megabits per second) downstream and 448 Kbps upstream (832 Kbps on professional lines). The technology is cheap, quick and easy to install (does not require an engineer) and reasonably reliable, although performance can suffer due to ISP congestion, distance from the local exchange (shorter lines are faster but anything over 6.5 km is usually slow), poor household wiring, and interference with other electrical devices. Each connection is fixed on a specific telephone line.

The latest ADSL2 + (ITU G.992.5) is capable of pushing download speeds up to 24 Mbps and downloads up to 1.4 Mbps, it also supports port linkage (connecting multiple lines together for faster speeds) and has improved range compared to ADSL. ADSL and ADSL2 + are “Best efforts“broadband services, which means that bandwidth is shared among many users and can be very variable – especially over long distances and off-peak hours (i.e. busy afternoons will slow down traffic). The following graph shows the impact distance can have on speeds.

High speed ADSL and ADSL2 range (meters)

Users of these services should learn to understand common router statistics, which can help diagnose line problems. The numbers for the following will change depending on the condition of your line (for example, expect them to be worse during thunderstorms that will cause additional interference):

ADSL / ADSL2 + Router Conditions and Statistics

* Line Profile / Synchronization Speed

This is the speed (usually in Kbps) at which your router connects to the local telephone exchange. Your line’s actual performance will typically be around 20% below these values, or even lower due to ISP limits or environmental factors (e.g. interference from Christmas tree lights, microwaves and AC adapters).

* Line attenuation (dB)

High attenuation usually means you live further away from your local exchange, so lower is better (for example, 60dB attenuation can give you a better speed of around 2Mbps).

* SNR / Signal to Noise Ratio (dB)

A high SNR usually means faster speeds are possible and upgrading to a faster connection will often cause the SNR to drop. The SNR can be improved by installing a special filtered faceplate on the line, such as BT’s iPlate (Broadband Accelerator) (not to be confused with splitters / microfilters).

* SNR margin (dB)

Related to the SNR above, an SNR margin is a measure of the difference between your current SNR and the SNR required to maintain a reliable connection speed. SNR margins tend to range from around 6 db to 12 db, with a higher value being better. The figure will fluctuate throughout the day, especially at peak times. Note that large fluctuations (eg going from 8db to 4db) can cause connection problems. Sometimes it is possible to change this number and keep it higher, depending on the brand of router you own. Using your home mastersocket for the broadband router is also helpful.

Elsewhere, some companies still use Symmetric DSL (SDSL) technology, which is similar to HDSL over a single twisted pair cable and can carry 1.544 Mbps (US and Canada) or 2.048 Mbps (Europe) in each direction over a duplex line (the speed is the same in both ways) . In addition, those looking for information on VDSL and VDSL2 (FTTC) services should consult our Hybrid-Fiber section above because it’s kind of a mixed technology.

In addition, BT also introduced a special style of long range DSL called Technology enabling broadband (BET), which is based on the High Speed ​​Symmetric Digital Subscriber Line (SHDSL). This is designed to provide speeds of up to 2 Mbps in rural areas that may reside up to 12 km from their local telephone exchange, where normal DSL services would fail. But this method is extremely expensive (up to £ 1,094 + VAT to install), poorly supported by ISPs, and requires two phone lines (twisted pairs) to get the best performance.

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