Bits per Tone Graph
Interpreting the Results
The lower frequency tones represent the bandwidth used for the upstream signal. The higher frequency tones at the right
show the frequencies used for the downstream signal.
The bits should drop to zero after the end of the upstream. This is the buffer between the upstream and downstream
signals. The actual size of this buffer varies.
If there is a major drop in bits in either the downstream or upstream section, this may indicate that there is a possible
interferer at that frequency. For example, if there is a major drop at 772 kHz, this may represent an interfering T1 signal.
Tone Allocations in ADSL/2+
6.3.2 SNR per Tone
This feature measures the Signal to Noise Ratio (SNR) for each downstream tone used by the modem to transmit the
provisioned rate. This can be displayed as a graph or as a table.
During initial modem link up, a signal to noise measurement is made for each tone and the bit distribution is optimized to deliver
the desired bit rate. During showtime, modems constantly monitor the SNR/tone and the bit distribution is adjusted to optimize
bandwidth based on noise, interference and cross talk. If a tone degrades in quality, a bit swap command adjusts the bit
allocation for that particular tone and these bits are either re-allocated to another tone or removed completely.
As with all transmission lines, signal quality depends primarily on the attenuation and SNR. The frequency of the bin can
therefore determine how many bits can be encoded, i.e. SNR can differ from bin to bin. Generally speaking, 1 bit can be encoded
reliably for each 3dB of available dynamic range above the noise floor e.g. a bin with a SNR of 21dB would be able to carry 7
MTTplus-523_e-manual, D07-00-117P RevA00
