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Wire and Wireless Communication Applications Chap. 8
frequencies. G.Lite has a data rate of 1.5 Mbs downstream and 512 kbs upstream over
an 18,000-ft line. It uses 128 DMT carriers with up to 8 bits, 256 QAM, modulated on
each carrier.
G.DMT and G.Lite may be the answer to the problem of how to provide economical
high-speed internet access. G.DMT is described in ANSI standard T.413 and ITU document
G.992.1. G.Lite is described in ITU document G.992.2.
Video On Demand (VOD)
VDSL technology allows telephone companies (TELCO) to provide a cable-TV type of
program service to the consumer using twisted pair lines. An HDTV (high-definition
TV) signal requires a data rate of 20 Mbs for compressed video, and SDTV (standarddefinition
TV) can be delivered with a 6 Mbs data rate. (See Sec. 8–9 on digital TV for
more details.) With VOD, the subscriber selects the desired program using a set-top box
(STB). The TELCO then sends the data for the selected program to the user via VDSL,
and the STB converts the data to the video signal that is viewed on the TV set. First-run
movies and other pay-per-view programs could also be provided. The VOD subscriber can
have access to almost an unlimited number of TV channels (one channel at a time) and
other video services.
Integrated Service Digital Network (ISDN)
The integrated service digital network (ISDN) uses a DSL to deliver digital data (no VF
signal) to the subscriber. The ISDN subscriber can demultiplex the data to provide any one
or all of the following applications simultaneously: (1) decode data to produce VF signals
for telephone handsets, (2) decode data for a video display, or (3) process data for telemetry
or PC applications.
ISDN uses twisted-pair telephone lines to connect to a circuit-switched network
system, which also provides access to packet-switched systems. This digital connection
provides much better VF quality than corresponding analog signaling over the twisted-pair
line, and it also provides the capability of including multiplexing digital data along with the
VF data.
There are two categories of ISDN: (1) narrowband or “basic rate,” ISDN, denoted by
N-ISDN, and (2) broadband, or “primary rate,” ISDN, denoted by B-ISDN. B-ISDN has an
aggregate data rate of 1.536 Mbs (approximately the same rate as for T1 lines), consisting of
23 B channels (64 kbs each) and one D channel (64 kbs). The B channels carry user
(or bearer) data, which may be PCM for encoded video or audio. The D channel is used for
signaling to set up calls, disconnect calls, or route data for the 23 B channels.
N-ISDN service can be found in Europe but is rarely used in the United States. B-ISDN
service can be found throughout the world, including the United States, and is used to connect
private branch exchanges (PBX) to the PSTN.
The standard implementation of N-ISDN is shown in Fig. 8–6. The N-ISDN subscriber
is connected to the RT of the telephone company by a twisted-pair telephone line.