ADSL for dummies

Telkom yesterday launched asymmetric digital subscriber lines (ADSL) into a market that has little understanding of the technology. With the exception of technology professionals and hard-core Internet users, many of whom have been anxiously awaiting it, few South Africans know the acronym and fewer care.

Yet if the experience in other countries is anything to go by, a significant percentage of the population will be using it in time.

Figures released by specialist digital subscriber lines (DSL) analysis group Point Topic this week claim there are now more than 25 million DSL lines in use around the world. Although growth slowed recently, attributed to the troubles of telecommunications companies and seasonal fluctuations, line growth was 35% in the last six months.

It is not only highly industrialised countries that account for this growth. Japan recorded 1.8 million new DSL lines this year, an increase of more than 100%, and in North America the figure was 1.1 million or 19%. The highest penetration of DSL in relation to the number of telephone lines is in South Korea.

The growing popularity of ADSL is not difficult to understand; it is a technology both popular and useful to consumers and profitable for providers.

Essentially, DSL allows more data to be sent over existing copper telephone lines with little capital cost. ADSL is a form of DSL specifically aimed at consumers. It splits the available data stream in two and optimises the downward path. A typical user will send a request of a few characters to a server and receive hundreds or thousands of times more data as the Web page downloads, so a ratio of 2:1 or more is used, with download speeds twice that at which data can be sent.

Although special, and fairly expensive, equipment must be installed at both ends of the copper line, it is orders of magnitude cheaper than installing a special data line with all the trench-digging this typically entails.

While it creates an always-on connection, the same phone line can also still be used for normal telephone calls. The phone line is effectively split into two frequency ranges, with frequencies below 4KHz used for voice and the rest for data.

The technique makes the quality of the physical copper cable and its length important. A line that can support an acceptable quality voice call can be too poor for DSL, and problems crop up if the distance between the user and the local exchange is more than 4km.

Where a normal modem creates a circuit all the way to the Internet service provider`s computer, a DSL modem connects to the local telephone exchange. From there, the data is carried on the backbone telecommunications network that typically has huge capacity.

Theoretical data speeds of up to 9Mbps, hundreds of times faster than a normal dial-up connection, can be reached up to this point. However, even the backbone network could not sustain many users at such speeds, leading providers such as Telkom to limit speeds to a fraction of what is possible.

New DSL users are also often frustrated by the fact that Web sites can still load at the same speeds experienced with a traditional modem. DSL can only guarantee high speeds up to the local exchange and if any part of the route used to connect to a Web site suffers from congestion, the result is a slower surfing experience.

However, the telecommunications backbones, international connections and networks hosting popular Web servers are constantly being upgraded and data capacity improved. DSL promises to give users the maximum possible speeds for some time to come as the rest of the Internet catches up to it.

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