Re: Bearers in mixed CDMA+LTE modems

[Marcel Holtmann <marcel holtmann org>]

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.
> > 
> 
> Needless to say that I would love to see the logs :-)
> 
> > 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...
> > 
> 
> Having read the document, it really seems that the whole eHRPD setup
> makes the handoff to/from LTE much much *much* more transparent than
> what I thought (keeping the IP addresses during the handoff is just one
> detail), and that we don't really need to take care of exposing them
> apart from just notifying about the change of technology being actively
> used (if possible to notify that). So for the generic case of "do LTE or
> CDMA, with LTE preferred" the setup is quite automatic.
> 
> The other thing that comes to be a bit more clear is that for mixed
> LTE+CDMA modems, the need of requesting a CDMA-only connection may be a
> bit unclear: mixed LTE+CDMA modems don't really need
> to support CDMA-only connections (Optional Rm-interface protocol setup
> plus ATDT#777), as the CDMA network may really be handled just for
> handoff from LTE via eHRPD. Could you try to connect your LTE+CDMA modem
> using the standard CDMA connection sequence and see if it works?
> 
> So, for the generic implementation of the mixed LTE+CDMA modem, I guess
> we should follow your suggestion of creating a generic Bearer that will
> do 3GPP-based connection sequence (PDP context setup and activation)
> when 4G is allowed, and otherwise try CDMA-based connection sequence
> (optional Rm-interface protocol setup plus ATDT#777). This single
> mutable object is probably the most dynamic/extensible implementation
> that we could give to plugin implementations.

unless you have a really really smart modem firmware, the ATD#777 can
not work out to setup any kind of auto-handover connection. The PPP
connection is terminated at the network and not the modem level as in
3GPP. There is a theory that can terminate PPP in the network for 2G/3G,
but even that I have never seen used in any network. And I doubt anybody
with a 4G network falling back to this. Way to expensive on the network
equipment to handle LTE loads via PPP.

So once you fall back to any kind of ATD with PPP, I think you are stuck
in one technology. Especially if it termination endpoints of your PPP
connection are different.

Regards

Marcel