t by using two pairs of wires, it can achieve a 30% increase in the distance allowed. Today, newer standards than HDSL, such as G.SHDSL (discussed later in this chapter), are preferred and used in new installations. SDSL involves a single twisted copper pair that can be up to 3.5 miles 5.5km) long. It is a symmetrical, full-duplex service, and it supports multiple data rates- up to T-1 or E-1 rates-so you can subscribe to varying bandwidths, up to 1.5Mbps or 2Mbps. Symmetry can be very important, depending on the application. If your only goal is to surf the Internet and browse Web sites, then most of the bandwidth you will need is in the downstream direction-from the network to you-in which case solutions such as ADSL are appropriate. But if you're telecommuting or oper- ating in a small office/home office (SOHO) and you need to transfer large files or images or engage in videoconferencing, you need a great deal of bandwidth in the upstream direction as well as in the downstream direction, and in these cases, sym- metrical services are best. So if your major purpose for wanting broadband access goes beyond Internet surfing, to a use such as increased productivity with school or professional work, then SDSL is probably a better option than ADSL. It is more costly than asymmetrical options, but it provides a better performance guarantee. DSL Technology Applications of SDSL include replacement of local repeatered T-1/E-1 trunks, use as fractional T-1/E-1, interconnection of PBXs, support of multirate ISDN, support for switched 384Kbps service (and therefore appropriate bandwidth for lower-level videoconferencing), support for local Frame Relay options, traffic aggre- gation, and high-speed residential service. SDSL was not standardized until the ITU-T standardized G.SHDSL (discussed in the following section). Unfortunately, this leads to some confusion because in Europe G.SHDSL was standardized by the European Telecommunication Standards Institute (ETSI) under the name SDSL. This ETSI variant is compatible with the