 exchanges and each connection minute increases the subscriber's telephone bill. In the case of DSL, data and speech are separated at the local exchange site. Then the data portion is connected to the data network for Internet access. Customers pay a fixed monthly fee for a high-data-rate connection that is always on. We review now a few DSL techniques and their applications. 6.4.3.1 Applications of DSL The carriers or network operators are aiming their DSL services mainly at residential users. For them it provides high-data-rate access to the Internet Introduction to Telecommunications Network Engineering and at the same time an ordinary telephone connection over a local loop. In these applications, ADSL, which transmits at a higher data rate downstream than upstream, and its variants are preferred. Corporate network managers can also take an advantage of the benefits that DSL offers. For the interconnection of LANs between offices in the same region, DSL offers a low-cost, high-data-rate access option. In these applications symmetrical DSL or HDSL is often preferred. Figure 6.16 illus- trates some applications of DSL: remote access to a data center, Internet access, and interconnection of LANs. DSL replaces the ordinary local loop, and DSL modems are needed at both ends of the line. If an ordinary telephone connection is to be available simultaneously, the lowpass filter, splitter, at the carrier's central office, splits off the voice channel and routes it to the PSTN. A DSL access multiplexer (DSLAM) terminates the data channel at the other end of the subscriber loop and sends traffic onto the carrier's backbone data network, implemented by IP, ATM, frame relay technology, or fixed data circuits, where it heads to a remote data center or the Internet. DSL is mainly designed to improve the utilization of subscriber cables in the access network. However, because it requires fewer intermediate repeaters, system cost is reduced and DSL will replace conventional primary rate, 1.5- or 2-Mbps, copper cable transmissi