Digital Subscriber Line


DSL is a means of providing high-speed connections over installed copper wires.
Several years ago, Bell Labs identified that a typical voice conversation over a local loop only required bandwidth of 300 Hz to 3 kHz. For many years, the telephone networks did not use the bandwidth above 3 kHz. Advances in technology allowed DSL to use the additional bandwidth from 3 kHz up to 1 MHz to deliver high-speed data services over ordinary copper lines.
As an example, asymmetric DSL (ADSL) uses a frequency range from approximately 20 kHz to 1 MHz. Fortunately, only relatively small changes to existing telephone company infrastructure are required to deliver high-bandwidth data rates to subscribers. The figure shows a representation of bandwidth space allocation on a copper wire for ADSL. The blue area identifies the frequency range used by the voice-grade telephone service, which is often referred to as the plain old telephone service (POTS). The other colored spaces represent the frequency space used by the upstream and downstream DSL signals.
The two basic types of DSL technologies are asymmetric (ADSL) and symmetric (SDSL). All forms of DSL service are categorized as ADSL or SDSL, and there are several varieties of each type. ADSL provides higher downstream bandwidth to the user than upload bandwidth. SDSL provides the same capacity in both directions.
The different varieties of DSL provide different bandwidths, some with capabilities exceeding those of a T1 or E1 leased line. The transfer rates are dependent on the actual length of the local loop, and the type and condition of its cabling. For satisfactory service, the loop must be less than 5.5 kilometers (3.5 miles).
Service providers deploy DSL connections in the last step of a local telephone network, called the local loop or last mile. The connection is set up between a pair of modems on either end of a copper wire that extends between the customer premises equipment (CPE) and the DSL access multiplexer (DSLAM). A DSLAM is the device located at the central office (CO) of the provider and concentrates connections from multiple DSL subscribers.

Transceiver - Connects the computer of the teleworker to the DSL. Usually the transceiver is a DSL modem connected to the computer using a USB or Ethernet cable. Newer DSL transceivers can be built into small routers with multiple 10/100 switch ports suitable for home office use.
DSLAM - Located at the CO of the carrier, the DSLAM combines individual DSL connections from users into one high-capacity link to an ISP, and thereby, to the Internet.
The advantage that DSL has over cable technology is that DSL is not a shared medium. Each user has a separate direct connection to the DSLAM. Adding users does not impede performance, unless the DSLAM Internet connection to the ISP, or the Internet, becomes saturated.
The major benefit of ADSL is the ability to provide data services along with POTS voice services.
When the service provider puts analog voice and ADSL on the same wire, the provider splits the POTS channel from the ADSL modem using filters or splitters. This setup guarantees uninterrupted regular phone service even if ADSL fails. When filters or splitters are in place, the user can use the phone line and the ADSL connection simultaneously without adverse effects on either service.
ADSL signals distort voice transmission and are split or filtered at the customer premises. There are two ways to separate ADSL from voice at the customer premises: using a microfilter or using a splitter.
A microfilter is a passive low-pass filter with two ends. One end connects to the telephone, and the other end connects to the telephone wall jack. This solution eliminates the need for a technician to visit the premises and allows the user to use any jack in the house for voice or ADSL service.
POTS splitters separate the DSL traffic from the POTS traffic. The POTS splitter is a passive device. In the event of a power failure, the voice traffic still travels to the voice switch in the CO of the carrier. Splitters are located at the CO and, in some deployments, at the customer premises. At the CO, the POTS splitter separates the voice traffic, destined for POTS connections, and the data traffic destined for the DSLAM.
The actual device is the network interface device (NID). This point is usually where the phone line enters the customer premises. At this point, a splitter can be attached to the phone line. The splitter forks the phone line; one branch provides the original house telephone wiring for telephones, and the other branch connects to the ADSL modem. The splitter acts as a low-pass filter, allowing only the 0 to 4 kHz frequencies to pass to or from the telephone. Installing the POTS splitter at the NID usually means that a technician must go to the customer site.
Because of this additional labor and technical support, most home installations today use microfilters, as shown in the figure. Using microfilters also has the advantage of providing wider connectivity through the residence. Since the POTS splitter separates the ADSL and voice signals at the NID, there is usually only one ADSL outlet available in the house.
One wire would go directly to the DSL modem, and the other would carry the DSL signal to the telephones. If you roll over the splitter box on the graphic, a typical wiring scheme will be revealed.

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