What is DSL?

DSL is a leased digital service that works over your existing phone line, and is considerably cheaper than similar leased digital services. The reason for its economy is the fact that no additional wiring needs to be installed on the end user premises. DSL uses the additional bandwidth that your copper phone line has available above the human range of hearing to transmit data rates of up to 8Mb/s (the maximum rate depends on your local carrier). This technology is similar to ISDN but is more versatile due to the fact that it allows voice and data down a single line simultaneously. To achieve this, a special piece of hardware at the telephone company's central office combines the data and voice information onto one line for transmission to your home or office. Likewise, a small splitter or in-line filters must be installed on your premises to decode the voice and data. DSL is an "always-on" service, in that dialing an external phone number is not required for connectivity. DSL is also capable of transmitting voice and data simultaneously. In other words, users can talk on the telephone while browsing the Internet using a single phone line.

The service is always active once installed. It does however require a DSL terminal adapter (modem or router) for communication. Its purpose is to convert DSL signals into signals that your computer understands (Ethernet, USB), and vice versa.

You may have seen the term xDSL. This designates that there are many different varieties of DSL services, as shown below:

ADSL: Asymmetric Digital Subscriber Line - This technology allows more bandwidth (speed) downstream (from the provider to you) than upstream (from you to the provider). Downstream, full-rate ADSL supports between 32 kbps and up to 6 Mbps. Upstream, the rate is between 32 kbps and 640 kbps and the operating range is up to 18,000 feet from a central office. ADSL is ideal for consumers who primarily download more information then upload over the Internet.

SDSL: Symmetric or Single Line Digital Subscriber Line - This technology provides the same bandwidth in both directions upstream and downstream. It provides transmission speeds within a T1/E1 range, up to 1.5 Mbps at a maximum range of 12,000 - 18,000 feet from a central office, over a single-pair copper wire. Ideal for small and medium size businesses that have an equal need to download and upload data over the Internet.

IDSL: ISDN Digital Subscriber Line - Similar to SDSL, this technology is symmetrical and provides speeds from 64 -144 kbps over a single-pair copper wire. Ideal for small and medium size businesses beyond serving range of SDSL. Uses ISDN customer equipment.

Limitations

DSL is not available to everybody. DSL is based on copper technology. With that said, the wiring between your location and the telephone company's central office must be 100% copper. Most large phone installations in the last 5 years have involved utilizing fiber optic technology to transmit phone calls to the customer, and only a small amount of copper is used to actually connect the lines. This situation will not work with DSL. Older phone line installations use 100% copper all the way back to the C/O. In the case of DSL, older phone lines are better. Interference, splices, and extraneous loop length are all important factors in the equation. DSL has a length limitation of about 15,000 – 16,000 feet. Length limitations are due to what is called attenuation. Attenuation is the decrease in signal strength as the signal travels farther away from the source. What that means to you is that if you are farther away from the C/O than 16,000 feet, your signal will not be strong enough to decipher and will not be able to achieve the maximum speed offered by your telephone company. This automatically rules you out of service availability. Line noise is a big factor. The more line noise there is, the worse your chances are. Splices, taps, and poorly shielded cabling all cause line noise. The phone company will do a qualification of your line when you decide to sign up for DSL, and they will inform you if these factors are going to affect your availability.

Speed

DSL has a large range of bandwidth options, and it is important to understand what you are paying for. The DSL link between your location and the C/O is guaranteed at the data rate you sign up for. From that point on, however, you share your bandwidth with all other DSL customers. The telephone company transmits your DSL information to your Internet service provider over a Frame Relay or Asynchronous Transfer Mode link. Both of these mediums are shared with all other DSL customers that your telephone company provides service to. Signing up for a 1.5Mb/s DSL link does not necessarily mean you will get 1.5Mbps all of the time. You may only be guaranteed 128kbps. This means that you have the ability to burst up to your full potential, but, depending on the number of customers online at the same time and the amount of traffic they are generating, you may see a decrease in your overall speed.

Internet Service Providers

All of the issues we have discussed deal with physical connections and telephone company problems. No DSL line is complete without a content provider or Internet service provider. It's like having a car without any gas; you can't go anywhere. Most ISPs follow the same bandwidth scheme as their local telephone company, and have a distinct fee that is not related to the physical line. Usually the prices are based on the amount of bandwidth a customer signs up for.