ws full-duplex transmission to occur over a single electrical path, and it relies frequency splitting to enable you to derive separate voice and data channels from one wire. DSL also retains power in the event of a power failure; if the electricity goes out, you lose your high-speed data services, but you retain your voice services. DSL Technology Finally, it is important to consider one more factor before selecting the broad- band access technology that will work best: contention ratios. In the case of DSL, contention ratio refers to the fact that users of the system compete for use of the same facility at the same time-in this case, the number of people connected to an ISP who all have to share a set amount of bandwidth. The following sections look at each of the DSL family members in turn. Table 12.2 summarizes some of the characteristics of xDSL. Keep in mind that the rates shown in Table 12.2 vary depending on the loop length, loop conditions, con- tention ratios, and so on. Table 12.2 DSL Technology Options Deployment Symmetrical/ Number of Designation Ratified Maximum Speed Distance Asymmetrical Twisted-Pairs G.991.1 Up to 2Mbps in 2.2 miles Symmetrical One or two 1980s each direction (3.6 km) Up to 2Mbps in 3.5 miles Symmetrical each direction (5.5 km) SHDSL G.SHDSL G.991.2 2001; 5.6Mbps in 3.5 miles Symmetrical One or two updated each direction (5.5 km) in 2003 G.dmt G.992.1 7Mbps down- 3.5 miles Asymmetrical stream, 800Kbps (5.5 km) upstream G.lite G.992.2 1.5Mbps down- 3.5 miles Asymmetrical stream, 512Kbps (5.5 km) upstream continued Chapter 12 Broadband Access Alternatives Table 12.2 DSL Technology Options (continued) Deployment Symmetrical/ Number of Designation Ratified Maximum Speed Distance Asymmetrical Twisted-Pairs ADSL2 G.dmt.bis G.992.3 12Mbps down- 3.5 miles Asymmetrical stream, 1Mbps (5.5 km) upstream G.lite.bis G.992.4 12Mbps down- 3.5 miles Asymmetrical stream, 1Mbps (5.5 km) upstream ADSL2+ ADSL2+ G.992.5 24Mbps down- 3.5 miles Asymmetrical stream, 1Mbps (5.5 km) upstream ADSL2-RE G.992.