on systems inside the core net- work as well. PC with internal ADSL modem Splitters Local loops subscriber Carrier network, modem DSLAM IP, ATM, frame relay, or data channels Carrier central Branch office office Ethernet modem Router Router Router DSL = digital subscriber line DSLAM = DSL access multiplexer ISP = Internet service provider Internet PSTN = Public switched telephone network Data center Figure 6.16 DSL in the local loop. Data Communications 6.4.3.2 DSL Techniques DSL technologies are still evolving and many alternative technologies are available today and new ones are under standardization. The most important technologies, their transmission distances, and data rates are presented in Table 6.1. We introduce these technologies here. Note that these technolo- gies are evolving and the characteristics given in the table are not final. The data rates and distances in Table 6.1 are approximate maximum figures and they are given just for the comparison of the different DSL technologies. We may expect that some of the technologies introduced here will disappear and some of them will receive wide acceptance in a few years time. Which technology a network operator chooses for its service depends on many things, for example, the subscriber loop lengths and cable network characteristics in the operator's network. In Europe more than 90% of all subscriber loops are less than 3 km and most technologies in Table 6.1 seem to be feasible. However, the higher data rate and/or the longer the distance, the more effort is required for installation and troubleshooting, which may make DSL less attractive, especially when competition has pressed service fees low. 6.4.3.3 ISDN DSL and Consumer DSL For residential markets, some carriers in the United States offer low-speed ISDN DSL (IDSL) access. IDSL is essentially ISDN without the ISDN Table 6.1 DSL Technologies, Access Distances, and Service Rates Reach Downstream Upstream Analog Technology Data Rate Data Rate Phone Market 144 Kbps 144 Kbps Residential G.lite ADSL 1.5 Mbp