Digital Subsribers Line Technology ( DSL)

What is DSL?
DSL is a very high speed connection that uses the same wires as that of a normal telephone line.
DSL is the acronym for digital subscriber line.

The advantages of a DSL are as follows:
1. The speed is higher when compared to a regular modem.
2. You can use your telephone to make voice calls while the internet connection is still open.
3. DSL does not need any extra wiring. It can use the phone line you already have.
4. You need not purchase a modem separately. The company that provides you DSL will also provide you the modem.

Disadvantages:
1. The performance of a DSL connection depends on how far you are to the provider's central office.The farther you are, the weaker the speeds are.
2. The connection is faster for receiving data than sending data.
3. The service is not available everywhere.

But how does DSL allow you to browse the internet as well as make voice calls on the telephone?

Keep Reading....


A standard telephone installation involves a pair of copper wires that the phone company installs in your home. These copper wires are capable of carrying much more information than just voice. They can handle much more bandwidth and range of frequencies ( infact millions of Hz). Human voice can be carried in frequency range of 0 - 3400 Hz. DSL makes use of this extra bandwidth without compromising the line's ability to carry voice conversations. The plan is to match particular frequencies to particular tasks.

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.

One of the variety in DSL technology is ADSL.

ADSL (Asymmetric DSL) : Asymmetric DSL is based on the assumption that people use the internet more to download than to upload. So usually the download speeds are three-four times more when compared to the upload speeds. Again this is a distance-sensitive technology. The farther you are from the provider's central office the lower the speeds are. The limits of ADSL services is around 5 km. ASDL can provide maximum download speeds of 8 Mbps ( within a range of 1.5 Km and upload speeds of 640 Kbps. In pracitce its around 1.5 Mbps upstream and 64 - 640 Kbps upstream. Some improved versions of ASDL are also availabe like ASDL2 which offers a download speed of 12 Mbps and upload speed of 1 Mbps and ASDL2+ which offers download speeds of 24 Mbps and upload speed of 3 Mbps.

How is the signal split?
There are two standards that are commonly used. One is the discrete multitone (DMT) and the other is the carrierless amplitude/phase (CAP). Remember, both are incompatible with each other and of course competing with each other.

CAP: CAP was the earlier standard used. It divides the signals on the telephone lines into three distinct frequency zones. The first zone 0 - 4 KHz is meant for carrying voice signals. The next zone from 24 KHz - 160 KHz is for upstream signals. The third zone from 240 KHz to 1.5 MHz ( max) is for downstream signals. However this depends on various factors like line noise, line length and the number of telephone users in a particular telephone company switch. So as this system has three discrete channels widely separated, the possibility of interference between the channels on one line or between signals on different lines is minimized.

DMT: This is a new but more complex standard than CAP. In this case the signals are divided into 247 channels of 4 KHz each. Each channel is monitored. For example, assume that the phone company has divided the copper line into 4 KHz lines and there is a modem connected to each one of them. While monitoring if the signal is impaired, it is shifted to another channel. Thus the system constantly keeps shifting the signals from one channel to the other thereby maintaining the quality of the signal.

Filters:

Ever wondered what the small socket to which the telephone line and the DSL modem line is connected does?

That is called as the low-pass filters. Since voice conversations take place below 4KHz, these low pass filters blocks everything above 4KHz, thereby preventing the data signals from interfering with the voice signals.

DSL Equipments:

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.

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 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.

Now ADSL is not the only variety in DSL technology. There are other variations as well. Few of them are as follows.

Very high bit-rate DSL : This provides higher speed than ADSL but it works only for a shorter distance. It can handle internet access and HDTV and on demand services at the rate of 52 Mbps downstream and 12 Mbps upstream.

Symetric DSL : The speed is the same, but you cannot use the phone while accessing the internet. Useful for small buisnesses.

Rate-adaptive DSL : In this case the modem can adjust the speed of the connection depending on the lenght and the 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.

References : DSL Basics