What is ADSL !!

Broadband is one of the primary ways in which a majority of us access the Internet today. But what very few of us know is that it works on a technique called ADSL. Let us take a look at what it is.

Digital Subsriber Line is a broadband connection technology which enables computers to connect to each other over Internet using existing copper wired telephone networks. The main idea of DSL technology is that it works by splitting the existing telephone line signal into two: one for voice and the other for data. “A” stands for Asymmetric DSL which indicates that the data can flow faster in one direction than the other. This is the reason why the download speed is much higher than the upload speed. One of the other characteristics of ADSL is that it is an always ON connection which is automatically established once the PC and modem are switched on. This takes about a minute or two and is the reason why we need to wait for the yellow LED (on the modem) to stop blinking before we can access the Internet. In addition, ADSL reserves bandwidth so that a telephonic line can be simultaneously used with the data access service.

ADSL can be visualized in the following manner:

The splitter in reality is a digital signal processor that works equivalent of 250 QAM (Quadrature Amplitude Modulated) Modems operated in parallel. The task of the splitter is to separate (low frequency) voice signals from the (high frequencies) data. The splitter although small plays a very important role. 

One of the other important terminologies that we should be aware of is contention ratio. To provide a cost effective service - all ADSL services are contended (shared). These contention ratios are applied in the backbone and international part of the network. A commonly applied contention rate is 50: 1. All customers are guaranteed the plan rate up to the local exchange or the ADSL equipment to which they are connected. On the shared part of the network (meaning the part of the network from the telephone exchange to your premises) a 50:1 contention ratio would mean that a 2 Mbps connection would be shared by 49 other users around us. In reality this is very unlikely to happen and the customer would find the connection much faster than dial-up modems. ADSL relies on individual users not making unreasonable traffic demands on the network to provide fast access speeds for all. Contention ratio is one of the reasons we are not able to avail the download rate as promised by the ISP. 

A newer variant called ADSL2 provides higher downstream rates of up to 12 Mbit/s for spans of less than 2.5 km (8000 ft). Higher symbol rates and more advanced noise shaping are responsible for these increased speeds.

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