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Content: • General • How Does it work • What can you use DSL for • Future of DSL • Links for more information General The DSL technology has been in the minds of Telecommunication Services providers since more than a decade (it must have been born sometime around 1985). Initially it was thought as the technology, which will allow the Telecom, companies to deliver video to their customers. This deployments did not really happen and the technology needed to wait another few years when the demand for high speed internet access brought her back to life. How does it work Basically, DSL is a broadband communication technology that provides for high-speed access to the Internet and remote networks (e.g. the network in your office) using the phone lines that are already present in your home. The magic is in utilising frequencies that are out of the frequency band dedicated to the plain old telephone service (POTS) -- PSTN or ISDN, as normal voice and fax services are called. On the figure bellow the separation of technologies into the frequency domain is depicted.  There are country variations in the exact frequency ranges used, e.g. in Europe: • PSTN 0-4 kHz • ISDN 0-80 kHz (Germany -120 kHz) • ADSL 138-1100 kHz As xDSL utilises more of the bandwidth on the copper phone line than what is currently used for POTS, xDSL can encode more data to achieve higher data rates than would otherwise be possible in the restricted frequency range of a POTS network. In order to utilise the frequencies above the voice audio spectrum, and provide both POTS and DSL services on the same phone lines, special equipment must be installed on both ends (i.e. in your home and in the Telecom operator Central Office (CO)). In addition, if the promised data rates are to be really achieved some requirements are imposed on the copper wire lying in between, e.g. bandwidth limiting devices must be removed or avoided, and the distance between the two ends shall not exceed some limits (currently, and depending on the type of DSL, about 6 km). The device usually installed in your home is called a splitter. It is used to separate the frequencies of the telephone audio spectrum from the frequencies of the xDSL signals, that is to allow you to use the line for your telephone as before when using it to access the Internet.  As already said a splitter is normally required at both the customer premises and at the far end (CO). However, whether a splitter is required or not in your home depends on the xDSL service being provided, for example the special version of the DSL called G.lite does not require splitter, if DSL is offered over an analog line splitter is not needed either. A splitter-less scenario is shown on the figure bellow.  There are several forms of xDSL, each designed around specific goals and needs. Some forms of xDSL are proprietary, some are simply theoretical models and some are widely used standards. The table bellow depicts the difference in data rate speed and distance from the Central Office:
Currently, the most popular DSL form in use is the Asymmetric Digital Subscriber Line (ADSL). The basic characteristic of ADSL is the difference between the upstream and downstream bandwidth, hence asymmetric, or uneven. In practice the bandwidth from the provider to the user (downstream) will be the higher speed path. This is in part due to the limitation of the telephone cabling system (e.g. the so called near-end crosstalk and attenuation - have you ever been on the phone and heard some other conversation, not yours, in the background, so, that is a crosstalk; as the attenuation is regarded to the fact that when an analog or digital signal traverses across a medium, it fades and this may lead to the inability to recover the signal on the far end.). Other reason for asymmetry is the desire to accommodate the typical Internet usage pattern where the majority of data is being sent to the user (programs, graphics, sounds and video) with minimal upload capacity required (keystrokes and mouse clicks). Downstream speeds in the offers from the DSL providers typically range from 256 Kbps, through 512 Kbps, 768 Kbps and possibly up to more than 8 Mbps (i.e. 8‘192). Upstream speeds typically range from 64Kbps to about 1 Mbps. Upstream speeds may be important for you if you are frequently uploading files from your PC to external server/PC, e.g. sending e-mails with huge attachments. What can you use DSL for At first, it does not require any change to your phoning and faxing habits - you can still use your existing telephone. The news is that now you can talk on the phone and use the Internet at the same time on a single phone line (of course if you had an ISDN line you were already accustomed to this feature - it is nice isn't it, specially if you have someone in the household that sleeps on-line...). Second, your connection to the Internet is much faster. What can you use fast connection for? Here some ideas: • Faster downloads of anything digital • CD-quality audio • Graphics-rich web-sites • Faster and better multimedia • High-speed multi-player games • Watch on-line movies (depends on your speed) And finally, your connection to the Internet is always on. What does this mean? You do not need to dial up your internet provider, no more need to logging on and off; no more busy signals; no more waiting… just open your browser and go! - you could even install up an own web server where you can host your web site. Future of DSL Apart of increasing the speed, as already said, initially DSL was developed for alternative to cable TV Video transmission to home. Though this service is maybe not available from your providers, who knows, may be one day you will get it. The next big spoken thing is the so called Voice over DSL (VoDSL), that is the possibility in addition to the data service to provide multiple "normal" telephone connections utilising the so called VoIP technology. This "normal" telephone connection in fact will utilise part of the DSL bandwidth providing you with the possibility to conduct multiple telephone calls at the same time. What you will need in this case is to connect one or more normal telephones (see the extensive palette of swissvoice telephones) to the specially provided for this purpose sockets in the DSL modem. Whether this service will be offered any soon depends basically on the balance between (a) the expenses the provider must undertake to introduce the service, (b) the price of this service the operator will ask you for (e.g. price per minute), and, (c) your willingness to pay for this service. Links for more information For more information on DSL you may visit the web site of your Telecom Company, Alternative service provider. For general DSL information check these links: http://www.dslforum.org/ (English) http://www.dslweb.de/ (German) http://www.adsldigest.fr.st/ (French) http://www.adsl.tlcweb.it/voodsl.htm (Italian) To find out more about the swissvoice IP products portfolio click here |
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