What is ADSL

Boston

Just some information i gathered. I just spent the last hour putting it to gether for someone, I might as well post it here.


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 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.
a normal telephone line is called POTS ( 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.

The Reason for this is basically C = F by wavelength, were C stands for velocity or speed. Greater the Frequency, the greater the speed. But there is a reason why POTS are kept below 3,400hz because to go higher puts a larger strain on the lines and you need signal relays, this is why the further away form the exchange the less speed you can get off DSL.
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 -- remember that 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 With the greatest preference to download rather then upload.
Precisely how much benefit you see will greatly depend on how far you are from the Exchange that owns the lines the ADSL service operates on. 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 exchange 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.

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 carrierless amplitude/phase (CAP) system, which was used on many of the early installations of ADSL.
CAP operates by dividing the signals on the telephone line into three distinct bands: Voice conversations are carried in the 0 to 4 KHz (kilohertz) band, as they are in all POTS circuits. The upstream channel (from the user back to the server) is carried in a band between 25 and 160 KHz. The downstream channel (from the server to the user) begins at 240 KHz and goes up to a point that varies depending on a number of conditions (line length, line noise, number of users in a particular telephone company switch) but has a maximum of about 1.5 MHz (megahertz). This system, with the three channels widely separated, minimizes the possibility of interference between the channels on one line, or between the signals on different lines.
DMT also divides signals into separate channels, but doesn't use two fairly broad channels for upstream and downstream data. Instead, DMT divides the data into 247 separate channels, each 4 KHz wide. One way to think about it is to imagine that the phone company divides your copper line into 247 different 4-KHz lines and then attaches a modem to each one. You get the equivalent of 247 modems connected to your computer at once! Each channel is monitored and, if the quality is too impaired, the signal is shifted to another channel. This system constantly shifts signals between different channels, searching for the best channels for transmission and reception. In addition, some of the lower channels (those starting at about 8 KHz), are used as bidirectional channels, for upstream and downstream information. Monitoring and sorting out the information on the bidirectional channels, and keeping up with the quality of all 247 channels, makes DMT more complex to implement than CAP, but gives it more flexibility on lines of differing quality.
CAP and DMT are similar in one way that you can see as a DSL user. If you have ADSL installed, you were almost certainly given small filters to attach to the outlets that don't provide the signal to your ADSL modem. These filters are low-pass filters -- simple filters that block all signals above a certain frequency. Since all voice conversations take place below 4 KHz, the low-pass (LP) filters are built to block everything above 4 KHz, preventing the data signals from interfering with standard telephone calls.

The individual channels are Quadrature Amplitude Modulated (QAM) and carry between 2 bits and 15 bits per second per Hz depending on the individual channels' characteristics. The way in which these rates are allocated is adaptive and depends crucially on the channel characteristics. During the initialisation process the individual carriers are allocated `signal spaces' depending on the noise in each corresponding channel. Discrete channels losses up to 50db can be tolerated. The larger the signal to noise ratio, the larger the signal space and thus the number of bits that can be represented by each transmitted symbol in that channel. (For the technically curious the modulations are: 128-QAM, 64-QAM, 32-QAM, 16-QAM, 8-QAM, QPSK).

Each channel in the combined signal is capable of carrying up 64.7 kbits/s. This yields a maximum capacity in excess of 16 Mbits/s (for 256 channels). In practice the higher frequency channels are attenuated and noisy giving significantly less usable bandwidth.

The Standard suggests two possible methods for channel allocation, straight forward FDM (Frequency Division Multiplexing) or ECC (Echo Cancellation and Compensation). The former reserves the first 26 channels (ISDN, 32) for `upstream' data and the remaining 230 (ISDN, 192) for `downstream' data. In the ECC method (known as category 2 ADSL), the first 26 channels are reserved for `upstream' data as before, however `downstream' data uses all 256 channels. This is possible because the data directions are independent, though a special echo cancellation processor is needed to ensure stable operation.

DSLAM
The DSLAM at the access provider is the equipment that really allows DSL to happen. A DSLAM takes connections from many customers and aggregates them onto a single, high-capacity connection to the Internet. DSLAMs are generally flexible and able to support multiple types of DSL in a single central office, and different varieties of protocol and modulation -- both CAP and DMT, for example -- in the same type of DSL. In addition, the DSLAM may provide additional functions including routing or dynamic IP address assignment for the customers.
The DSLAM provides one of the main differences between user service through ADSL and through cable modems. Because cable-modem users generally share a network loop that runs through a neighborhood, adding users means lowering performance in many instances. ADSL provides a dedicated connection from each user back to the DSLAM, meaning that users won't see a performance decrease as new users are added -- until the total number of users begins to saturate the single, high-speed connection to the Internet. At that point, an upgrade by the service provider can provide additional performance for all the users connected to the DSLAM.

Each end of the `copper loop' has an ADSL modem to manage the conversion of digital traffic into the analogue carriers on each channel. Incoming data (w.r.t each modem) is applied to an encoder which allocates data to a specific channel depending upon its respective data carrying capability (established during setup). and adds Forward Error Correction (FEC) using a Reed Solomon code similar to that used in digital TV. The data streams are then individually encoded onto their respective carriers using an Inverse Fast Fourier Transform (IFFT) and framing data added. The data is then serialised and converted to an analogue form using a Digital to Analogue converter (ADC). The decoding process is the exact reverse of this.

The encoding/decoding process results in a transparent digital data channel without rigorous data protection, framing standards or control protocols. For this purpose the well tried ATM (Asynchronous Transfer Mode) transport layer is used, resulting in a connection that can then take `useful' data streams such as TCP/IP or real video data.

dahamsta

*koff*

Howstuffworks "How DSL Works"

*koff koff*

adam

Moriarty

Originally posted by dahamsta
*koff*

Howstuffworks "How DSL Works"

*koff koff*

adam

Maybe he wrote the article for them

Boston

In fariness Dahamsta thats a tad bite misleading, i Resourced the information from several sites of which how things work was but one. Its not like a copied and pasted the information.

Moriarty, see where i said.

Just some information i gathered

Never claimed i wrote it

Btw One source of this information(some of it) is the eircom user guide hand book on adsl.

dahamsta

Like I said before Boston, I have no problem with you posting valid and helpful information, I'm sure it's most welcome to some people. But you should cite your sources, because you're misleading the readers. You're also breaking the law -- I /could/ have pointed DeVore to this thread and to the copyright notice on the HowStuffWorks website, and he would have had to remove it. But I didn't, I dropped a hint.

Please, cite your sources. People put a lot of hard work into writing stuff like that, they deserve the credit for it.

adam

Boston

fair enough

dahamsta

Good man Boston.

adam

Keeks

Originally posted by dahamsta

Please, cite your sources. People put a lot of hard work into writing stuff like that, they deserve the credit for it.

adam

Not only that, but if it is from several sources, it is only your interpetation of what is relevant. Other ppl migh need more info or just to check validity of the information. Thats why u see of the references in manuals, along with giving credit where credit is due.

its handy info tho.