Subject: Re: [MOBILE-IP] IPv6 handover document

From: "Hesham Soliman (EPA)" <[email-masked]>
Date: Fri, 18 Aug 2000 03:19:37 +1000

=> obviously there is something like overlapping cells here.

Sure but that doesn't necessarily mean you can receive two
separate traffic streams.

PS: is there a specialized mailing list for this topic?

=> Don't know !

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From: Linfeng Yang <[email-masked]>
Date: Thu, 17 Aug 2000 21:04:40 +0300

> separate traffic streams.

Yes my assumption is, there must be some form of overlapping, otherwise you
cannot achieve zero-delay.

-Linfeng

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From: Mattias Pettersson <[email-masked]>
Date: Fri, 18 Aug 2000 10:09:02 +0200

> cannot achieve zero-delay.

Hi,

Yes, overlapping is of course necessary, but that's not what Hesham is pointing
at. The nature of radio link layers is not exactly like an Ethernet, where
everyone can hear everyone else. Since bandwidth is limited and transceivers
are interfering, different transceivers will use different physical and/or
logical channels.

If the link medium is using different physical channels (different frequencies,
not CDMA technology, for instance), then a receiver can usually only lock onto
one single of these channels, since (due to cost) it is only equipped with one
transceiver. If access point A is on subnet Na and using channel Ca, a mobile
node locked onto channel Ca won't be able to hear or measure anything of a
possible access point B on subnet Nb on channel Cb. The only way for the mobile
node to detect the presence of B would be to start a scan over all channels
(and do so occasionally) or to be informed of its existence by the network over
channel Ca.

The few radio technologies I've fought with do this and if anyone else have
better news I would be very happy to hear of it.

The point is: we must understand these limitations inherited from radio links
and take them into consideration when designing a handoff scheme.

Otherwise, if we assume the opposite with the ability to hear routers
regardless of radio channels and have good enough overlap plus add some
heuristics in reachability detection of routers and maybe crank up the RA rate
a bit, good old Mobile IPv6 will do the work perfectly without our help!

/Mattias Pettersson

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From: Gerald Maguire <[email-masked]>
Date: Fri, 18 Aug 2000 15:39:55 +0200

You stated:

This is not quite true, as it is possible for the receiver to have
only enough circuitry to determine the power level in a band - and
thus detect activity in other frequency bands -- even though it is not
listening to the actual data in these bands (for example DECT
receivers do this).

In addition, it is also possible to use wideband front ends to the
receiver and do the decoding of multiple narrow bands within (for
example using software radio techniques). Again, a useful aspect is
that you might not need to actually decode all the narrowband
contents, but just look for some feaure -- such as a pilot tone or the
freq. spectrum or perhaps only decode it once in a while.

All of the above means that you don't necessary have to have a second
full receiver nor do you have to necessary time multiplex this
receiver (and hence lose the current communication channel) by having
it scan the other channels.

However, I think we are rapidly reaching the point where it will be
the case that we have multiple receivers in many devices and it will
be nature to use one for the current communication and the others
either for additional communication channels or to (occasionally) look
for better communication channels. This is certainly the case with
cellular handsets which have multiple distinct radio receivers (such
as GSM and DECT or the long awaited GSM and Bluetooth).

Further, it does not cost that much to listen to multiple channels --
vs. the cost to both listen and transmit on multiple channels.

I've often thought of having CDMA beacons that annouce what resources
are available -- in analogy with the signs you see on many highways
which tell you want are the local radio stations (for traffic/public
safety information). Such a beacon could be design to have a very low
data rate (and very long spread code) - so that it could be very
readily detected, but minimize its interference.

Chip