[asterisk-users] PRI TBCT - Practical Experience, Anybody?

[Matthew Fredrickson]

Tilghman Lesher wrote:
> On Friday 15 August 2008 13:45:11 Jay R. Ashworth wrote:
>> On Fri, Aug 15, 2008 at 02:37:46PM -0400, Matt Florell wrote:
>>> Most carrier sales people don't know what TBCT is unfortunately, and
>>> even if a carrier is capable of doing it, it is a possiblity that not
>>> all of their equipment is capable of doing it. One client of mine
>>> tried to get TBCT working across all 16 of their PRIs(all on the same
>>> carrier) and it only worked on 4 of them, supposedly because not all
>>> of the telco equipment was capable of the feature.
>> I expect to fight this battle, yes.  :-)
>>
>>> This actually depends on the kind of PRI service you have. For
>>> instance with DMS100 circuits you can only do TBCT with calls that
>>> come in to your circuit, not with outgoing calls.
>>>
>>> As for connecting two incoming calls, since that is not possible in
>>> Asterisk(to natively bridge two incoming calls together) I can't see
>>> how you would get that to work even if it is possible in TBCT.
>> To be more clear, what I'm after is to have *someone else besides me*
>> place calls out their PRI, and then TBCT those placed calls to my DN.
>>
>> By the time the calls get to me, they should just be standard phone
>> calls.
>>
>> So I expect the call-placing-party to need TBCT, but not me.
>>
>>> I believe that only DMS100, NI2 and 5ESS PRI signalling protocals are
>>> capable of TBCT with the current zaptel code-base. Also, the two B
>>> channels involved in the TBCT have to use the same D channel.
>> And I'm probably not concerned with whether Asterisk can deal with
>> TBCT, because Asterisk probably won't be involved at that stage; just
>> once the call's transferred to me.
>>
>> But before I inquire of said second party whether they *can* do that, I
>> wanted to confirm it was possible.
> 
> 2BCT works when the telco originates the call and Asterisk is hairpinning
> the call back out the same PRI circuit.  However, Asterisk does not support
> the opposite direction.  That is, a call originated from Asterisk that comes
> back in via the same PRI circuit cannot be 2BCT.  I'm not certain whether this
> is a limitation of Asterisk alone or of the protocol, but it cannot be done.
> 
> Similarly, Asterisk cannot complete a 2BCT request, if Asterisk is on the NET
> side of the PRI circuit.  That might could be added in the future, but it is
> not supported now.
> 
> So in summary, Asterisk can request 2BCT, but it cannot perform a 2BCT if
> requested from the other side.
> 


Let me clarify some of this.

Under no circumstances can Asterisk receive a TBCT request.  We just 
ignore them.  We can initiate them however.

There are different TBCT implementations, dependent on which switch type 
is used, with different restrictions associated with each switch type 
selected.

For true TBCT (on switchtypes of NI2 and 5ESS, AFAIK), you can have any 
combination of inbound and/or outbound channels (one inbound/one 
outbound, two inbound, two outbound) and transfer them to the upstream 
switch.  The protocol doesn't care.

For DMS100's version of TBCT, called RLT, one leg *must* be inbound and 
the other *must* be outbound.  No other combination is going to work. 
This is explicitly mentioned in the protocol in RLT.

Hope that helps a bit.

Matthew Fredrickson
Digium, Inc.