Triple-Play Service Deployment

Chapter 9: Troubleshooting High Speed Data Service
POTS/
ISDN
ADSL signal encoding
Traditional POTS uses a narrow 4-kHz base band frequency to
transmit analog voice signals. This means that even with
sophisticated modulation techniques, current dial-up modem
technology can only achieve data throughput up to 56 kbps. DSL
obtains much higher throughput by using a much wider
frequency spectrum. Frequency division multiplexing (FDM)
creates multiple frequency bands to carry the upstream and
downstream data.
Upstream Downstream
Figure 9.3: Frequency bands
ADSL1&2
ADSL2+
0.14 1.1 2.2 MHz
The American National Standards Institute (ANSI) and the
International Telecommunications Union (ITU) chose discrete
multi-tone modulation (DMT) as the standard line code for ADSL.
DMT divides the frequency spectra into parallel channels where
the center of each channel is represented by a modulated
quadrature amplitude modulation (QAM) sub-carrier. Each carrier
is orthogonal to the other sub-carriers so there is no interference
between sub-carriers. ADSL is a two-band system where one part
of the frequency spectrum is used for upstream transmission and
the other part (up to 1.1 MHz) is used for downstream
transmission. QAM employs a combination of amplitude
modulation and phase shift keying. For example, a signal that
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