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This article describes a method of monitoring the modem speed in a network in which each modem is provided with nominal and fall-back speeds, based on the measurement of a normalized line quality.
English (United States)
This text was extracted from a PDF file.
This is the abbreviated version, containing approximately
86% of the total text.
Page 1 of 1
Modem Speed Monitoring in a Multispeed Modem Network
This article describes a method of monitoring the modem speed in a network
in which each modem is provided with nominal and fall-back speeds, based on
the measurement of a normalized line quality.
One approach to characterize the quality of the signal received from the line
consists of measuring the quadratic error, which is the average distance the
received signal misses the theoretical target point. The bit error rate (BER) is
strongly related to line quality Q; the smaller Q is, the lower is the bit error rate.
Let Q(o) be the line quality corresponding to a BER of 10/-5/ when the
modem operates at its nominal speed. If Q < Q(o), BER at nominal speed will be
less than 10/-5/; If Q > Q(o), BER at nominal speed will be greater than 10/-5/.
Assuming that noise immunity is about 6 dB higher at fall-back speed than
that at nominal speed, we will have: If Q < 2Q(o), BER at fall-back speed is less
than 10/-5/; If Q > 2Q(o), BER at fall-back speed is greater than 10/-5/.
Since Q(o) depends on the modem type and a network may comprise various
modem types, it is convenient to define "normalized line quality" as: L = K(o) Q
with K(o) = 1 over Q(o).
Whenever a modem is requested by the host system to perform a line quality
measurement, it measures Q and encodes the normalized line quality in
accordance with its operating speed, which is sent to the host, together with a bit
indicating its operating speed.