Using many modems

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Hi!

Gert Doering wrote:
> On Thu, Nov 05, 1998 at 04:05:40PM -0500, Brian McGovern wrote:
> > Multiple B channels on ISDN lines (either BRI, PRI, or E-1) do not get
> > "Ring" at the same time. 
> 
> Uh, no B channel will get a RING at all.
> 
> The switch will send a SETUP (or was that ALERTING?) message on the *D*
> channel, which all devices will get at approximately the same time.  This
> is what will be reported as "RING".

The switch sends a SETUP message which will be answered by the
equipment with a SETUP ACKNOWLEDGE. Then the equipment will start to
ring the extensions and send back an ALERTING message to the switch.
As soon as someone picks up the phone the equipment sends a CONNECT
message to the switch.

> So, what happens is that one of the devices accepts the SETUP message
> before the other one, and *this* device will get the call.  The other
> will get something like "there is no call anymore".
> 
> Any real ISDN experts out there that can clarify this stuff better?

Hmmm, let's have a look into ITU-T Q.931...

The switch sends a SETUP message. All connected devices send back a
SETUP ACKNOWLEDGE message (if they decided based on the information
in the SETUP message, that they should answer that call at all). Now
these devices start to alert the user and send back an ALERTING
message. As soon as the first user picks up, that device sends a
CONNECT message to the switch. The switch acknowledges this with a
CONNECT ACKNOWLEDGE message. It will only send this message to one
device. All other devices, that have sent a CONNECT message will get
a RELEASE message as the indication, that they didn't receive the
call in the end. So it's the job of the switch to make sure, that
only one device is selected. These devices react with a RELEASE
COMPLETE message and go back to the initial state (which is waiting
for the next connection or a dialout by the user). See ITU-T Q.931
section 5.2.9 "Non-selected user clearing" for details.

Does that answer the question?

Bye,
Marc
_______________________________________________________________________________
Email: Marc.Eberhard@Poseidon.ThPhy.Uni-Duesseldorf.DE
WWW:   http://Poseidon.ThPhy.Uni-Duesseldorf.DE/~marc/
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