Chapter 9: Technical Reference

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, 10-7BER means that on average one error occurs per 107 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 10-7BER. A margin of 6 dB, for example, means that the SNR can degrade by 6 dB and still provide a performance of 10-7BER. 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 10-7BER.

Megabit Modem 400F User Manual

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