ADSL Basics (DMT)
Asymmetrical digital subscriber line (ADSL) is emerging as the optimal solution to high-speed Internet access technology. ADSL matches the asymmetric pattern of Internet traffic with speeds of up to 8 Mb/s downstream from the network to the end user, and up to 640 kb/s upstream from the end user to the network. Because ADSL can transmit both voice and data simultaneously over an existing, single copper pair up to 18,000 feet long, it is the perfect solution for service providers to meet the increasing customer demand for faster Internet access. With its amazing speed and economical use of the installed base of copper cable, ADSL keeps the service cost low for both service providers and end users. This Technical Note provides an overview of ADSL technology, a detailed discussion of how ADSL works, and recommended physical layer test procedures that can guarantee consistent performance. This Technical Note focuses on the discrete multi-tone (DMT) version of ADSL. MDF POTS Switch Pedestal POTS POTS Splitter ADSL NID Cross Box
An ADSL system consists of the following components: • ADSL transceiver unit-central office (ATU-C) • ADSL transceiver unit-remote (ATU-R), also referred to as an ADSL modem • Splitter – low pass filter for separating POTS from ADSL • Digital subscriber line access multiplexer (DSLAM) – Multiplexes many ADSL copper lines into one asynchronous transfer mode (ATM) fiber and may include the splitter and ATU-C in the same frame Standard ADSL loop architecture is illustrated in Figure 1.
A DS L
ATM DATA Network NID DLC Cross Box Splice Case
Figure 1 ADSL loop architecture.
From the Loop to the Customer Premise
The combined POTS and ADSL signal travels downstream from the loop to the network interface device (NID) at the customer premise. A splitter in, or adjacent to, the NID splits the signal and sends two distinct signals to the customer premise. The splitter is a passive, low-pass filter that always allows POTS to go through, guaranteeing uninterrupted voice service even if ADSL fails. The splitter also protects the ADSL signal from POTS transients originating from hand sets going on-hook and off-hook. The first output sends a POTSonly signal to the standard telephone sets in the house. The second output sends the combined ADSL and POTS signal to the ATU-R, which contains a high-pass filter that screens out the POTS signal. This resulting data-only signal is then converted to a standard 10BaseT or ATM.25 output for connection to a PC network interface card. This process is illustrated in Figure 2. Depending on the application, existing house wiring is often used to carry the ADSL signal. This is typically referred to as the pair of wires used for line 2 —the yellow and black wires. However, some applications may require the installation of a dedicated pair, sometimes called a homerun, to carry the ADSL signal. For example, faulty wiring that will not carry the ADSL signal or two existing POTS lines would require a dedicated pair.
From the Customer Premise to the Central Office
The combined POTS and ADSL signal travels upstream from the customer premise to the main distribution frame at the central office. The signal then proceeds to the splitter where the POTS signal is routed to the voice switch to be handled via the public telephone network. The ADSL signal continues to the ATU-C, which, along with the splitter, is usually located in the DSLAM. The DSLAM contains many ATU-Cs where the ADSL signals are multiplexed onto an ATM fiber network. This ATM fiber network connects to Internet service providers, which provide data connections to the Internet and other services such as video-ondemand. The Internet service provider is chosen by the end user. Customers who are located many miles from a central office usually...