Re: Bearers in mixed CDMA+LTE modems

[Dan Williams <dcbw redhat com>]

On Fri, 2012-01-13 at 12:34 +0100, Aleksander Morgado wrote:
> Hey Marcel,
> 
> > 
> >>>> 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.
> 
> There is no IP connection detail stored in the ModemManager bearers, so
> that wouldn't be a big deal for us I guess. Maybe I'm missing something.

Except for Static mode :)

> > 
> > Before everybody goes crazy here you might wanna check if Verizon even
> > provides the same IP address when falling back to CDMA from LTE.
> > 
> 
> They probably don't give the same IP address, so if the hand off is
> really transparent (i.e. not getting disconnected and then connected
> again), I assume we rely on the modem itself to detect that and handle
> the IP switch at PPP session level. I cannot really do any such test, so
> cannot tell :-/

Again, I'm still trying to find out, but I think there are cases where
we may get the same IPv4 and IPv6 address.

Dan