[Asterisk-Users] yet another question on DID trunks

[Scott Stingel]

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!

Cheers
Scott Stingel
 

Scott M. Stingel 
Emerging Voice Technology Inc.

Email:          scott "at"  evtmedia.com  
URL:            www.evtmedia.com 



-----Original Message-----
From: asterisk-users-admin at lists.digium.com
[mailto:asterisk-users-admin at lists.digium.com] On Behalf Of john lawler
Sent: Wednesday, January 07, 2004 7:38 PM
To: asterisk-users at lists.digium.com
Subject: [Asterisk-Users] yet another question on DID trunks


Hey Steven,

Sorry to bother you yet again w/ a question on my seemingly endless 
quest to get DID trunks setup for a customer.

If you don't know anything about this issue or would rather I looked 
elsewhere (including the Asterisk list, I suppose), please just let me 
know right off the bat.  I'm having great difficulty finding good 
resources on the web that explain this technology.

I'm still trying to examine a DID solution for this customer but don't 
understand how a single trunk (whatever that is--I assumed just a single 
pair of wires like a POTS line, but I'm thinking now it must not be) can 
support multiple (incoming) phones calls.  I've also been told how many 
phone calls per trunk depends on your equipment.  Do you know anything 
about equipment that interfaces with DID trunks?  Are there special 
devices that do this?  I think I've heard about FXS port devices 
supporting DID.  But how could a single FXS port (say on a channel bank, 
or even one of the Digium PCI FXS cards) support multiple 
conversations?  Asterisk would certainly just see it as a single channel.

Anyway, those are the questions I've got at the moment based on my 
(lacking) understanding of how DID (and other) trunks operate.  I've got 
two Digium T1 cards, a Rhino Equipment FXS channel bank and a Carrier 
Access channel bank w/ 2 - 12 port FXO cards in it.  And I'm just about 
ready to start testing that system, but I want to determine the 
additional difficulty + cost of using DID trunks for inbound calls 
instead of POTS lines.  Oh, and the other disclaimer is, fractional T1 
is not an option for me, so these'd have to be "analog" DID trunks, or 
whatever the default way of receiving them is.

Thanks a bunch for your help, and please refer me elsewhere if you don't 
have the time for this query.

jl
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