Topic:Chapter 16 xDSL

Digital subscriber line (DSL, originally digital subscriber loop) is a family of technologies that provide internet access by transmitting digital data over the wires of a local telephone network. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line (ADSL), the most commonly installed DSL technology. DSL service is delivered simultaneously with wired telephone service on the same telephone line. This is possible because DSL uses higher frequency bands for data separated by filtering. On the customer premises, a DSL filter on each outlet removes the high frequency interference, to enable simultaneous use of the telephone and data.

                                                                        A DSL modem

How It Works

Traditional phone service (sometimes called POTS for "plain old telephone service") connects your home or small business to a telephone company office over copper wires that are wound around each other and called twisted pair . Traditional phone service was created to let you exchange voice information with other phone users and the type of signal used for this kind of transmission is called an analog signal. An input device such as a phone set takes an acoustic signal (which is a natural analog signal) and converts it into an electrical equivalent in terms of volume (signal amplitude) and pitch (frequency of wave change). Since the telephone company's signalling is already set up for this analog wave transmission, it's easier for it to use that as the way to get information back and forth between your telephone and the telephone company. That's why your computer has to have a modem - so that it can demodulate the analog signal and turn its values into the string of 0 and 1 values that is called digital information. 

Types of  DSL:

ADSL -The variation called ADSL (Asymmetric Digital Subscriber Line) is the form of DSL that will become most familiar to home and small business users. ADSL is called "asymmetric" because most of its two-way or duplex bandwidth is devoted to the downstream direction, sending data to the user. Only a small portion of bandwidth is available for upstream or user-interaction messages.

CDSL -CDSL (Consumer DSL) is a version of DSL, trademarked by Rockwell Corp., that is somewhat slower than ADSL (1 Mbps downstream, probably less upstream) and has the advantage that a "splitter" does not need to be installed at the user's end. 

G.Lite or DSL Lite - G.Lite (also known as DSL Lite, splitterless ADSL, and Universal ADSL) is essentially a slower ADSL that doesn't require splitting of the line at the user end but manages to split it for the user remotely at the telephone company.

HDSL - HDSL (High bit-rate Digital Subscriber Line), one of the earliest forms of DSL, is used for wide band digital transmission within a corporate site and between the telephone company and a customer. The main characteristic of HDSL is that it is symmetrical: an equal amount of bandwidth is available in both directions.

IDSL - IDSL (ISDN DSL) is somewhat of a misnomer since it's really closer to ISDN data rates and service at 128 Kbps than to the much higher rates of ADSL.

RADSL - RADSL (Rate-Adaptive DSL) is an ADSL technology from Westell in which software is able to determine the rate at which signals can be transmitted on a given customer phone line and adjust the delivery rate accordingly. Westell's FlexCap2 system uses RADSL to deliver from 640 Kbps to 2.2 Mbps downstream and from 272 Kbps to 1.088 Mbps upstream over an existing line.

SDSL - SDSL (Symmetric DSL) is similar to HDSL with a single twisted-pair line, carrying 1.544 Mbps (U.S. and Canada) or 2.048 Mbps (Europe) each direction on a duplex line. It's symmetric because the data rate is the same in both directions.

UDSL - UDSL (Unidirectional DSL) is a proposal from a European company. It's a unidirectional version of HDSL

VDSL- VDSL (Very high data rate DSL) is a developing technology that promises much higher data rates over relatively short distances (between 51 and 55 Mbps over lines up to 1,000 feet or 300 meters in length). 

x2/DSL - x2/DSL is a modem from 3Com that supports 56 Kbps modem communication but is upgradeable through new software installation to ADSL when it becomes available in the user's area. 3Com calls it "the last modem you will ever need."

Here are 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.

But there are disadvantages:

-   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.  

Difference Between ISDN and DSL

Speed - In terms of speed, DSL is faster than ISDN. DSL sends data packets with speeds ranging from 128Kbps – 1.5Mbps. On the other hand, ISDN comes in two different speeds i.e., 64Kbps and 128Kbps.

Price - In terms of price, ISDN is somewhat more expensive than DSL. The main reason is that DSL utilizes wires that are already installed into homes or businesses, and there is no special line installation needed

Technology - ISDN is a dial-up service and transmits voice and data through a single line. There are two types of ISDN: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI is used mostly for residential homes and comes with three channels.

DSL connections are often referred to as "always on" connections, so don't do not need to dial up a number. In DSL, there is only a single route for carrying voice, data and video. Two types of DSL connections are widely: Symmetric DSL (SDSL) and Asymmetric DSL (ADSL).These two types of DSL connections differ in their data carrying capacities i.e., upload and download. For more downloading, ADSL is a better choice.

ISDN and DSL are both distance sensitive. To get either service, your place should not be more than 18,000 feet away from the central office.

Sources:

http://en.wikipedia.org/wiki/Digital_subscriber_line

http://www.ehow.com/facts_5472956_dsl-benefits.html

http://whatis.techtarget.com/definition/0,,sid9_gci213915,00.html

http://www.tech-faq.com/difference-between-isdn-and-dsl.html