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Chapter 9: Technical Reference
Megabit Modem 400F User Manual 63
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 400F 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 400F 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.
Reach, Data Rate, SNR Margin, and Noise Environment
The maximum transmission rate of the Megabit Modem 400F is determined by distance, SNR
margin, and the condition of the line (wire gauge, condition noise environment). The figure on
page 73 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.