asterisk users - Jan 2004 - yet another question on DID trunks

> -----Original Message-----
> From: john lawler [mailto:maillist@tgice.com]
> Sent: Wednesday, January 07, 2004 1:38 PM
> To: asterisk-users@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.  
Trunk Lines can only support one call per trunk.  It's called a trunk
line because the telco provider assigns a Trunk Group to the Trunk Line.
They then assign numbers (DID's) to that trunk group which then come
over the trunk line.  Thus, you can have multiple numbers come over the
same line, they just give more information to the equipment on the
customers side.  Thus DID (direct inward dialing) information is passed
in some amount of digits to the customers equipment depending on how
many digits the customer wants to receive.  Usually it's all 10 digits
of the number.


A PRI would come across on 2 Pair and has 23 channels for voice.  You
will usually plug it straight into the card of a phone system.  For
Asterisk, you would plug it into the T100P (single port) or the T400P (4
port) cards directly.
> I've also been told how many
> phone calls per trunk depends on your equipment.  
It does.  You have to have FXS cards to read the information that is
usally delivered from an FXO device.
> Do you know anything
> about equipment that interfaces with DID trunks?  
You just have to have an FXS card that is able to talk to the telco's
Trunk Lines.  I don't see why the channelbanks you have listed below
will not be able to work with a Trunk Line w/ DID's on it.
> 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?  
It does not and can't.

>Asterisk would certainly just see it as a single channel.
> 
You are correct, each trunk line can only handle one conversation, but
it can have multiple DID's assigned to it.

> 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.
> 
How many trunk lines are you talking about getting.
> Thanks a bunch for your help, and please refer me elsewhere if you
don't
> have the time for this query.
Another note you might have to deal with is that sometimes telcos make
these "incoming" only lines.  If you want to dial out them, you will
need to make sure they are "incoming/outgoing" Trunk Lines and then
you
will be able to pass the CID to telco on where the call is coming from.
Usually if you don't pass the information, they just attach the
main/billing number to the call.

Hope that helps a little.

> 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.
As another attempt to help clarify this, I can add that DID is a service you
subscribe to with your local carrier as a range of numbers that are shared over
a smaller number of physical DID trunks brought into your premise by the
carrier.  For example, you might buy a block of 50 numbers ranging from
650-555-5200 to 650-555-5249 and subscribe to 5 DID circuits.

A special signalling method over these trunks delivers the actual number that
was dialed (the last 4 digits) to the PBX on each inbound call. This is not the
same as ANI.  This is information on the called number, not the calling number. 
The PBX then interprets the dialed number information and rings the proper
station.  That is a general description of how the classic DID service works
over inband/analog signalling.  Each DID circuit can only handle 1 incoming call
at a time.  Pure DID circuits are used only for inbound calls, not outbound.

Variations on this exist in the digital signalling to support the same result
(ringing a specific phone with 10-digits over shared trunks).  You can do this
with ISDN or T1 or Inward 800 service (aka DNIS) or other means.  In some of
these cases, the same lines may be used for 2-way traffic (inbound and
outbound).  None of these are the same services from the carrier and don't
interface with the PBX the same way. They all have a much different pricing
structure ranging from, say, $50/mo per DID line, to hundreds or thousands per
month for T1 or ISDN PRI (24 or 23 channels).