The RFL 93B CC uses three squelch rules to determine whether communication quality is below that required
by the RFL 9300. They are identified as Squelch Rule 1, Squelch Rule 2, and Squelch Rule 3 (SQ1, SQ2, and
SQ3):
SQ3 Under Squelch Rule 3 (SQ3), the RFL 93B CC module will squelch the channel if the long-term noise
produces a BER of -3 or worse. This represents one bad bit in one thousand bits, and is equivalent to
an S/N ratio of about 16 dB. When SQ3 is on, the RFL 93B CC module alarms the RFL 93B SV module
and blocks all communication messages from going to the RFL 93B SV module.
To come out of SQ3, 3 the BER must be better than -4 (one bad bit in ten thousand). This is equivalent
to an S/N ratio of about 18 dB. It takes over 4 seconds to measure the 2000 word segments used to cal-
culate SQ3; a fast exit from SQ3 occurs if 1000 error-free word segments in a row are received within 2
seconds. A fast exit can also occur if the EQM (Eye Quality Monitor) indicates that the S/N is better than
18 dB. The EQM value could return to normal in less than 100 ms if the average noise is not too great.
To enter SQ3, 30 bad word segments out of a total of 2000 word segments were measured. If 30 bad
word segments are received in a row, SQ3 is entered without waiting for the full 2000 word segments.
After the RFL 93B CC has entered SQ3, less than 3 bad segments out of 2000 segments are required
to drop SQ3.
SQ2 SQ2 relates to short error pulses. It is the trigger that frames the local receiver hardware, and sends re-
train request messages to the remote terminal. SQ2 will also alarm the RFL 93B SV module and block
messages from being sent to the rest of the terminal. If eight bad words are received out of a total of 25,
the RFL 93B CC enters SQ2. It takes ten good words in a row to exit SQ2.
If SQ2 is on for 100 ms and the EQM value indicates that the channel is good, the hardware receiver is
initialized or framed. If the channel is bad for over 500 ms and SQ2 is on, a request is sent to the re-
mote terminal and the remote terminal sends a training sequence. After the training sequence has been
received, the hardware receiver is initialized for framing. For short noise bursts (less than 50 ms), the
QAM reference vector is not lost and the channel recovers within 10 words after the noise turns off.
SQ1 SQ1 blocks messages from going to the RFL 93B SV module, but does not alarm the module. SQ1 is
entered if two bad words in a row are received, and turned off if three good words in a row are received.
29.10 CHANNEL DELAY LIMITS
The maximum total delay that the supervisor module will accept is 25 ms. There are several delays other than
the channel delay which must be accounted for in this 25-ms interval. The modem has a through delay of about
9 ms which changes slightly each time the modem module is trained. There are several messages in the shift
queue, and there are interboard delays within the RFL 9300 itself.
Adding up all delays other than the channel delay equals about 19 ms, which leaves about 6 ms for channel
delay. A reasonable upper limit on the channel delay would be 5 ms. The ping-pong delay displayed on the front
panel is the total delay, and should be 24 ms or less.
RFL 9300 RFL Electronics Inc.
May 5, 2002 29 - 6 (973) 334-3100
To Next Page
Loading...