Scott Stingel wrote:
Hi John-
I'll try and give you a brief outline of DID signalling:
Many businesses have several incoming telephone numbers used for different
purposes, for example customer service, sales, etc. Some have individual
telephone numbers for each user in the system. In a home setting on the
other hand, as you know, each telephone number comes in on a different pair
of wires typically. This is not practical in a business enviroment that has
many telephone numbers.
So DID ("direct inward dialing") was invented as a way to re-use a limited
number of physical phone lines to handle calls to different published
numbers. In a business with DID, the phone company uses DID signalling to
identify the number they are about to connect to the business's PBX.
Historically, this was done by pulsing the last 3 or 4 digits of the number
being dialed before connecting the number. The PBX would use these DID
digits to switch the call to the right recipient.
In modern PBX's, typically, digital methods (ex: ISDN) are used to do the
same thing. But many business's still have old PBX's which use the analog
signalling I mentioned before. The type of telephone lines used for DID are
different than regular home telephone lines. Usually, battery voltage is
supplied by the business PBX instead of the telco. Also, the telco signals
a new call by bridging the line briefly instead of ringing the line. The
receiving PBX signals back that it's ready to take the call by momentarily
reversing polarity of the voltage on the line (the is called "winking" the
line)
Anyway, this is much more detail than you want or need!
If that's the case, I'm sure someone else will need it. Updated DID page:
http://www.voip-info.org/tiki-index.php?page=DID
Thank You, Scott!
/Olle
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