seminars report on dsl technology

Presented By-
Prashant Ashok Karandikar

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CHAPTER 1
INTRODUCTION
1.1 Internet
Today is the era of the computer. Every one is having a PC at his/her home or office and he/she can interact with anybody in the world with the excellent facility provided to the pc called as Internet. Due to Internet, world has come closer. Everybody thinks of the Internet as some magnificent, powerful tool for communication among many corners of the world. Just see the miracle!! A man sitting at home can watch anything on his PC within just a few seconds of time. Internet is a very good facility which uses telephone lines laid down between a telephone set of a subscriber and telephone office nearby and using the telephone network, makes him able to communicate with the entire world.
But every coin has two faces. Can anybody of you think of the disadvantages of the Internet facility provided by ISP or NSP? Obviously one is that nobody can use his telephone set when the Internet is on. Also the main practical problem occurs while downloading any document is the huge time. Time factor is also important today. To overcome these problems new technologies are being developed, one of which is DSL technology, which is a topic of discussion
1.2 DSL (Digital Subscriber Line)
Digital Subscriber Line technology provides transport of high-bit-rate digital information over telephone subscriber lines. Telephone lines, whose heritage dates back to Alexander Graham Bell’s invention of the telephone in 1875, can now transport data at millions of bits per second. This is accomplished via sophisticated digital transmission techniques, which compensate for the many transmission impairments common to telephone lines. The digital transmission techniques involve complex algorithms that
have recently become practical due to the enormous processing power of digital signal processors based on very –large-scale (VLSI) circuits. Marketing alchemists claim that DSLs turn copper into gold.
DSL technology has added a new twist to the utility of telephone lines. Telephone lines, which were constructed to carry a single voice signal with a 3.4 kHz bandwidth channel, can now convey nearly 100 digitally compressed voice signals, or a video signal with quality similar to broadcast television. High-speed digital transmission via telephone lines requires advanced signal processing to overcome transmission impairments due to signal attenuation, cross talk noise from the signals present on other wires in the same cable, signal reflections, radio frequency noise and impulse noise.
The twisted-wire-pair infrastructure connects to virtually every home and workplace in the world, but DSLs have their limitations. Approximately 15% of the telephone lines in the world will require upgrade activity to permit high-speed DSL operation. A corrective measure for long loops includes installation of mid-span repeaters, installation of fiber-remoted multiplexers, and removal of load coils.
CHAPTER 2
HOW DSL WORKS?
2.1 Skinny Voice, Broad Band
A standard telephone installation consists of a pair of copper wires that the phone company installs in your home. The copper wires have lots of room for carrying more than your phone conversations they are capable of handling a much greater bandwidth, or range of frequencies, than that demanded for voice. DSL exploits this "extra capacity" to carry information on the wire without disturbing the line's ability to carry conversations. The entire plan is based on matching particular frequencies to specific tasks.
To understand DSL, you first need to know a couple of things about a normal telephone line, the kind that telephone professionals call POTS, for Plain Old Telephone Service. One of the ways that POTS makes the most of the telephone company's wires and equipment is by limiting the frequencies that the switches, telephones and other equipment will carry. Human voices, speaking in normal conversational tones, can be carried in a frequency range of 0 to 3,400 Hertz. This range of frequencies is tiny. For example, compare this to the range of most stereo speakers, which cover from roughly 20 Hertz to 20,000 Hertz. And the wires themselves have the potential to handle frequencies up to several million Hertz in most cases. The use of such a small portion of the wire's total bandwidth is historical the telephone system has been in place, using a pair of copper wires to each home, for about a century. By limiting the frequencies carried over the lines, the telephone system can pack lots of wires into a very small space without worrying about interference between lines. Modern equipment that sends digital rather than analog data can safely use much more of the telephone line's capacity. DSL does just that.
Most homes and small business users are connected to an asymmetric DSL (ADSL) line. ADSL divides up the available frequencies in a line on the assumption that most Internet users look at, or download, much more information than they send, or upload. Under this assumption, if the connection speed from the Internet to the user is three to four times faster than the connection from the user back to the Internet, then the user will see the most benefit (most of the time).
2.2 Voice and Data
Precisely how much benefit you see will greatly depend on how far you are from the central office of the company providing the ADSL service. ADSL is a distance-sensitive technology: As the connection's length increases, the signal quality decreases and the connection speed goes down. The limit for ADSL service is 18,000 feet (5,460 meters), though for speed and quality of service reasons many ADSL providers place a lower limit on the distances for the service. At the extremes of the distance limits, ADSL customers may see speeds far below the promised maximums, while customers nearer the central office have faster connections and may see extremely high speeds in the future. ADSL technology can provide maximum downstream (Internet to customer) speeds of up to 8 megabits per second (Mbps) at a distance of about 6,000 feet (1,820 meters), and upstream speeds of up to 640 kilobits per second (Kbps). In practice, the best speeds widely offered today are 1.5 Mbps downstream, with upstream speeds varying between 64 and 640 Kbps.
You might wonder, if distance is a limitation for DSL, why it's not also a limitation for voice telephone calls. The answer lies in small amplifiers called loading coils that the telephone company uses to boost voice signals. Unfortunately, these loading coils are incompatible with ADSL signals, so a voice coil in the loop between your telephone and the telephone company's central office will disqualify you from receiving ADSL. Other factors that might disqualify you from receiving ADSL include:
2.2.1 Bridge taps
These are extensions, between you and the central office, that extend service to other customers. While you wouldn't notice these bridge taps in normal phone service, they may take the total length of the circuit beyond the distance limits of the service provider.
2.2.2 Fiber Optic Cables
ADSL signals can't pass through the conversion from analog to digital and back to analog that occurs if a portion of your telephone circuit comes through fiber- optic cables.
2.2.3 Distance
Even if you know where your central office is looking at a map is no indication of the distance a signal must travel between your house and the office.
2.3 Splitting the Signal
There are two competing and incompatible standards for ADSL. The official ANSI standard for ADSL is a system called discrete multitone, or DMT. According to equipment manufacturers, most of the ADSL equipment installed today uses DMT. An earlier and more easily implemented standard was the carrier less amplitude/phase (CAP) system, which was used on many of the early installations of ADSL.