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ADSL2+ Tester
b. ADSL Line Modulation
There are three line code modulations in use: Discrete Multi-Tone (DMT), Carrierless
Amplitude Phase Modulation (CAP), and Quadrature Amplitude Modulation (QAM).
DMT is the official standard adopted by ANSI, ITU and ETSI. DMT are supported by
more and more new ADSL modems since T1.413 adopts DMT as the national standard
in USA.
c. Architecture of ADSL
Since the ADSL access system is built on the base of existing telephone twisted line to
support both the data and voice services, the system consists of DSLAM, Splitter (for
ATU-C), Splitter (for ATU-R), Local Loop Line and RTU. The architecture of ADSL is
shown as Fig 31.
Network
Interface
DSLAM
ATU-C
ATU-R
d. New ADSL Standards: ADSL2 and ADSL2+
ADSL2
n
ADSL2 (ITU G.992.3 and G.992.4) adds new features and functionality targeted at
improving performance and interoperability, and adds support for new applications,
services, and deployment scenarios. Among the changes are improvements in data
rate and reach performance, rate adaptation, diagnostics, and stand-by mode.
ADSL2 has been specifically designed to improve the rate and reach of ADSL largely
by achieving better performance on long lines in the presence of narrowband
interference. ADSL2 accomplishes this by improving modulation efficiency, reducing
framing overhead, achieving higher coding gain, improving the initialization state
machine, and providing enhanced signal processing algorithms. As a result, ADSL2
mandates higher performance for all standard-compliant devices.
ADSL2 provides better modulation efficiency by mandating fourdimensional, 16-state
trellis-coded and 1-bit quadrature amplitude modulation (QAM) constellations, which
provide higher data rates on long lines where the signal-tonoise ratio (SNR) is low. In
addition, receiver-determined tone reordering enables the receiver to spread out the
72-0046-03A
ATU-C
ATU-C
Splitter
ATU-C
PSTN
ADSL Transceiver Unit Central Office
Transmission Unit at the customer premises end
Fig 31: Architecture of ADSL
5 Performing Measurements
Downstream
Splitter
Upstream
POTS
A
T
Consumer
U
Interface
ı
R
RTU
58
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