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From: sirbu@GAUSS.ECE.CMU.EDU (Marvin Sirbu)
Newsgroups: mod.telecom
Subject: Alternate circuits
Message-ID: <8601140633.AA12557@ucbvax.berkeley.edu>
Date: Mon, 13-Jan-86 13:47:28 EST
Article-I.D.: ucbvax.8601140633.AA12557
Posted: Mon Jan 13 13:47:28 1986
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	Another (more serious) problem I have experienced, is that when I use
	many of the LD svcs other than AT&T and a couple of others, after 
	being
	connected for a few seconds, the other person will dissapear for a
	fraction of a second.  This makes it impossible for me to use my modem
	because carrier gets lost after being connected.  This doesn't seem
	to happen to all areas...617 is one of them.  The pitch of the static
	seems to change after that happens too, and often a lot of the static
	goes away.  
	
Telco circuits frequently have automatic monitoring ewuipment which is
checking the quality of a line.  Microwave systems in particular are subject
to periodic fading or dropout due to atmoshperic conditions.  When a poor
microwave link is detected, the entire group of channels running over that
link may be switched to a different tranmission facility -- thus both the
temporary drop out and the better quality when you come back.