Asymmetric digital subscriber line
Asymmetric digital subscriber line (ADSL) is the most mature and standardized of all asymmetrical DSLs in terms of available documentation, service trials, and open specifications.
The visual shows the basic ADSL system architecture, from the ADSL Forum. Like most architecture diagrams, this one establishes a number of standard interfaces between major components. The internal functioning of these components is left up to individual product vendors. It is not necessary to detail all of the interfaces and functional groupings. Only the highlights are mentioned here.
Several features of ADSL are quite important. First of all, note that provision is made to support analog voice service, or POTS. A combination high pass/low filter device called a splitter enables the 4 kHz analog channel from switch to customer premises and back to exist “under” the packet “data” bandwidth on the ADSL link. Even in splitterless ADSL architectures, support for analog voice requires at least a splitter in the CO/LE.
Most commonly, the customer interface on the DSL modem (ATU-R) is a 10BASE-T Ethernet interface running at 10 Mbps. It is possible to make the ADSL modem a simple board in a PC, or the modem can be connected to the PC through a universal serial bus (USB) connection.
Many services are envisioned for ADSL systems, including digital broadcast, broadband (i.e., video, Internet access), and network management. All of these services are accessed outside of the normal voice switch, neatly solving the trunking and switch congestion problems. Many ADSL links are serviced by a single ADSL access node in the CO/LE. This access node is usually called a DSL access multiplexer (DSLAM) but it is technically the ADSL access node.
Finally, the implementation of ADSL on the customer premises can take a variety of forms. Under the architecture, these schemes form the premises distribution network. This could be as simple as individual wire pairs running to devices such as TV set-top boxes or PCs, or as elaborate as a full LAN such as Ethernet in the home.
ADSL from ATU-C to ATU-R
Perhaps the best way to understand how ADSL works is to break the overall ADSL architecture down into more manageable components. The visual isolates the ADSL architecture between ADSL termination unit-central office (ATU-C) and ADSL termination unit-remote (ATU-R), or DSL modem. Between ATU-C and ATU-R, the ADSL line code is most important. Although the splitter is part of the ATU-C–ATU-R network, splitters warrant their own considerations. The most important topic here is how the single pair of wires between ATU-C and ATU-R should carry digital information.
This level of ADSL should address the following issues.
- Line code: Current products use carrierless amplitude phase modulation (CAP), quadrature amplitude modulation (QAM), or discrete multitone modulation (DMT), but only DMT is the standard line code.
- ADSL frame structure: ADSL specifications establish both a frame and superframe structure that is quite flexible.
- ADSL frame contents: ADSL can carry IP packets, ATM cells, or frame relay frames. ATM cells are the most common ADSL frame content.