Subject: Re: [MOBILE-IP] CDMA misinterpreted and

From: K Malki <[email-masked]>
Date: Fri, 6 Aug 1999 19:32:19 +0100

Hi Alessio

I actually have not "misinterpreted" CDMA, but rather you clearly have
your interpretation :)

> By the way, if the target application of this is to do something applicable
> to
> CDMA access networks, CDMA access networks for cellular systems are
> already mostly defined and do a good job at handling micromobility.

They don't do a good job if the operator decides to split the network
into subnets within the same administrative domain. That's the point
behind the draft.

If the operator decides to have flat adressing and micromobility (layer2)
then MIP's importance is largely reduced. However, personally I feel
this is a limitation which operators may not want, therefore fast
handoffs would be useful in supporting inelastic IP-telephony-like apps
in CDMA networks.

Cheers
Karim

---

From: Pete McCann <[email-masked]>
Date: Mon, 9 Aug 1999 11:48:39 -0500

Karim,

I think you may still have some misunderstanding about the way CDMA
networks are architected.  This is related to my question about "frame
selectors" right after your talk: this is a more-or-less centralized
location in a CDMA network where signals from multiple base stations
are received and the best one is "selected", as the name implies.

The point Alessio was trying to make is that even though CDMA
technology is apparently multipoint, because multiple base stations
receive the transmission simultaneously, these transmissions are
actually grouped into a single stream back at the frame selector.
This is also the first point at which IP packets can be formed up from
the data stream and is the logical place to put a Mobile IP foreign
agent (or in any case, it can't be done any closer to the base
stations than this).  So, if you want to have true multi-point
connectivity, that is, you want to have two points of IP connectivity,
you need to actually replicate a lot of hardware so that you can talk
on two different CDMA spreading codes at once.  This is typically not
done in the CDMA transmitters of today, and, although it may be a
possibility in the future, my understanding is that this is not really
being planned for (someone please correct me if I'm mistaken).

There may actually be solutions that allow you to talk to two
different frame selectors without replicating the transmission
hardware, but these still need a lot of work before they can become
reality.  In the meantime, we have a pretty good "micromobility"
solution in the CDMA access network itself, especially if it supports
"soft handoff" features.  Granted, there will be cases of handoff
between foreign agents, and these should be handled as quickly as
possible, but I think this will happen somewhat less often than you
indicate and certainly less often than at every base station.

When trying to build a data-over-cellular network, it's important to
keep in mind that traditional voice is still the most important
service and will likely be that way for some time to come.  Therefore,
you should try to piggyback the service as much as possible on the
existing voice infrastructure, which has already solved many of these
problems in a cost-effective way.  The more changes or additions to
the infrastructure you require, and the closer these changes/additions
are to the "fringes" of the network (base stations), the more
expensive and harder to deploy it will be.  This is a lesson that
should have been learned from CDPD.

-Pete

K Malki <[email-masked]> (KM) writes:

KM> Hi Alessio
KM> I actually have not "misinterpreted" CDMA, but rather you clearly have
KM> your interpretation :)

>> already mostly defined and do a good job at handling micromobility.

KM> They don't do a good job if the operator decides to split the network
KM> into subnets within the same administrative domain. That's the point
KM> behind the draft.

KM> If the operator decides to have flat adressing and micromobility (layer2)
KM> then MIP's importance is largely reduced. However, personally I feel
KM> this is a limitation which operators may not want, therefore fast
KM> handoffs would be useful in supporting inelastic IP-telephony-like apps
KM> in CDMA networks.

KM> Cheers
KM> Karim

---

From: Gerald Maguire <[email-masked]>
Date: Mon, 9 Aug 1999 19:36:32 +0200

Pete McCann <[email-masked]> stated:

    ... So, if you want to have true multi-point

Assuming that the CDMA system is using a single frequency for these
two channels (i.e., that it is only a code difference betwen the two)
then the is very little that needs to be replicated to _receive_ two
different channels of CDMA - since the RF section can be shared, all
you need to have is an extra correlator (or three - depending on if
the user is moving or not) for the additional spreading code. Thus the
cost of receiving on multiple codes is low and you can easily go from
chips to bits in the basestations. If these are IP packets in some
sort of link frame, then you could immediately put these frames into a
routed IP infrastructure right from the base station.

The transmission problem is more complex if you are going to do
accurate power control - since the different chip streams may have to
have different power levels when transmitted. However, in the case
where the power can be identical (for example indoors over short
distances) - then there is no problem simply combining (or-ing) the
chip streams and then feeding them into the same RF section {since
after all it is the codes which separate the streams not their
freq. etc.}. [Note that in this case I have assummed that you are
generating the chips for the different streams synchronously - which
just makes things simpler.] Once again there is no reason not to do
this as close to the basestation as possible - since it conserves
bandwidth on the typically wired link to put off the bit to chip
conversion until as late as possible.

Unlike the voice case there is no problem (other than the transport of
duplicate packets) with each of the basestations which can receive a
mobile sending on the IP traffic that it receives. These extra copies
can be eliminated by administratively deciding which basestations(s)
will be forwarding traffic coming from a given mobile. There are
clearly cases (such as when the mobile is on the fringe of two cells)
when it may be quite reasonable for several basestations to be
forwarding traffic to or from a given mobile.

Pete further states:

There is an important distinction of if this data-over-cellular
network is being built by an incumbant who is already operating an
voice oriented cellular system or not. In the case where there is no
existing voice oriented cellular system - there is no need to use the
typical voice oriented infrastructure and you can build an IP oriented
infrastructure from the start. Clearly if you have an existing voice
oriented cellular infrastructure you want to preserve its value for as
long as possible - therefore you are more likely to take an incremental
approach rather than forklift replace it. However, I don't see that a
voice oriented infrastructure is inherently more cost-effective in the
wireless setting (just as it is not in the wired setting).

For example, consider an operator who does not have an existing GSM
infrastructure but decides to go into the GPRS business. Is there any
reason why they should build a circuit switched infrastructure? By
utilizing VOIP from the handset to the remote end - you can
potentially remove 60% of the processing required since you don't have
to transcode voice packets to a mu-law or A-law stream into/out of a B
channel. Of course if you do have to go through a voice gateway at the
national gateway or where you interconnection to other operators you
will have to do the transcoding, but thanks to the digital modems used
to create dialin modem pools (such as in the Ascend Max) you can do
this with very high density devices (i.e., 2-4 E1 connections per 10cm
of rack space).

From a practical point of view there is evidence that putting all of
the processing in the center (i.e., not distributing it) is not always
the preferred solution - otherwise _everyone_ would use Ericsson's
AXEs to make their infrastructures (which they don't).  Even though
this was the case at the start of many cellular voice networks it is
not the case today.

Regards,
G. Q. "Chip" Maguire Jr.