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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.

	Here are some additional explanations for speech clipping, where the
other end of the conversation will "diasppear" for a fraction of a second:

1.	Faulty or maladjusted echo supressors.  Four-wire toll circuits
	greater than 500 miles in length generally have what are called echo
	supressors, which literally do just that - suppress talker "echo".
	Echo is the result of unavoidable impedance mismatches at the hybrid
	ends of toll circuits where four-wire lines (i.e., separate transmit
	and receive circuits) are converted to two-wire lines (which eventually
	go to the subscriber location).  In general, an echo suppressor
	is an analog circuit which detects speech in one direction and then
	inserts an attenuation in the opposite direction.  The concept of
	an echo suppressor allows speech in only one direction at one time,
	which is why you may not be able to "interrupt" someone on a toll
	circuit - because they can't hear you until they stop talking!
	Echo suppressors would be an anathema to full-duplex data usage were
	it not for the fact that echo suppressors are designed to be disabled
	when frequencies in the range of 2.0 to 2.3 KHz are detected.  Modems
	either have enough energy distribution in this frequency range, or
	they send pilot tones for this particular purpose.

2.	The use of a "statistical" method of voice circuit multiplexing called
	TASI (Time Assignment Speech Interopolation).  TASI makes use of the
	fact that audible speech is present on a one-way voice channel only
	about 45 percent of the time.  Therefore, using speech detection and
	switching circuitry, it is possible to create more input channels
	than physical channels; when speech is detected on a given input
	channel, that channel is switched to an idle physical channel, with
	the corrsponding action taking place at the opposite end of the
	circuit.  During low traffic periods, a conversation may keep the
	same phsyical channel for its duration.  When traffic is high, each
	successive word may go over a different physical channel.  It is
	unavoidable that several milliseconds is speech is lost during the
	speech detection process.  TASI was originally developed for undersea
	cables; there has been a resurgence in TASI use in recent years since
	integrated circuits have reduced the cost of manufacturing the required
	apparatus.

==>  Larry Lippman @ Recognition Research Corp., Clarence, New York        <==
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