[Ecrit] PSAP Call Control features

[<g.caron at bell.ca>]

Dear ECRIT,

NENA has been working on mechanisms to provide PSAP control over
emergency calls. This is the first of two posts that NENA is submitting
which targets the "Switch-hook Status" and "Ringback" features. A
distinct post targeting "emergency call termination" will be submitted
separately. 

These kinds of controls are implemented (and sometimes mandated) in some
existing emergency call systems and NENA is desirous of maintaining
current capability as we move to new IP-based systems. It is also NENA's
desire that such mechanisms be implementable in migratory environments
such as i2 for forward-compatibility purposes.

Here are short descriptions of the features as currently defined in the
PSTN:

Switch-hook Status (SHS): When the emergency call is established between
the caller and the PSAP, changes in the "line" state (on/off hook) are
signalled to the PSAP through a special audible tone.

Ringback (RB): During an established emergency call, the PSAP can
re-invite a caller that has gone on-hook by invoking RB which triggers
ringing at the callers' end. If the caller is still off-hook but do not
react to the PSAP attendant commands, invoking RB will supply a special
tone (usually ROH) towards the caller.

Currently, -framework (v.05) and -phonebcp (v.04) acknowledge that a
caller cannot terminate an active emergency call (section 12 of the
documents). However, in order to support the other features, additional
signaling may be required beyond the "don't send a BYE" language in the
current -phonebcp. All mechanisms being discussed within NENA utilize
existing capabilities defined in the SIP suite; only changes in
behaviour are sought. Please find a short description below:  

SHS: In addition to not sending a BYE when a caller is trying to
terminate the call, the endpoint sends a re-INVITE with SDP=inactive.
The PSAP interprets this action as an on-hook state at the caller's
device. If the caller comes back online, the endpoint sends a re-INVITE
with SDP=sendrecv. The PSAP interprets this action as an off-hook state
at the caller's device and can resume the conversation.

On-hook RB: the PSAP issues a re-INVITE with Alert-Info=<ringing>
towards the caller's endpoint.

Off-hook RB: The PSAP supplies a ROH tone in-band of the media path. 

The following changes are submitted for your consideration:

-framework:
=========

At the end of section 2:
-------------------------------

Further, the following capabilities may be required at the PSAP:

o A PSAP may require to be notified of a change in state (aka on/off
hook) of the caller's device so it can react accordingly.

o The PSAP may have the ability to withhold an active emergency
communication.

o Once an emergency call has been initially setup, the PSAP attendant
may choose to re-invite a caller into a conversation through ringback
independently of the caller's device's state (on/off hook).

Changes to section 12:
-------------------------------

Change the section title to: Call Termination and call control [a new
section specific to call control could be considered]

Add the following:

A PSAP may have the capability to re-invite a caller into conversation
through the ringback feature. Ringback can be invoked while the caller's
device is in an on-hook state by supplying a ringing tone to the
caller's device. Ringback can also be invoked while the caller's device
is still off-hook by supplying a special audible tone (usually ROH) to
the caller's device.

It is desirable to signal changes in the caller's device's state to the
PSAP so it can react accordingly. By allowing this, the PSAP may choose
to hold the call, invoke ringback or dispose of the call at its
discretion.

-phonebcp:
=========

Appendix A

[I will leave to the group to determine where they best fit in the body
of the document if those get acceptance]

A.1

Change the ED-65 as follows:

ED-65 UACs with an active emergency call (i.e. SIP Dialog) MUST NOT
generate a BYE request (or equivalent for other non-SIP signaling). The
PSAP must be the only entity that can terminate a call. If the user
"hangs up" an emergency call, the device MUST alert the PSAP by putting
the media on hold (through a re-INVITE with sdp=inactive), and if the
user responds by attempting to pick up the call, the device MUST
reconnect the caller to the PSAP.

Add the following requirement:

ED-84 A PSAP UAC SHALL invoke Ringback within an existing emergency call
by sending a re-INVITE towards the caller. If the caller's UAC is in an
on-hook state, the re-INVITE MUST include a ringing URI in the
Alert-Info header. If the caller's UAC is in an off-hook state, a
special tone (usually ROH) MUST be supplied inband of the media.

Any feedback would be greatly appreciated.

Note that the NENA WG is copied on this post. Please keep this mail
address CC'ed so the group can follow the discussions.

Thanks,
Guy Caron
On behalf of NENA
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