Re: Bearers in mixed CDMA+LTE modems

[Dan Williams <dcbw redhat com>]

On Fri, 2012-01-13 at 11:58 +0100, Marcel Holtmann wrote:
> Hi Aleksander,
> 
> > >> I believe we need a MMBearerType enum in the 0.6 API, so that we can
> > >> tell in CreateBearer() whether we want a 3GPP or CDMA (well, or POTS)
> > >> bearer. This property would be redundant for 3GPP-only, CDMA-only or
> > >> POTS-only modems, but would be mandatory if we have a mixed
> > >> 3GPP(LTE)+CDMA bearer. This value would also be shown as a property in
> > >> the Bearer interface, so that we can know the type of the bearer behind
> > >> a given DBus path. Another possibility to avoid the new enum would be to
> > >> assume that if "apn" is given when creating the bearer, we want a 3GPP
> > >> bearer, while if no "apn" is given we really want a CDMA bearer. But not
> > >> sure I like to rely just on this "apn"-based logic. What do others think?
> > > 
> > > The problem with that approach is handoffs.  If you create a 3GPP/LTE
> > > bearer and then leave LTE coverage where the device hands off to EVDO,
> > > now your 3GPP bearer is a CDMA bearer.  In this scenario there's no
> > > interruption of packet data service and you don't even know anything
> > > happened except that the access technology changed from LTE to EVDO.
> > 
> > Well, that is already some indication that we can use. If we had a 3GPP
> > bearer connected, and suddenly the access technology changed to EV-DO,
> > then we could internally mark the CDMA bearer as connected and mark the
> > 3GPP one as disconnected. If done in that order, we wouldn't be issuing
> > any state change notification. This, assuming that for mixed technology
> > modems we have different technology-specific bearers. The only drawback
> > of having technology-specific bearers is that for the user not using the
> > Simple interface, it would mean needing to create two bearers with two
> > CreateBearer() calls. But I don't think that that is a big deal; if the
> > user of a mixed CDMA+LTE modem just creates a 3GPP bearer and gets it
> > connected, and then we detect the connection handed off to CDMA, we can
> > request the disconnection of the bearer and that's it. If the user
> > didn't create a CDMA bearer, we would need to assume she didn't want a
> > CDMA connection. If using the Simple interface, all that would be
> > automatic, different bearers would be created automatically.
> 
> there is no guarantee that the IP connection details stay the same.
> 
> Before everybody goes crazy here you might wanna check if Verizon even
> provides the same IP address when falling back to CDMA from LTE.

It's supposed to work that way according to the eHRPD docs.  I tried to
drivetest this Friday but due to my own stupidity I forgot to take the
modem out of LTE+HRPD mode and into AUTO+eHRPD so I couldn't capture the
handoff and then I ran out of battery.  My bad, I'll try again.

But at least the UE is supposed to make this transparent according to
3GPP2 X.S0057-A.  If the ME already has IP address information from the
network, in the VSNCP Configure-Request packet it sets the Attach-Type
configuration option to "handoff" and includes the existing IP
information (10.1.4.2).

Section 13 (Handoff from E-UTRAN to eHRPD) states:

"For optimized handoff, when the UE accesses eHRPD via the E-UTRAN radio
and the S101 tunnel, it shall send a VSNCP Configure-Request message
with Attach-Type set to handover to the HSGW for each of it's existing
PDN connections in the EPS system that it intends to maintain in eHRPD."

Section 13.1.1 step 7 says:

"The UI exchanges VSNCP messages with the HSGW for each PDN connection
that it currently has attachments to within E-UTRAN and that it wants to
maintain on eHRPD.  The UI sets the Attach-Type to "handoff" in the
VSNCP Configure-Request message.  Also, the UI includes the IP
address(es) it obtained via LTE in the VSNCP Configure-Request message."

See also section 13.1.1 where it details what happens for optimized
handoff; non-optimized handoff is supposed to be the same, more or less.

So let's assume that the IP address is supposed to stay the same.  Next,
the standard talks in various places about separate bearers for EPS and
eHRPD, like 13.2.1: "When the UE returns to eHRPD to resume the existing
eHRPD session, the PDN connections are created per the context that the
UI had on E-UTRAN.  Likewise, bearers are established to  match those
that were available on E-UTRAN."

Basically, it appears that bearers may change at various times, but the
IP addresses may stay the same across bearer changes in some cases too.
The problem is that we don't really want to expose that to clients much,
because it's not really that useful to know that bearers are dancing
around.  You really just want to know if one of your existing bearers
*changed* attributes like IP addressing or QoS/TFT, since the modem and
network appear to do all they can to maintain characteristics between
E-UTRAN and eHRPD.  I also still don't know how these changes are
presented via AT, WMC, or QMI, and how much of this the modem does
internally and hides from these interfaces but I'm still trying to
figure out.  Unfortunately the end of my LTE coverage is about 30+
minutes away in all directions...

Dan