EXFO CableSHARK P3 VF/DSL Cable Qualifier - Manual (Page 18)

VF / DSL Cable Qualifier
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Figure 2D – Asymmetric Nature of ADSL
There are actually 32 bins allocated for Upstream and up to 250 bins allocated for Downstream; 250 can
only be used with echo cancellation. For the most part, however, DMT implementations will use 218 bins for
Downstream. Guard bands that separate the POTS signals from the ADSL signals use some of the possible
carrier allocations. There is also a guard band between the upstream and downstream carriers.
Figure 2E – ADSL DMT
ADSL uses 256 amplitude modulated carriers. The dark bars represent a “snapshot” of the modulation at any one point in
time. The light bars represent the maximum modulation state amplitude available for each carrier frequency. Notice how at
higher frequencies there are fewer available positions due to the diminishing signal to noise ratio.