The transmission windows are used to deliver data services to and from a subscriber
modem. Several flavors of DSL lines have been developed over the years, such as basic
digital subscriber line (bDSL), targeting backward compatibility with integrated
services data network (ISDN) equipment; high-speed digital subscriber line (HDSL),
compatible with the T1 rate of 1.544 Mbps; asymmetric digital subscriber line (ADSL),
which is currently the most widely deployed flavor of DSL with short range transmissions
reaching 16 Mbps in the downstream direction (toward the subscriber); and finally
very high-speed digital subscriber line (VDSL), boasting 24 Mbps in the downstream
direction, though with very short reach. Recent years brought also the development of
VDSL2/2+ (specified in the framework of ITU G.993.2), which permits the transmission
of asymmetric and symmetric (Full-Duplex) aggregate data rates up to 200 Mbit/s on
twisted pairs using a bandwidth up to 30 MHz.
174 Chapter 7
CATV networks were originally designed to deliver analog broadcast TV services to
subscriber TV sets, thus adopting a standard tree-and-branch topology and allocating
most of the transmission channel bandwidth for downstream analog channels.
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