Triple-Play Service Deployment

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transmits at three bits per baud requires eight binary
combinations to represent the signal. This example assumes two
possible measures of amplitude and four possible phase shifts,
which allow for eight possible waves. Using this method, a large
bit stream is broken down into a few bit words per DMT channel
or tone.
Figure 9.4 ADSL and VDSL bits-per-tone graphs
Chapter 9: Troubleshooting High Speed Data Service
ADSL2+
In July 2002, the ITU completed the ADSL2+ standard which
provides major improvements in data rate and reach
performance, rate adaptation, and diagnostics. ADSL2+ specifies a
downstream frequency spectrum of up to 2.2 MHz providing a
substantial increase in downstream data rates of over 25 Mbps at
shorter loop lengths. ADSL2+ also extends the lower frequency
spectrum which increases upstream data rates to about 1 Mbps.
In addition to increased spectrum usage, other enhancements are
made. As opposed to ADSL where overhead bits are fixed and
always consume 32 kbps per frame of transmission capacity, in
ADSL2+ the overhead data rate can be reduced to 4 kbps,
providing an additional 28 kbps for payload data. Improved
Reed-Solomon error correction coding yields higher coding gain,
and the modifications to the initialization state machine deliver
data rate increases.