DSL is short for Digital Subscriber Line and is a method to move information over phone lines. Phone lines can handle much larger bandwidth than what is used by voice transmission. In the United States, a standard phone line is a pair of copper wires installed by the phone company. While DSL uses a subscriber’s copper phone lines, it is faster than a 30-50 times faster than a dial up connection. To use a DSL connection, the lines must be configured to connect two specific locations. Since the DSL information is carried on a higher frequency than voice, both can be used at once.
On the user end, there is one major piece of equipment. A DSL transceiver, or a "DSL modem," as it is often called, can connect to a customer's equipment in several ways. The two most common residential connections are USB and Ethernet. DSL modems for businesses may also include networking capabilities.
On the side of the access provider is DSLAM, the equipment that really allows a DSL connection to work. The DSLAM takes the connections from many customers and aggregates them onto a single, high-capacity connection to the Internet. This is where DSL differs greatly from cable modems. On a cable modem, a whole neighborhood shares a network loop. Adding new users to the loop can lower performance noticeably. Adding new users on a DSLAM causes no noticeable change.
There are several types of DSL connections. The first is Asymmetrical DSL (ADSL). Typically, in this configuration the download speed is faster than the upload speed. In most cases this is a favorable set up because the majority of users download more information than they upload. Rate Adaptive DSL (RADSL) changes download and upload speeds depending on line quality and length. Single-line DSL (SDSL) is common in small businesses. This configuration provides the same speed rate for both downloads and uploads. One drawback to SDSL is that the phone can’t be used at same time.
There are also High-data-rate DSL (HDSL) and Very-high-data-rate DSL (VDSL) but are only effective over small distances.
There are two competing and incompatible standards for a DSL service. Carrierless Amplitude/Phase (CAP) is an easier and earlier system. With CAP, information is divided into three channels that are widely separated. This minimizes the possibility of interference. Discrete Multitone (DMT) is the method used most often today. DMT divides the data into 247 separate channels, each 4 KHz wide. Each channel is monitored and if the quality is impaired, the signal is shifted to another channel. 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.
Overall there are many factors you should consider when debating a DSL line for your home or business. DSL is faster than ISDN and less costly than a leased line. Connection speeds can be as high as 1.5Mbps and above, but the speed you are able to receive depends greatly on line quality and the distance to your local phone company's central office. DSL lines are distance sensitive; as connection length increases, signal quality decreases significantly. The maximum distance for DSL service is around 18,000 feet, but an individual provider may stick to a lower limit to preserve quality. Another disadvantage to DSL is that if section of the wires between you and the local phone company’s central office are fiber optic cables.
