563489578934

Sec. 8–3 Digital Subscriber Lines (DSL) 579
Splitter
4 kHz low-pass filter
VF signal
Telephone
VF plus DMT signal
Telephone
Twisted pairline from RT
26 kHz high-pass filter
G. DMT signal
DSL Modem
Ethernet
or USB Signal
Computer
(a) Customer Premises Equipment (CPE)
Power spectral density
VF spectrum
Unused tones due to line conditions
Upstream
carriers
Downstream
carriers
(b) VF plus DMT Spectrum
0 4 26 138
Figure 8–5
1,100
f (kHz)
G.DMT digital subscriber line.
upstream for line length up to 12,000 ft. As shown in Fig. 8–5b, the DMT carrier spacing is
4.3125 kHz. The upstream data are on carriers in the 26–138 kHz band and the downstream
carriers are placed in the 138–1,100 kHz band. In an alternative implementation, both the
upstream and the downstream carriers are placed in the 26–1,100 kHz band, and a hybrid
(or echo canceller) is used to separate the upstream and downstream carriers. DMT has the
advantage of allowing the modulation to be customized to accommodate the transmission
and noise characteristics of a particular line. That is, carriers are deleted at frequencies
where there are nulls in the frequency response of the line caused by reflections and at frequencies
where the noise is large. This allows a BER of 10 -7 to be achieved at a data rate
that the line can support.
G.Lite uses no splitter. (The telephone and data wires are wired directly together in
parallel.) This has the advantage of “no truck roll” (i.e., no installation of a splitter) for
the TELCO, but has the disadvantage of old telephone sets loading down (shorting out)
the data signal and transmission nulls in the spectrum caused by reflections from
telephone line bridge taps (i.e., extension phones). Consequently, G.Lite may not work
for some customers unless offending telephones are replaced or a splitter is installed.
G.Lite uses fewer carriers than G.DMT to avoid the spectral impairments at the higher