

The Empire Strikes Back: Or AT&Ts Micro Cell Strategy Pt2--Response from ATT - jboydyhacker
http://www.blindreason.org/2010/06/empire-strikes-back-or-at-micro-cell.html

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tptacek
I like the notion that someone at AT&T thinks we should appreciate the fact
that they don't micro-bill us for usage on our own wifi networks. You just
know somebody at AT&T has a Word doc with 1000 revisions in it outlining
exactly how they might do that.

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grogers
The MSC is part of the core network, and it handles circuit switched
calls/data, the SGSN is also part of the core network, and it handles packet
switched data.

When AT&T is telling you not to use your femtocell for data, it isn't just
because it puts an extra strain on their network, its also significantly
slower than if you just used the wifi you probably already have to begin with.
The extra hops it has to go through - from the femtocell, through your ISP, to
AT&T's SGSN, GGSN, then out to AT&T's ISP, make it seem silly to want to use a
femtocell for data.

In addition, I don't see any reason they can't charge you for the femtocell,
even though it is using your ISP's resources you already paid for, it still
adds value to your service.

The SIP gateways that providers like T-Mobile use seem like a better
alternative to femtocell's anyways. With these, your voice and data both go
over wifi, and only the voice will enter the core network. When you need to be
paged on a mobile termination, the core network knows you are registered as a
SIP guy, so it just sends you a SIP INVITE which reaches you over your wifi.
When you roam back into the cell coverage area, it goes back to normal.

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tjarratt
I haven't read much about this, but I don't see how AT&T can justify charging
you for the base station hardware as well as its continued use. If using the
device does not use any of their services or network and is ONLY between your
cell phone and ISP, what benefit does AT&T bring that warrants them charging
you for using two goods/services (the micro-cell base station and your ISP's
bandwidth) that you have already paid for?

This is a classic bait and switch, as far as I can tell.

~~~
wmf
The prevailing theory is that AT&T is too incompetent to meter traffic based
on source; since the double-billing is not intentional they don't have to
justify it. Of course, now that they've been informed of the problem they have
to either fix it or somehow justify not fixing it (I predict that "it's not
cost-effective to rewrite the billing system to save a few dollars for a few
people using femtos the wrong way").

~~~
chrisbolt
They can definitely meter traffic based on source. You can get unlimited free
calling with your microcell for $20/month.

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jonah
It seems this is the core of the question: Is your 3G DATA going through
AT&T's core systems on its way to the internet or is it splitting out at the
microcell and going directly to the wider internet?

If it's the latter - then it's nuts that they are charging against your data
cap for usage that doesn't even traverse their network.

If it's the former, (which I suspect it is), whether it's just or not, they'd
be within their right to charge for the transit load on their systems.

~~~
grogers
When you send packets of data out of your phone, there are several layers that
are added onto your IP packet. Here is a basic diagram of the different nodes
in the RAN and CN and which layers they operate on:

UE<->NodeB (Uu): <your data> | PDCP | RLC | MAC | <L1 radio stuff>

NodeB<->RNC (Iub): <your data> | PDCP | RLC | MAC | FP | <L2 stuff depending
on IP/ATM/etc>

RNC <-> SGSN (Iu): <your data> | GTP | UDP | IP | <L2 stuff>

SGSN <-> GGSN (Gn): <your data> | GTP | UDP | IP | <L2 stuff>

GGSN <-> Internet (Gi): <your data> (finally!)

At some points your data's UDP/TCP headers might be compressed/uncompressed
but thats the basic idea. Something needs to perform the functions performed
by all these nodes. "Traditionally" the femtocell is a combination of the
RNC/NodeB (aka the Radio Access Network).

Clearly it is possible to add core network functionality to the femtocell, but
not only is it probably not worth it, the security implications are tricky
(you would have to expose your HLR directly to the internet to be able to talk
to the femtocell - instead of just your SGSN, which you would likely have
special CN nodes for talking to femtocells).

So it is almost guaranteed to be the former. And it makes AT&T's
recommendations make alot more sense. You don't even need to assume any malice
in them.

~~~
jonah
Googling madly ...

<http://en.wikipedia.org/wiki/GPRS_Core_Network>
<http://en.wikipedia.org/wiki/Network_switching_subsystem>

... OK, makes some sense now.

Their recommendations make sense. It's actually nice of them to recommend
using WiFi where available (though you'd hope people would be smart enough to
figure that out on their own.)

Looking at it from a bottleneck/financial perspective I wonder where the
constraints are that cause them to want/need to limit data usage. If it's just
the UE<->NodeB (Uu) bandwidth, then the femtocell addresses that and maybe
they could prorate the data coming through it, if it's farther up the string
then it's understandable.

