RFL 9300 RFL Electronics Inc.
October 29, 2001 2 - 8 (973) 334-3100
2.4.2 TERMINALS EQUIPPED WITH SINGLE-CHANNEL MODEM INTERFACES
2.4.2.1 MODEM OPERATION
The RFL 93B MO Modem Module receives voice-band analog signals (300 Hz to 3300 Hz), and converts these
signals into a 7.2-Kbps stream. The method used in the conversion is "Quadrature Amplitude Modulation," or
"QAM." QAM means that the phase and amplitude are changing. The rate of change is 2400 times a second,
and each QAM vector represents three bits. Since the QAM vectors are changing 2400 times a second (baud
rate) and each change equals a three-bit value the data rate equals 7,200 bits per second (3 times 2,400).
To decode these vectors, the modem's receiver must equalize the incoming analog signal to compensate for
changes in phase and amplitude that could be introduced by the communication path. The reference vector
used by the transmitter must also be learned by the receiving modem. A training sequence is sent by the trans-
mitting modem; the receiving modem recognizes this sequence of tones and learns the reference vector while
equalizing the line. After training is complete, the serial data bits inputted to the transmitter appear out of the
receiver (after a delay).
An automatic gain control (AGC) circuit adjusts the incoming level over a dynamic range of 0 to -37 dBm. An
additional measure is available in the modem, since the value of each QAM vector is known. The error from the
ideal point can be measured, and the value of error is called EQM (Eye Quality Monitor). This value is read by
the RFL 93B CC module if the digital words received are in error.
2.4.2.2 COMMUNICATIONS CONTROLLER OPERATION
The RFL 93B CC module controls the RFL 93B MO Modem Module, and receives its incoming serial data. The
incoming serial data is framed into words and decoded. If the words are valid, they are passed on to the RFL
93B SV Supervisor Controller Module for distribution to the other modules in the terminal.
A hardware receiver in the RFL 93B CC module counts the bits and tests each word for a valid CRC code and
framing bit. In normal operation, the receiver hardware is framed correctly and tests the word at a count of fif-
teen (8 data bits + 6 CRC bits + 1 frame bit). If the framing is not correct, the microcontroller goes into a special
sub-routine that resets the counter after five good words are observed.
After the counter is initialized, the burden of counting the bits and strobing each word into the microcontroller
falls on the hardware receiver. When a word is received, the hardware signals the microcontroller and indicates
whether the word passed the CRC test. The software in the microcontroller then applies squelch rules. These
rules determine if the channel is good enough to give messages formed from the words to the RFL 93B SV
module.
2.4.2.3 SQUELCH RULES
The microcontroller counts the number of good and bad words. It uses this information to decide if the channel
is good or bad. The word segments are binary, and the errors that are counted yield a bit error rate (BER) which
is a measure of communication quality. The noise on a channel may be long-term average noise, or the noise
may come in pulses for a short duration.
The quality of the analog signal received by the modem is expressed as a signal-to-noise (S/N) ratio. If the line
coming into the modem is a Type 3002 line, the spectrum is not flat and equalization is required. The link be-
tween S/N and BER (Bit Error Rate) changes slightly with added equalization (2 dB). If the S/N is better than 16
dB, the dependability of the relay is 100 percent with a 3002 conditioned line.
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