Stephen is a Cisco qualified network administrator for a leading University and is currently studying towards an MSc in Telelecommunications.  | Thousands of homes around the world are utilising DSL technology. ADSL being the most popular form of DSL connectivity in the residential homes of today. Ever wondered just how DSL works? Stephen explains all in this article..... |
Hey there, and welcome to my article on just how exactly DSL works. I've tried my best to explain as much as I can within this article, but if you are serious about understanding and/or working with DSL, I'd strongly recommend that you have a read of the highly reviewed and saught after book: Fundamentals of DSL Technology
Short for Digital Subscriber Line the xDSL technology is an extremely popular type of broadband Internet connection available for most homes and businesses. Just like cable modems an average xDSL connection currently has transfer speeds of 512 kbit/sec downstream and 128 kbit/sec upstream, however speeds up to three times greater than these are available.
An xDSL modem doesn't require any extra cabling to be installed into your house, unlike a cable modem that is! The reason for this is that xDSL works using the existing telephone lines already installed, however because of the way the xDSL technology works you are still able to use the same telephone line to make calls, even whilst connected to the Internet.
There is however a drawback with xDSL technology, this is that in order to be able to use the service you must be within a specific distance from your telephone companies central office so that the signal strengths reaching your modem are within specific levels. If your too far away then there is little that the telephone company will do to help.
So how does it work?
To understand how xDSL works it is important to look at the technology behind the standard telephone. The technical term for your plain old telephone is POTS, which actually stands for Plain Old Telephone Service! When you make a telephone call your handset sends signals along the copper wires running out of your house, these signals carry your voice and are directed via the telephone exchange to the telephone handset of the person you are calling. When the POTS was originally conceived and implemented the technology was less advanced, less reliable and vastly more exensive that it is today. The result of this is that the telephone companies had to make the service as efficient as possible and had to cram together vast amounts of wires within switch boxes and switch boards. This lead to a problem as when a signal travels down a wire it gives off whats termed EMI (Electro Magnetic Interference) this normally wouldn't be a problem but due to the sheer volume of wires packed together it lead to high levels of interference on the telephone lines. This problem was overcome by limiting the frequencies of the signals used to a range just below that of a human voice, this maintaned quality but mean't that some of the subtle tones that individualise a persons voice were simply chopped off, this is why many people sound somewhat similar on the phone.
Todays technology and the introduction of digital communications has lead to major advances in the way that we communicate and this has allowed the full potential of the now dated telephone lines to be unleashed.
The CAP method works by taking the entire bandwidth of the copper wires and simply splitting them up into 3 distinct sections or bands seperated to alleviate interference. Each signal band is then allocated a particular task. The first band is in the signal range of 0 to 4 khz and is used for telephone conversations. The second band occupies the range of 25 to 160 khz which is used as an upstream channel, whilst the third band covers from 240 khz up to a maximum (depending on conditions) of 1.5 mhz and is used as a downstream channel. This method was simple and effective as poor quality wires or large amounts of interference wouldn't affect the xDSL from working, instead it would just limit the range of the third band and result in slightly reduced speeds.
The DMT is system is much more complex. It works by splitting the entire frequency range (bandwidth) into 247 channels of 4 khz each and allocating a range of the lower channels, staring at around 8 khz, as bidirecitonal to provide upstream and downstream channels. By splitting the bandwidth up in this way it effectively allows one connection to operate as if there were 247 modems connected to it, each of which operating at 4 khz. The technology used in the DMT system is vasltly more complex than that required for the CAP method as each of the 247 channels requires constant monitoring and assessment. If the system detects that a specific channel or range of channels are suffering from interference or a degredation in quality then the data stream must be automatically transferred to different channels. Finally, it is most likely that if you have the DMT system installed then you will need to place low pass filters into any telephone socket that you wish to make voice calls from. This is because voice calls take place below the 4 khz frequency and the filters simply block anything above this to prevent data signals interfering withe ur telephone call.
DSL Equipment
With the fundamental technology now covered all that is left is to now look at the equipment required to utilise an xDSL connection. In essence there are only two main pieces of hardware required and these are the DSL transceiver and the DSL Access Multiplexer (DSLAM).
The DSL transceiver is basically the equipment that is used to connect your computer to the DSL line. In the case of home users this will be the modem that is connected to your computer using either USB or an Ethernet adaptor however business users may have the DSL transceiver technology built into a switch or router that is a part of their company network.
The DSLAM is a piece of equipment at the central office. It takes all of the individual xDSL connections from the region and combines them into one high capacity link to the Internet. The DSLAM is a flexible piece of equipment and can combine both CAP and DMT based DSL lines. One great feature that xDSL has over cable modem technology is that cable modems operate using a shared bandwidth conneciton meaning as more users are added the performance of existing users can drop slightly. xDSL works differently and each customer has their own direct connection to the DSLAM and therefore adding extra users will not impede performance, unless of course the number of users connected to the DSLAM gets so great that its high capacity connection to the Internet become saturated. In which case it is simply a task of increasing the bandwidth on the DSLAM to resolve the problem.
So why can't everybody have xDSL?
As mentioned in the introduction there are contributing factors that decide who is able to use xDSL. The biggest and most important of these is the distance you are from your service providers central office. As we know a signal travelling down a wire is subject to the resistance of the wires themselves and elso external interference, the result is that the signal gets progressively weaker the further it travels. For the xDSL service to be effective the distance from the CO to your home or business is limited to 5,460m (18,000 feet), however to provide speed and reliability most service providers limit this further as customers nearing the distance limits will experience very slow speeds. In theory xDSL is capable of a maximum download speed of 8 megabits/second and an upload speed of 640kilobits/second, however this is at a maximum distance of 1,820m (6,000 feet) from the CO. In the real world however the best speeds offered at the moment are 1.5megabits/second downstream and between 64 and 640kilobits/second upstream. You may however be wondering why standard calls aren't affected by this problem of distance and this is due to the use of amplifiers that boost your call as it travels long distances. These amplifiers however are not compatible with the signals used in xDSL and if an amplifier exists between your home or business and the CO then you will be unable to use the xDSL service. Also telephone lines that use fibre optics are incompatible with xDSL as the signals cannot be correctly converted between the transmission types.
The future of xDSL is bright for a long time to come. Currently the service is restricted to speeds of around 1.5 mbit/second however we know that potentially the speed can be several times greater than this. For the time being at least xDSL can hold its own against cable modems and satellite Internet in the broadband battle, however how long its stanima will last is something that only time will tell.