End-to-End DSL Architectures

xDSL Varieties

There are two broad divisions of xDSL—asymmetric (ADSL) and symmetric (SDSL). Current and future types are summarized in Table 1-1.

Table 1-1. Summary of xDSL Types and Standards

Asymmetric DSL	Symmetric DSL (Multiple Varieties of Symmetric DSL Are Marketed as “SDSL”)
CAP— No standard	SDSL— No standard
G.DMT— (Worldwide) ITU G.992.1	HDSL— No standard
ADSL/ISDN (ADSL over ISDN)— No standard	HDSL2— (North America) ANSI T1.418
DMT2-(North America) ANSI T1.413.2 “G.Lite” or “half-rate DMT”— (Worldwide) ITU G.992.2	IDSL— ISDN over DSL
G.dmt.bis (ADSL2)— (Worldwide) G.992.3	SHDSL— (Worldwide) ITU G.991.2
G.lite.bis (half-rate ADSL2)— (Worldwide) G.992.4
VDSL— No standard

The difference between asymmetric and symmetric service is simple: Either the ...