INTRODUCTION:
When you connect to the Internet, you might connect through a regular modem, through a local-area network connection in your office or home, through a cable modem or through a digital subscriber line (DSL) connection.
DSL is a very high-speed connection that uses the same wires as a regular telephone line.To understand DSL, you first need to know a couple of things about a normal telephone line.
To Know about telephone line please go through the given link
ABOUT DSL:
Asymmetric DSL:
Precisely how much benefit you see from ADSL 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.
Splitting the Signal:
The CAP System
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.
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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.
The DMT System
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.
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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.
Filters
CAP and DMT are similar in one way that you can see as a DSL user.
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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.
DSL Equipment:
ADSL uses two pieces of equipment, one on the customer end and one at the Internet service provider, telephone company or other provider of DSL services. At the customer's location there is a DSL transceiver, which may also provide other services. The DSL service provider has a DSL Access Multiplexer (DSLAM) to receive customer connections
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The Transceiver
Most residential customers call their DSL transceiver a "DSL modem." The engineers at the telephone company or ISP call it an ATU-R. Regardless of what it's called, it's the point where data from the user's computer or network is connected to the DSL line.
The transceiver can connect to a customer's equipment in several ways, though most residential installation uses USB or 10 base-T Ethernet connections. While most of the ADSL transceivers sold by ISPs and telephone companies are simply transceivers, the devices used by businesses may combine network routers, network switches or other networking equipment in the same platform.
The 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.
Alternatives to ADSL:
There are lots of variations in DSL technology -- many of them address DSL's distance limitations in one way or another. Other types of DSL include:
- Very high bit-rate DSL (VDSL) - This is a fast connection, but works only over a short distance. It is capable of handling Internet access, HDTV and on-demand services at rates of 52 Mbps downstream and 12 Mbps upstream.
- Symmetric DSL (SDSL) - This connection, used mainly by small businesses, doesn't allow you to use the phone at the same time, but the speed of receiving and sending data is the same.
- Rate-adaptive DSL (RADSL) - This is a variation of ADSL, but the modem can adjust the speed of the connection depending on the length and quality of the line.
- ISDN DSL (IDSL) - This is a combination of the Integrated Services Digital Network (ISDN) and DSL technology. ISDN was the solution to dial-up Internet -- it allowed voice, text graphics, video and other data to share one telephone line. This made it possible to talk on the phone and use the Internet at the same time. IDSL is faster than ISDN connections but slower than DSL. It can travel a longer distance of 5 to 6 miles, so it is usually a good option for people who can't get DSL in their area.
- Universal DLS (Uni-DSL) - This emerging technology, developed by Texas Instruments, is backwards compatible with all existing versions of DSL. It offers somewhat of a middle ground between ASDL and VDSL -- at longer distances, it can reach the speeds of ASDL, but it can provide greater speeds than VDSL at shorter distances. In some locations, Uni-DSL can provide four times the amount of speed as VDSL.
Advantages and Disadvantages of DSL:
Some advantages of DSL:
- You can leave your Internet connection open and still use the phone line for voice calls.
- The speed is much higher than a regular modem
- DSL doesn't necessarily require new wiring; it can use the phone line you already have.
- The company that offers DSL will usually provide the modem as part of the installation.
- A DSL connection works better when you are closer to the provider's central office. The farther away you get from the central office, the weaker the signal becomes.
- The connection is faster for receiving data than it is for sending data over the Internet.
- The service is not available everywhere.
