[Asterisk-Users] ECHO discussion & (TLP) Transmission Level points for *

[David Boyd]

Hello all, I have been following the recent flurry of emails on echo with a
great deal of curiosity, and have a question for anyone who may have the
answer.

What is the equivalent decibel level of the RTP stream being transported
through *.  This may sound like a funny question but I will try to show what
I mean by a diagram below:

PSTN Receive level into FXO card		Center point level after FXO or FXS  Gain
adjustment

PSTN		>-----Transmit in -8db				>---Receive  0db
PSTN		<-----Receive out -5db				<---Transmit 0db


When you attempt to make a connection from analog line to analog line the
idea is to have both channels achieve a 0/0db level at what would be termed
the center of the SWITCH> This way the call is optimized for connectign both
legs of the call.  A typical tlp (Test Level Point) for PSTN lines would be
to send -5db into the PSTN from your FXO card and expect the receive level
to be -8 or -9 db at the input of your FXO card. This is the same concept
used when dealing with channel banks the digital stream level should be
0/0db.

It stands to reason( at least to me:) ) that if digital levels are not 0/0
for the codecs used for transport of voice within the digital stream then
when you convert sip/iax and send it out over an analog line you will have a
gret potential for reflected power and will experience an echo issue.

Does any of this make sense to anyone out there?


Thanks,
Dave