Subject: [Sipping] sip-911, Preventing dialog termination

From: "Farahmand, Fardad" <[email-masked]>
Date: Thu, 28 Feb 2002 13:43:37 -0500

In draft-schulzrinne-sip-911-01 there is a brief mention of some existing
E911 features for disabling call-waiting, three-party-calls and not allowing
the caller to hang up the call during an E911 call.  But the document has no
recommendation of how to do this with SIP.  
In particular, consider the scenario where someone calls an emergency number
and then hangs up.  In PSTN E911 case, the operator is notified that the
caller has hung up but the call is not released.  The operator has the
option of (1) initiate a request by which the caller's device starts ringing
or (2) release the call.  
Is there any plan to cover recommendations for this feature in this (or
another) draft?  
Thanks,
Fardad Farahmand

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---

From: "Dean Willis" <[email-masked]>
Date: Thu, 28 Feb 2002 15:11:59 -0600

Fardad says:
> That's no different from PSTN unplugging the phone.
>
> The point is that in E911, even if the phone is unplugged, the call stays
up
> (to the nearest switch).  Allowing the operator to keep the call ringing,
> the police to trace the address, the ambulance to arrive at the door etc.
>
> The idea is that in case of SIP maybe the dialog should not be terminated
up
> to the nearest UA, stateful proxy or B2BUA which is E911 aware.
>
> This maybe more applicable in some cases such as when the UA is on a
> SoftSwitch controlling the user's device.  This is probably less
applicable
> in other cases such as a softclient UA making a call through the internet.

Actually, it is different. In the wireline network, unplugging the phone
merely takes the analog instrument off the end of the circuit and does not
affect the actual circuit condition. Recovery consists of plugging the
analog device back in -- all state was "in the network" and not affected by
the loss of the terminal.

In the mobile case, taking the battery out drops the radio carrier hard,
instituting a network-initiated termination process that affects systems at
the radio and application layer. Restoring power to the handset does not
provide the restoration of state required to recover the emergency signaling
association. Even if the network state were somehow preserved, the mobile
itself no longer has the context, and some sort of recovery process is
indicated.

--
Dean