ADSL, SDSL and optical fibre are three different internet connection types.
Asymetrical Digita Subscriber Line was the first broadband technology introduced to the mass market in the late 1990s. While providing relatively fast connection speed for the time (1.5 megabits per second originally), they did not need the installation of a specific network as they relied on the old copper telephone wires to carry data.
If downstream connection speeds were improved to reach as much as 25mb/s, upstream speeds have never achieved more than about 3mb/s. Also, the further the end user is located from the telephone exchange, the lower the connection speed - and it goes down pretty quickly.
SDSL was introduced shortly after ADSL to cater the needs of business users who require faster upload speeds. Symmetrical Digital Subscriber Lines provide the same upstream speeds and downstream speeds, but at a much higher price than ADSL connections. As users need a discrete installation from their phone line, coming from a distinct infrastructure network, as performance has not improved much since it was introduced, and as ADSL now allows faster upstream speeds, this technology is being phased out.
Optical fibre is the newest of the three technologies, but it is based on the old engineering field of fibre optics. Electrical data is converted into light signals transmitted over very thin transparent cables called optical fibre. Signals carried over optical fibre suffer very little disruption and can therefore be brought much further than ADSL signals. Optical fibre also means much faster connections, with theoretical bandwidth as 111 gigabits per second.
Optical fibre networks are different networks which need to be dug out and installed, which requires heavy investments. Network coverage is therefore still limited compared to ADSL.
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