Introduction to Digital Subscriber Lines (DSL)

      by: Lawrence Harte and Robert Flood

Digital subscriber line is the transmission of digital information, usually on a copper wire pair. Although the transmitted information is in digital form, the transmission medium is usually an analog carrier signal (or the combination of many analog carrier signals) that is modulated by the digital information signal. The reason that this technology is important is that it allows the telephone company to use existing investment in copper wires to each home and business to deliver higher speed data services. So for the customers that are in a service area for DSL services, the two copper wires that provided their basic tele

This article is Part 1 of a 5 Part Series

 phone service can be used to deliver higher speed data services as well. DSL service requires special electronic equipment (Digital Subscriber Line Access Module [DSLAM]) to be connected to the two copper wires both at their origination point at the telephone company offices and at the customer's location (DSL Modem).
A DSL network is composed of several key parts; this includes a local access line provider, DSL access provider, backbone network aggregator, ISP provider, and other media providers. DSL services can be provided by a single service provider or may result from the combination of processes from different service providers. The communication network can be divided into several parts; local access lines 

DSL systems are primarily installed and operated by post, telephone, and telegraph (PTT’s) and local exchange carriers (LEC's) are the established telephone network operators or companies that provide local telecommunications services. For some countries, PTTs are government operated telephone systems. In the United States, LEC's are granted franchises to provide telephone services to certain geographical areas as mandated by the Federal Communication Commission (FCC). Recently, deregulation and privatization of telecommunication systems worldwide have allowed the creation of new competing local exchange carriers (CLECs). CLEC's provide similar services as LEC's and PTTs. In some cases, CLECs provide services by leasing existing lines from incumbent local exchange carriers (ILECs) and reselling services on these lines. In other cases, CLECs install new communication lines or provide connection by wireless service. 

Figure 1.1 shows a simplified ADSL communication system that