[Bug 28378] Channel handover

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=28378

--- Comment #4 from Mikhail Zabaluev <mikhail.zabaluev at nokia.com> 2010-12-10 05:23:23 PST ---
There are few more potential cases where handover may be more practical than
implementing each heterogeneous protocol pair in one CM.

Google Voice
------------
An HTTP request is issued to the service to establish an outbound call. The
service makes a VoIP call to the destination, then makes a telephony call (or
in likely future, a SIP call, or a Jingle call) to the initiator to complete
the call "circuit".

Handover-aware implementation: Instead of adding Google Voice request support
to three CMs for possible call landing protocols, there could be one CM that
does the Google Voice setup and then hands over to the landing CM, signalling
the client to expect an incoming channel with certain properties as a
continuation.

Handover-unaware client behavior: The outbound Google Voice call quickly ends,
but an incoming call soon follows. This would suck in a handset without head
proximity detection, but presumably handset vendors that integrate Google Voice
will ensure that the call client can handle it properly.

Enterprise VoIP with cellular call fallback
-------------------------------------------
A company or a service provider offers a mobile VoIP service with a feature
that calls can be seamlessly resumed as cellular calls when the user leaves a
WLAN network ("seamlessly" here mostly means that the users need not care about
billing issues; from the signalling and media perspective, the two calls are
completely separate).

Handover-aware implementation: The client at the caller gets properties to
request a continuation channel in case when the original channel/connection
goes down due to loss of network connectivity.
The client at the callee side can expect and quickly handle an incoming channel
with certain identifying properties.

Handover-unaware client behavior: Caller: the call simply terminates without
continuation. Callee: the original call terminates, the continuation call by a
fallback-enabled client is received as an ordinary cellular call.

SR-VCC in VoLTE
---------------
3GPP IMS specifications describe continuity scenarios where an IMS
packed-switched call session can be handed over to a CS call when the cellular
network dictates a downgrade to 2G/3G.

Handover-aware implementation: unlike the VoIP->CS fallback case, it can be
driven by events from the cellular modem, so there is no need to make a channel
request. Like with Google Voice, a signal to expect a channel with particular
properties can be used.

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