Subject: RE: [SIP] SIP and soft hand-off

From: James Kempf <[email-masked]>
Date: Fri, 14 Apr 2000 16:02:39 -0700 (PDT)

I'm coming into this discussion late so please excuse me if I'm repeating
anything. I've just spent the last three months working with the
Mobile Wireless Internet Forum on how to introduce IP into the radio
access network (RAN) in 3G CDMA systems, and we will be continuing this
discussion until a concluding report is made in August. In March, I
posted some notes to the mobile IP mailing list where a discussion
of "micro-mobility" was underway. I`ve been planning to write up these thoughts 
in an ID sometime soon, since the question of replacing soft handoff with IP 
mobility solutions keeps coming up, but in the interim, here is a summary.

Within the RAN, there is no one to one mapping between
the octet stream of a call coming into the RAN and the octet stream
coming out into the IP stack on the mobile. The RAN controller
breaks the incoming octet stream into multiple (up to 6) streams that are
then sent to multiple base stations. In the mobile, the multiple
incoming radio signals are combined at L1 to produce the packet
that appears in the IP stack. Similarly, on return, multiple copies of
a packet from the mobile flow from the multiple base stations to the
RAN controller where the best one is selected (called frame selection or 
macrodiversity combination). So it is not possible to assign a single base 
station (and thus a single SIP registrar server or mobile IP FA in
the RAN) to the mobile. 

Exactly how to utilize IP in the RAN is an open question, but nobody
I've talked to thinks that the IP packets coming out of the mobile
will be running directly on L2 over the RAN anytime soon. In current
networks, the RAN controller takes the incoming voice and shapes
it into packets that the base station can quickly take out and
send over the air. Similarly, in the other direction, the base
station takes packets off the air and sends them largely unmodified
to the RAN controller. So the traffic in the RAN looks like radio packets. 
There is alot of very sophisticated (and proprietary) control protocol
between the base stations and the RAN controller, involving signal
processing, for doing soft handoff. There are also very tight
delay and jitter constraints. The RAN controller has time slots
between 5 and 80 ms to reach the base station, if it misses a time
slot, the base station tells it that it needs to do better, and the
packet may be dropped, potentially resulting in a degradation of
call quality. Most proposals for using IP in the RAN involve simply
replacing the L2 transport as specified with IP, so the current
proprietary RAN signalling and data protocols would run over IP
unmodified. Indeed, both 3GPP and 3GPP2 (European and North American
3rd generation) cellular designs have hooks for IP as control transport 
in the RAN, though not for IP data transport.

Given these constraints, I am skeptical about replacing soft handoff
with either L3 (mobile IP or any of the pre-frame selection
micromobility proposals) or L7 (SIP) mechanisms. Should the radio
manufacturers and cellular operators desire it, it would be interesting to look
at an Open RAN protocol that would take the current proprietary soft handoff 
mechanisms and standardize them. But, for now, I think any IP mobility 
solutions need to be restricted to post frame selection, i.e. hard handoff 
only.

Note that the situation is different in TDMA systems like GSM. In that
case, there is no soft handoff and I believe that the mobile can
only be in contact with a maximum of two base stations at a time, and then only
until handoff is complete between one and the other.

		jak

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From: Faramak Vakil <[email-masked]>
Date: Sun, 16 Apr 2000 21:08:36 -0400

James Kempf wrote:

> mechanisms and standardize them.

The goal is NOT to replace soft hand-off with any L3 or L7 mechanisms,
but to devise L3 and/or L7 mobility mechanisms that utilize the soft hand-off
mechanism correctly (i.e., create multiple streams that satisfy delay jitter
requirements and contain identical information).

> But, for now, I think any IP mobility

More work is needed before arriving at a conclusion.

Regards, Faramak