EXFO CableSHARK P3, User Manual, Page: 118 / 321

VF / DSL Cable Qualifier
106
8.3.5 DMT Test Application Hints
1 Single-Ended DMT test results provide EXTRAPOLATED data rates only!
2
The DMT test can be used to analyze the problem in which a pair of ADSL modems or an ADSL service
fails with the cable. This is done by looking at the signal level and noise level at the ADSL modem
synchronization frequencies. For example, a pair of ADSL modems compliant with the ANSI T1.413
standard employs some twelve tones ranging from 34.5kHz to 310.5kHz at a level of -1.65dBm to -
3.65dBm at 100 ohm to handshake and to maintain synchronization with each other. If a pair of ADSL
modems fails to synchronize, one should look at the cable noise, interference and/or attenuation at any
of those frequencies. Normally, to ensure reliable ADSL service on a cable, the tone signal level at
those synchronization frequencies should be at least 20 dB above the noise level.
3 The insertion loss or attenuation level of the cable at a tone frequency can be obtained by subtracting
the measured tone signal PSD in dBm/Hz at that tone from the normal transmitting tone signal PSD of –
40 dBm/Hz as specified by the T1.413 and G.Lite standards.
4
The DMT test can also be used to predict the data rates for other standards of xDSL applications where
applicable.
5 The powerful noise PSD measurement can be used for applications where the PSD measurement of
noise is required for the frequency range available. Even with one CableSHARK unit, the DMT test can
complete the noise PSD test while it is showing the message of “No Signal Detected! (showing noise
only)”.
8.4..Noise Measurements
Noise on the line is the greatest negative factor in delivering xDSL services to a subscriber. Noise can be
caused by a variety of things including bad splices, changes in wire gauge, adjacent twisted pairs, and a T1
or 2 Mbit/s (E1) line or another xDSL technology bundled in the same cable. The CableSHARK can measure
up to 2.2 MHz or 6 MHz.
NOTE: When turning on the backlight on a monochrome unit, the backlight inverter
will induce some low level interference onto the PSD measurement. This will not affect
the measurements.