When you access the Internet with a high-speed broadband connection, you typically use a cable modem, digital subscriber line (DSL), or local-area network (which ultimately may use a DSL or cable connection). DSL and cable modems use the same physical wires as the telephone line or cable TV. Internet access is also becoming more popular using cellular or dedicated wide-area wireless networks as the cost and performance of these alternatives improves.
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To understand DSL, you first need to know a couple of things about a normal telephone line — the kind that telephone professionals call POTS, for Plain Old Telephone Service. One of the ways that POTS makes the most of the telephone company’s wires and equipment is by limiting the frequencies that the switches, telephones and other equipment will carry. Human voices, speaking in normal conversational tones, can be carried in a frequency range of 0 to 3,400 Hertz (cycles per second How Telephones Work). This range of frequencies is tiny. For example, compare this to the range of most stereo speakers, which cover from roughly 20 Hertz to 20,000 Hertz. The phone wire has the potential to handle frequencies up to several million Hertz in most cases. The use of such a small portion of the wire’s total bandwidth is historical — the telephone system has been in place, using a pair of copper wires to each home, for about a century. By limiting the frequencies carried over the lines, the telephone system can pack lots of wires into a very small space without worrying about interference between lines. Modern equipment that sends digital rather than analog data can safely use much more of the telephone line’s capacity. DSL does just that.
Most homes and small business users are connected to an Asymmetric DSL (ADSL) line. ADSL divides up the available frequencies in a line on the assumption that most Internet users look at, or download, much more information than they send, or upload. Under this assumption, if the connection speed from the Internet to the user is three to four times faster than the connection from the user back to the Internet, then the user will see the most benefit (most of the time).
Precisely how much benefit you see will greatly depend on how far you are from the central office of the company providing the ADSL service. ADSL is a distance-sensitive technology: As the connection’s length increases, the signal quality decreases and the connection speed goes down. The limit for ADSL service is 18,000 feet (5,460 meters), though for speed and quality of service reasons many ADSL providers place a lower limit on the distances for the service. At the extremes of the distance limits, ADSL customers may see speeds far below the promised maximums, while customers nearer the central office have faster connections and may see extremely high speeds in the future. ADSL technology can provide maximum downstream (Internet to customer) speeds of up to 8 megabits per second (Mbps) at a distance of about 6,000 feet (1,820 meters), and upstream speeds of up to 640 kilobits per second (Kbps). In practice, the best speeds widely offered today are 1.5 Mbps downstream, with upstream speeds varying between 64 and 640 Kbps.
You might wonder, if distance is a limitation for DSL, why it’s not also a limitation for voice telephone calls. The answer lies in small amplifiers called loading coils that the telephone company uses to boost voice signals. Unfortunately, these loading coils are incompatible with ADSL signals, so a voice coil in the loop between your telephone and the telephone company’s central office will disqualify you from receiving ADSL. Other factors that might disqualify you from receiving ADSL include:
- Bridge taps – These are extensions, between you and the central office, that extend service to other customers. While you wouldn’t notice these bridge taps in normal phone service, they may take the total length of the circuit beyond the distance limits of the service provider.
- Fiber-optic cables – ADSL signals can’t pass through the conversion from analog to digital and back 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.
If you have ADSL installed, you may have been given small filters to attach to the outlets that don’t provide the signal to your ADSL modem. These filters are low-pass filters — simple filters that block all signals above a certain frequency. Since all voice conversations take place below 4 KHz, the low-pass (LP) filters are built to block everything above 4 KHz, preventing the data signals from interfering with standard telephone calls.
ADSL uses two pieces of equipment, one on the customer end and one at the provider’s end. At the customer’s location there is a DSL transceiver, which may also provide other services. The DSL service provider has a DSL Access Multiplexer (DSLAM) to receive customer connections (see DSL Diagram on page 1).
Most residential customers call their DSL transceiver a “DSL modem.” The engineers at the telephone company or ISP call it an ATU-R. Regardless of what it’s called, it’s 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 USB or Ethernet connections. While most of the ADSL transceivers sold by ISPs and telephone companies are simply transceivers, the devices used by businesses may combine network routers, network switches or other networking equipment in the same platform.
The DSLAM at the access provider is the equipment that 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 in a single central office, and different varieties of protocol and modulation in the same type of DSL. In addition, the DSLAM may provide additional functions including routing or dynamic IP address assignment for the customers.
The DSLAM provides one of the main differences between user service through ADSL and through cable modems. Because cable-modem users generally share a network loop that runs through a neighborhood, adding users means lowering performance in many instances. ADSL provides a dedicated connection from each user back to the DSLAM, meaning that users won’t see a performance decrease as new users are added until the total number of users begins to saturate the single, high-speed connection to the Internet. At that point, an upgrade by the service provider can provide additional performance for all the users connected to the DSLAM.