Because RFL™ and Hubbell® have a policy of continuous product improvement, we reserve the right to change designs and specifications without notice.
4.13 SETTING THE SQEL FUNCTION
SQEL (Squelch) function is used to control data traffic on the multiplexer backplane and to control
fallback behavior in the event of loss of sync with the received DS1 data stream.
4.13.1 EFFECT OF SQUELCH ON DATA
The receiving Common Module drives received data onto the backplane, where is it used by channel
modules. It is also used by the other common module in the shelf in Drop-and-Insert configurations.
Without squelch enabled, when sync is lost, data driven onto the backplane may contain random or
unrelated data fragments. For some applications the system operator may elect to invoke SQEL to
substitute that random backplane data with All-Ones, in order to prevent random output of channel
modules or to prevent emitting of that data by the other side of a Drop-and-Insert configuration.
Squelch, when activated, will block data on the backplane during loss-of-sync.
4.13.2 EFFECT OF SQUELCH ON FALLBACK
Another issue addressed by the SQEL setting is that the loss-of-sync condition may not be stable. This
is more likely to happen with optical interfaces, as the detector increases its gain while attempting to
recover faint signals from a broken fiber and occasionally regains sync for a brief moment.
A loss-of-sync condition controls the fallback mode of the other common module in a Drop-and-Insert
application; if the receiving Common Module momentarily regains and loses sync, it may be forcing
the other Common Module out of and into fallback just as briefly. Since the fallback condition blocks
pass-through data traffic and uses fallback timing, these spurious switchovers may be undesirable.
To prevent this, the system operator may elect to stretch or debounce the signal on the backplane that
carries loss-of-sync condition to the other Common Module in a Drop-and-Insert shelf. This is also
accomplished by the use of Squelch, in the manner described below.
Squelch, when activated, will debounce (stretch) loss-of-sync signal on the backplane.
4.13.3 SQUELCH FUNCTION TIMERS
The SQEL function has two separate timers and corresponding settings. These are the Pre-Squelch
(PRSQ) timer and the Post-Squelch (POSQ) timer.
The Pre-Squelch timer determines how long the Common Module waits after encountering a loss-of-
sync condition before substituting backplane data with All-Ones. The Post-Squelch timer determines
how long the Common Module waits after recovery of sync before restoring data onto the backplane.
Post-Squelch also determines how long the signal announcing loss-of-sync to the other Common
Module in Drop-and-Insert is extended during recovery, thus delaying the return to normal conditions.
Note that this extension takes place only during recovery; loss-of-sync signal is always activated
instantly to ensure the fastest transition into a fallback condition.
M-DACS-T1 RFL Electronics Inc.
July 18, 2008 4-19 (973) 334-3100
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