[Scan-DC] FFX digital

[Kevin Loch]

Two days ago (on th 28th) the Fairfax County digital system briefly went
into "failsoft" mode.
This is where each of the 20 channels is assigned a dedicated
talkgroup.  At that time
I noticed that all 20 frequencies were transmitting at once.  All
digital channels had a
digital carrier, the three analog channels had an analog dead carrier. 
The control channel
was still doing it's thing.

I was surprised to hear all 20 channels on the air.  I rarely ever hear
traffic on 853.7875
and 853.9125.  It was the first time I had ever heard traffic on
852.9625.  If the
system is capable of using those three channels, why aren't they in the
regular rotation?

853.9625 and 853.4875 still seem to be in dedicated talkgroup mode. 
They are continuously 
transmitting p25 TDULC frames for talkgroups 17136 and 17328
respectively.  A TDULC frame
(Terminate Data Unit with Link Control) is normally transmitted after
the conclusion of a 
voice transmission to tell the subscriber unit that the voice
transmission is done and what 
talkgroup the channel is currently assigned to.  I suppose the system
uses those frames to
tell the subscriber units what channel is what in failsoft mode.

KL