1.0                                                       ABSTRACT

Digital Subscriber Line (DSL) is a communications medium used to transfer digital signals over standard telephone lines. Along with cable Internet, Digital Subscriber Line is one of the most popular ways Internet Service Providers provide broadband Internet access. When you make a telephone call using a landline, the voice signal is transmitted using low frequencies from 0 Hz to 4 kHz. This range, called the “voice band,” only uses a small part of the frequency range supported by copper phone lines. Therefore, Digital Subscriber Line makes use of the higher frequencies to transmit digital signals, in the range of 25 kHz to 1.5 MHz. While these frequencies are higher than the highest audible frequency (20 kHz), then can still cause interference during phone conversations. Therefore, Digital Subscriber Line filters or splitters are used to make sure the high frequencies do not interfere with phone calls. Most Digital Subscriber Line technologies require that a signal splitter be installed at a home or business, requiring the expense of a phone company visit and installation. However, it is possible to manage the splitting remotely from the central office. This is known as splitterless Digital Subscriber Line, “Digital Subscriber Line Lite”, G.Lite or Universal Asymmetric Digital Subscriber Line and has recently been made a standard. 

2.0                                                       INTRODUCTION

The digital signal can be separated or filtered, so that some of the bandwidth can be used to transmit an analog signal so that normal telephone calls can be made while a computer is connected to the internet. This gives “always-on” Internet access and does not tie up the phone line. No more busy signals, no more dropped connections, and no more waiting for someone in the household to get off the phone. Because analog transmission only uses a small portion of the available amount of information that could be transmitted over copper wires, the maximum amount of data that you can receive using ordinary modems is about 56 Kbps (thousands of bits per second). With ISDN you can receive up to 128 Kbps. This shows that the ability of your computer to receive information is constrained by the fact that the telephone company filters information that arrives as digital data, puts it into analog form for your telephone line, and requires your modem to change it back into digital. In other words, the analog transmission between your home or business and the phone company is a bandwidth bottleneck. DSL however offers users a choice of speeds ranging from 144 Kbps to 1.5Mbps. This is 2.5 times to 25 times faster than a standard 56 Kbps dial-up modem. This digital service can be used to deliver bandwidth intensive applications like streaming audio/video, online games, application programs, telephone calling, video conferencing and other high-bandwidth services. What different types are there? HDSL is the pioneering high speed format, but is not a commercially viable option due to its need for two twisted pairs and does not have support for normal telephone services. SDSL is symmetric DSL, and operates over a single twisted pair with support for standard voice transmission. The problem with this system is that it is limited to relatively short distances and suffers NEXT limitation due to the use of the same frequencies for transmitting and receiving. IDSL stands for ISDN DSL, and is in many ways similar to ISDN technology. It’s disadvantages are the lack of support for analog voice, and that its 128kbps rate is not much greater than that offered by standard 56kbps V90 modems. VDSL provides very high bit rate DSL, up to 52Mbps, but requires shorter connections lengths than are generally practical. It has been used in conjunction with an experimental project, FTTC (Fiber to the Curb), but development in this area has slowed due to commercial viability issues. ADSL is the most promising DSL technology, proving suitable for personal broadband requirements and allowing for the same channel to still act as a traditional POTS service. Rate Adaptive DSL, RADSL, is a further advancement which is able to automatically optimize the ADSL data rate to suit the conditions of the line being used.

3.0                                                       MAIN SECTION

Digital Subscriber Line (DSL) is a technology that brings high bandwidth information to homes and small businesses over the existing 2 wire copper telephone lines. Since DSL works on the existing telephone infrastructure, DSL systems are considered a key means of opening the bottleneck in the of the existing telephone network, as telephone companies seek cost-effective ways of providing much higher speed to their customers. DSL is a technology that assumes digital data does not require change into analog form and back.


Digital (DSL) is the major competitor to cable Internet access. Commonly offered by telephone companies, home service can share a telephone line with a voice line; business service requires a dedicated telephone line. Because DSL uses existing telephone lines, it would seem logical that DSL would be even more available than cable Internet access. Actually, the opposite is true. There are some significant distance limitations to DSL; as a result, many areas in North America do not have DSL access.

  • DSL provides digital transmission of data between the end-user premises and the first telephone company switch (known as the central office, or CO, although it may not be much of an office, but just a box of electronics). Because it uses telephone lines, the connection to the CO is dedicated to a single user’s premises.
  • DSL piggybacks onto the copper wire that runs from the CO to the end-user premises by using frequencies higher than those used by voice communications. The separation between the two is not perfect, however, and DSL installations therefore require that filters be installed between each telephone and its wall jack.

Note: The definition of a “telephone” can be very broad today. If you have an alarm system that uses the telephone to notify a monitoring company, for example, you need to get the alarm company to install a special DSL filter. Also, don’t forget filters for devices such as cable and satellite TV boxes that call their services providers occasionally with billing information.

  • DSL designed for home use is asynchronous (ADSL): It has faster download speed than upload speed. Typical speeds include 768 Kbps downstream and 128 Kbps upstream; a more costly, faster service is 3 Mbps downstream and 768 Kbps upstream. This provides only dynamic IP addressing. If you want to add a static IP address to the faster service, the price more than triples. Even faster service (7.1 Mbps downstream) with 768 Kpbs upstream) with static IP addressing more than quadruples the price.
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