ADSL uses two pieces of equipment: one on the customer end and one at the provider end:
- Transceiver - At the customer's location, there is a DSL transceiver, which may also provide other services.
- DSL access multiplexer (DSLAM) - The DSL service provider has a DSLAM to receive customer connections.
Most residential customers call their DSL transceiver a DSL modem. The engineers at the telephone company or ISP call it an ATU-R, which stands for ADSL Transceiver Unit - Remote. Regardless of what it's called, the transceiver is the point where data from the user's computer or network is connected to the DSL line. The transceiver can connect to a customer's equipment in several ways, though most residential installation uses Universal Serial Bus (USB) or 10BaseT Ethernet connections. Most of the ADSL transceivers sold by ISPs and telephone companies are simply transceivers, but the devices used by businesses may combine network routers, network switches or other networking equipment in the same box.
The DSLAM at the access provider is the equipment that really makes DSL 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, as well as provide additional functions such as routing and dynamic IP address assignment for customers. For more information about ADSL, check out How DSL Works.
DSL is a distance-sensitive technology: As the connection's length increases, the signal quality and connection speed decrease. ADSL service has a maximum distance of 18,000 feet (5,460 m) between the DSL modem and the DSLAM, though for speed and quality of service reasons, many ADSL providers place an even lower limit on the distance. At the upper extreme of the distance limit, ADSL customers may experience speeds far below the promised maximums, whereas customers close the central office or DSL termination point may experience speeds approaching the maximum, and even beyond the current limit in the future.
You might wonder why, if distance is a limitation for DSL, it's not a limitation for voice telephone calls, too. The answer lies in small amplifiers, called loading coils, that the telephone company uses to boost voice signals. These loading coils are incompatible with DSL signals because the amplifier disrupts the integrity of the data. This means that if there is a voice coil in the loop between your telephone and the telephone company's central office, you cannot receive DSL service. Several other factors might disqualify you from receiving ADSL:
- Bridge taps - These are extensions, between you and the central office, that service other customers.
- Fiber-optic cables - ADSL signals can't pass through the conversion from analog to digital to analog that occurs if a portion of your telephone circuit comes through fiber-optic cables.
- Distance - Even if you know where your central office is (don't be surprised if you don't -- the telephone companies don't advertise their locations), looking at a map is no indication of the distance a signal must travel between your house and the office. The wire may follow a very convoluted path between the two points.
Fiber-optic cables, one of the major disrupting factors of ADSL, is actually what enables VDSL technology. In the next section, you'll find out why.