gh-Bit-Rate DSL (HDSL) Asymmetrical DSL (ADSL, ADSL2, ADSL2+) Symmetrical (or Single-Line) DSL (SDSL) Symmetric High-Bit-Rate DSL (SHDSL) Rate-Adaptive DSL (RADSL) Very-High-Bit-Rate DSL (VDSL, VDSL2) Some of the members of the DSL family are symmetrical and some are asymmetri- cal, and each member has other unique characteristics. Twisted-Pair As in many other areas of telecommunications, with xDSL there is not one per- fect solution. One of the main considerations with xDSL is that not every form of xDSL is available in every location from all carriers. The solution also depends on the environment and the prevailing conditions. For example, the amount of band- width needed at the endpoint of a network-and therefore the appropriate DSL family member-is determined by the applications in use. If the goal is to surf the Web, you want to be able to download quickly in one direction, but you need only a small channel on the return path to handle mouse clicks. In this case, you can get by with an asymmetrical service. On the other hand, if you're working from home, and you want to transfer images or other files, or if you want to engage in video- conferencing, you need substantial bandwidth in the upstream direction as well as the downstream direction; in this case, you need a symmetrical service. The following sections briefly describe each of these DSL family members, and Chapter 12 covers xDSL in more detail. HDSL Carriers use HDSL to provision T-1 or E-1 capacities because HDSL deploy- ment costs less than other alternatives when you need to think about customers who are otherwise outside the permitted loop lengths. HDSL can be deployed over a dis- tance of about 2.2 miles (3.6 km). HDSL is deployed over two twisted-pairs, and it affords equal bandwidth in both directions (i.e., it is symmetrical). HDSL is deployed as two twisted-pairs, but some homes have only a single pair of wires running through the walls. Therefore, a form of HDSL called HDSL2 (for two-pair) has been standardized for consumer/residential acti