Asymmetric digital subscriber line
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30-07-2009, 04:17 PM
ADSL technology is asymmetric. It allows more bandwidth downstream -from an NSP s central office to customer site - than upstream from the subscriber to the central office. This asymmetry, companied with always-on access (which eliminates call setup), makes ADSL ideal for Internet/intranet surfing, video- on -demand, and remote LAN access. Uses of this application typically download much more information than they send. ADSL transmits more than 6 Mbps to a subscriber, and as much as 640Kbps more in both directions. Such rate expands existing access capacity by a factor of 50 or more with out new cabling. ADSL can literally transform the existing public information network from one limited to voice, text, and low-resolution graphics to a powerful, ubiquitous system capable of bringing multimedia, including full motion video, to every home this century. ADSL will play a crucial role over the next decade or more as telephone companies enter new markets for delivery information in video and multimedia formats. New broadband cabling will take decades to reach all prospective subscribers. Success of these new services will depend on reaching as many subscribers as possible during the first few years. By bringing movies, television, video catalogs, remote CD-ROMs, corporate LANs and the internet into homes and small businesses, ADSL will makes these markets viable and profitable for telephone company and application suppliers. Imagine yourself in a world where humans interact with computers. You are sitting in front of your personal computer that can listen, talk, or even scream aloud. It has the ability to gather information about you and interact with you through special techniques like facial recognition, speech recognition, etc. It can even understand your emotions at the touch of the mouse. It verifies your identity, feels your presents, and starts interacting with you .You ask the computer to dial to your friend at his office. It realizes the urgency of the situation through the mouse, dials your friend at his office, and establishes a connection.
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Digital Subscriber Lines (DSL) are used to deliver high-rate digital data over existing ordinary phone-lines. A new modulation technology called Discrete Multitone (DMT) allows the transmission of high speed data. DSL facilitates the simultaneous use of normal telephone services, ISDN, and high speed data transmission, e.g., video. DMT-based DSL can be seen as the transition from existing copper-lines to the future fiber-cables. This makes DSL economically interesting for the local telephone companies. They can offer customers high speed data services even before switching to fiber-optics.
DSL is a newly standardized transmission technology facilitating simultaneous use of normal telephone services, data transmission of 6 M bit/s in the downstream and Basic- rate Access (BRA). DSL can be seen as a FDM system in which the available bandwidth of a single copper-loop is divided into three parts. The base band occupied by POTS is split from the data channels by using a method which guarantees POTS services in the case of ADSL-system failure (e.g. passive filters).
The past decade has seen extensive growth of the telecommunications industry, with the increased popularity of the Internet and other data communication services. While offering the world many more services than were previously available, they are limited by the fact that they are being used on technology that was not designed for that purpose.
The majority of Internet users access their service via modems connects to the Plain Old Telephone System (POTS). In the early stages of the technology, modems were extremely slow by today's standards, but this was not a major issue. A POTS connection provided an adequate medium for the relatively small amounts of data that required transmission, and so was the existing system was the logical choice over special cabling.
Technological advances have seen these rates increase up to a point where the average Internet user can now download at rates approaching 50Kbps, and send at 33.6Kps. However, POTS was designed for voice transmission, at frequencies below 3kHz, and this severely limits the obtainable data rates of the system. To increase performance of new online services, such as steaming audio and video, and improve general access speed, the bandwidth hungry public must therefore consider other alternatives. Technologies, such as ISDN or cable connections, have been in development for sometime but require special cabling. This makes them expensive to set up, and therefore have not been a viable alternative for most people.
1.1 DIFFERENT VARIANTS OF DSL
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 and implimentation, 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.
1.2. WHAT MAKES DSL POPULAR
Digital Subscriber Line (DSL) technology provides high-speed Internet Access using regular telephone lines. It has the ability to move data over the phone lines typically at speeds from 256K to 1.5Mb - up to 25 times quicker than the fastest analog modems available today (56,000 bits per second).
1.3. WHAT ARE THE BENEFITS?
DSL is Always On, 24 hrs A Day
No Dial-Up Required
No Second Phone Line Required
Use the Phone At The Same Time You Are On-Line
No Dropped Connections
Super Fast Speeds
Flat Rate Billing
Upgrade Speed As Your Needs Change
The Bandwidth You Need To Truly Experience The Internet. In addition to its very high speed, DSL has many benefits over analog connections. Unlike dial-up connections that require analog modems to "dial-in" to the Internet Service Provider every time the user wants to retrieve e-mail or obtain access to the Internet, DSL connections are always on.
2. ASYMMETRIC DIGITAL SUBSCRIBER LINE (ADSL)
Asymmetric Digital Subscriber Line (ADSL), a modem technology, converts existing twisted-pair telephone lines into access paths for multimedia and high-speed data communications. ADSL can transmit up to 6 Mbps to a subscriber, and as much as 832 kbps or more in both directions. Such rates expand existing access capacity by a factor of 50 or more without new cabling. ADSL is literally transforming the existing public information network from one limited to voice, text and low resolution graphics to a powerful, ubiquitous system capable of bringing multimedia, including full motion video, to everyone's home this century.
ADSL will play a crucial role over the next ten or more years as telephone companies, and other service providers, enter new markets for delivering information in video and multimedia formats. New broadband cabling will take decades to reach all prospective subscribers. But success of these new services will depend upon reaching as many subscribers as possible during the first few years. By bringing movies, television, video catalogs, remote CD-ROMs, corporate LANs, and the Internet into homes and small businesses, ADSL will make these markets viable, and profitable, for telephone companies and application suppliers alike.
Asymmetric Digital Subscriber Line (ADSL) technology is asymmetric. It allows more bandwidth downstream from an NSP's central office to the customer site than upstream from the subscriber to the central office. This asymmetry, combined with always-on access (which eliminates call setup), makes ADSL ideal for Internet/intranet surfing, video-on-demand, and remote LAN access. Users of these applications typically download much more information than they send.
ADSL transmits more than 6 Mbps to a subscriber and as much as 640 kbps more in both directions (shown in Figure-1). Such rates expand existing access capacity by a factor of 50 or more without new cabling. ADSL can literally transform the existing public information network from one limited to voice, text, and low-resolution graphics to a powerful, ubiquitous system capable of bringing multimedia, including full-motion video, to every home this century.
3. ADSL CAPABILITIES
An ADSL circuit connects an ADSL modem on each end of a twisted-pair telephone line, creating three information channels: a high-speed downstream channel, a medium-speed duplex channel, and a basic telephone service channel. The basic telephone service channel is split off from the digital modem by filters, thus guaranteeing uninterrupted basic telephone service, even if ADSL fails. The high-speed channel ranges from 1.5 to 9 Mbps, and duplex rates range from 16 to 640 kbps. Each channel can be sub multiplexed to form multiple lower-rate channels.
ADSL modems provide data rates consistent with North American T1 1.544 Mbps and European E1 2.048 Mbps digital hierarchies (see Figure 21-2), and can be purchased with various speed ranges and capabilities. The minimum configuration provides 1.5 or 2.0 Mbps downstream and a 16-kbps duplex channel; others provide rates of 6.1 Mbps and 64 kbps for duplex. Products with downstream rates up to 8 Mbps and duplex rates up to 640 kbps are available today. ADSL modems accommodate Asynchronous Transfer Mode (ATM) transport with variable rates and compensation for ATM overhead, as well as IP protocols.
Downstream data rates depend on a number of factors, including the length of the copper line, its wire gauge, the presence of bridged taps, and cross-coupled interference. Line attenuation increases with line length and frequency, and decreases as wire diameter increases. Ignoring bridged taps, ADSL performs as shown