Subject: RE: [SIP] SIP Info Method and IS-41 Cellular Signalling Protocol

From: "Fairlie-Cuninghame, Robert" <[email-masked]>
Date: Fri, 14 Apr 2000 17:58:50 -0700

Hi James,

Have you considered the SUBSCRIBE & NOTIFY methods ("SIP for Presence"
draft)? These methods can be used by a UA to request future notification of
specified asynchronous events (according to my understanding of it).  This
seems to be what you are wanting in a least a few of your bullets.

Cheers,

Robert.

-----Original Message-----
From:	James Kempf [mailto:[email-masked]]
Sent:	Saturday, April 15, 2000 5:59 AM
To:	[email-masked]
Cc:	[email-masked]
Subject:	[SIP] SIP Info Method and IS-41 Cellular Signalling Protocol

I'm working on a design for replacing the IS-41 North American cellular
signalling protocol with IP protocols. I had a couple of questions about
how the SIP INFO method, or potentially another SIP method, might
be used.

1) In the introduction to draft-ietf-sip-info-method-03.txt, exchange
of radio signal strength information is explicitly mentioned as
an application of the SIP INFO method. The HandoffMeasurementRequest
and HandoffMeasurementRequest2 messages in IS-41 are sent by the anchor
Mobile Switching Center (MSC) to a potential serving MSC to obtain
information 
about the signal strength of a Mobile Station (MS) in order to determine
if the potential serving MSC is a better candidate for handling the
mobile's call (called "hard handoff" in cellular jargon). They could be 
replaced with a SIP INFO method. There are three problems:

	a) The radio information is always solicited by the anchor MSC,
	and from the ID, the SIP INFO method seems like it is 
	sent unsolicted. I suppose one could come up with two INFO
	methods, one to solicit the radio information and one to
	return it, but this seems like a hack somehow.
	
	b) In the ID, it explicitly states that the INFO method is 
	only applicable when there is a call in progress. Problem is,
	there isn't a call in progress between the anchor MSC and
	the serving MSC, that's why the anchor MSC is soliciting
	the radio quality information in the first place.
	
	c) The INFO method seems like it is meant to be sent between 
	SIP UAC's, while the HandoffMeasurementRequest(2) methods
	are primarily designed to be sent between network elements.
	A SIP replacement would probably be sent between the last hop SIP
	proxies acting as Call Agents (CAs) for the call, which replaces
	the MSC in an all IP network.

2) There are a bunch of IS-41 messages that involve transmitting
feature or other information between the MS's Home Location
Register (HLR) and the MS. These are initiated by either the serving
MSC or the HLR. Examples are InformationDirective, which is used
by the HLR to provide notifications to an idle MS, and FeatureRequest,
which sends particular features to a potentially idle MS (features
being things like messages or playing particular tones). Again, these
seem like candidates to replace with the SIP INFO message, but 
the potential lack of any call in progress would prohibit that.

The alternative I could see would be establishing a session in order
to then send the information via the INFO method, but that seems like
overkill. And in the case of HandoffMeasurementRequest(2), the
message isn't intended for a SIP UAC anyway.

Maybe SIP is the wrong protocol to consider, since it is dedicated
to setting up and maintaining sessions, and not to exchanging information
between network elements or between network elements and terminals?

Can anybody provide some guidance?

		jak

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