Asymmetrical Digital Subscriber Lines (ADSL), is a technology that enables high-speed data to be carried between modems located at the customer site and local telephone exchange via the copper lines normally used for telephone calls.

ADSL is asymmetrical because more data is delivered downstream (towards the user) than is returned to the exchange in the upstream channel (away from the user). Typically, downstream speeds for standard ADSL (also known as ADSL1) are between 0.256 Mbps (Megabytes per second) and 8.0 Mbps, with upstream speeds ranging from 0.064 Mbps to 0.384 Mbps.

Because it operates at higher frequencies than the much lower frequency needed for voice telephone calls, ADSL service is distance limited to between 4.5 km to 6 km, depending on line quality. If there is more than around 5 km (3.1 mi) between the customer and the local exchange, attenuation (signal loss) degrades operation. However, distances in excess of 7 km can be acheived in some rare cases - my own ADSL1 connection operates at a range of 7.2 km from the exchange, but I only get approx 2.3 Mbps on a 1.5 to 8.0 Mbps plan.

Attenuation is the energy loss of signal transmission over distance through a given medium, such as copper wire. Simply put, a person with normal hearing can hear normal speech at a distance of 10m from the speaker but they can't hear a thing at 100m.

This distance limitation makes ADSL less suitable for rural Australia where distance is a major problem in delivering Broadband services. Despite the roll out of ADSL in the Lockyer Valley and Somerset Regions, there are still numerous Broadband 'Black holes' where ADSL is not available.

Fernvale and Lowood exchange have recently been upgraded to ADSL2, which works at a higher frequency with slightly better distance penetration, but speed diminishes signifantly over distance. ADSL2 offers speeds from 1.5 Mbps to 20 Mbps, with an upload speed of 0.820 Mbps.

One stumbling block in the roll out of ADSL is the widespread use of pair gain technology that was used in the past to increase the capacity of existing telephone lines. This a major cause of so-called 'Broadband black spots' in our region.

Pair gain was originally introduced to increase the capacity of the telephone infrastructure long before ADSL was introduced. It is a technology that combines separate signals into a single signal, which is then transmitted through the existing copper cable telephone line. The signals are then separated into individual subscriber lines at the customer premises.

Rather like the introduction of the cane toad into Australia, pair gain may have seemed a good idea at the time, but it has since become a pest to the many people in our region who are deprived of access to ADSL because they are on a pair gain system.

For this reason, wireless or satellite Broadband technologies provide an excellent alternative solution in a rural environment where distance is a factor.

To summarise, ADSL is a technology that uses the copper wire telephone infrastructure to deliver Broadband. However, it has limitations as the signal will only travel up to between 4.5 km to 6.5 km. That‘s fine in an urban region with high density population, but in a rural region, many people live too far from the ADSL enabled exchange to access the service.

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