DSL is a means of providing high-speed connections over installed copper wires (Twisted pair).
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.
The figure shows a representation of bandwidth space allocation on a copper wire for ADSL. The area labeled POTS identifies the frequency range used by the voice-grade telephone service. The area labeled ADSL represents the frequency space used by the upstream and downstream DSL signals. The area that encompasses both the POTS area and the ADSL area represents the entire frequency range supported by the copper wire pair.
Another form of DSL technology is symmetric DSL (SDSL). SDSL provides the same capacity in both directions.
The different varieties of DSL provide different bandwidths, some with capabilities exceeding 40 Mb/s. For satisfactory ADSL service, the local loop length must be less than 5.46 km.
A DSL 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. A DSLAM is often built into an aggregation router.
The two components of a DSL connection are the DSL transceiver and the DSLAM:
- Transceiver (modem) – 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 therefore, 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.
ADSL Data and Voice separation
When the service provider puts analog voice and ADSL on the same wire pair, ADSL signals can distort voice transmission. For this reason, the provider splits the POTS channel from the ADSL modem at the customer premises 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.
The demarcation point is the point where the phone line enters the customer premises. The actual device that marks the demarcation point is the Network Interface Device (NID). 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.
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. Click the highlight in Figure 2 to see an image of a microfilter. This solution allows the user to use any jack in the house for voice or ADSL service.
A POTS splitter, separates the DSL traffic from the POTS traffic. The POTS splitter is a passive device. Click the highlight in Figure 3 to see a diagram of a splitter. 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. Installing the POTS splitter at the NID usually means that a technician must go to the customer site.