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Rate Adaptive Transmission
68 Megabit Modem 410F and 420F User Manual
RATE ADAPTIVE TRANSMISSION
The following definitions are useful for understanding the operation of the Megabit Modem:
• Bit Error Rate (BER) is the ratio of received bits that are in error relative to the total number
of bits received, measured over time. For example, 10
-7
BER means that on average one
error occurs per 10
7
bits received.
• Signal to Noise Ratio (SNR) is the ratio (typically expressed in dB) of the received signal
power to the received noise power. It is a measure of the quality of the transmission.
• Margin (SNR margin) is the amount of degradation in SNR that the system can tolerate
under the current conditions and still achieve 10
-7
BER. A margin of 6 dB, for example,
means that the SNR can degrade by 6 dB and still provide a performance of 10
-7
BER. The
Megabit Modem 410F and 420F have a margin configuration option that defaults to 3 dB,
but can be set anywhere between 0 to 15 dB.
• Reach is the longest loop length that the system can support with a given margin.
Rate Adaptation
With ADC's rate adaptive technology, the Megabit Modem 410F and 420F can automatically
startup or adjust to the fastest speed possible, given the transmission distance and line
conditions. Or, you can set the modem to a specific rate. For details, see the
Startup Option
parameter on page 53. For information about setting the maximum and minimum data rates for
upstream and downstream transmission, see the
Max Data Rate (kbps) and Min Data Rate (kbps)
parameters on page 54.
Reach, Data Rate, SNR Margin, and Noise Environment
The maximum transmission rate of the Megabit Modem 410F and 420F is determined by
distance, SNR margin, and the condition of the line (wire gauge, condition noise environment).
Thefigureonpage 78 shows the relationship between reach and data rate for a given set of
conditions. The plots can be used to determine the achievable reach at a given data rate, or they
may be used to determine the achievable data rates at a given distance. In all cases except the
no noise case, a margin of 4 dB was allocated above the SNR that provides a 10
-7
BER.