[Asterisk-Users] yet another question on DID trunks

[Olle E. Johansson]

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