Re: Bearers in mixed CDMA+LTE modems

[Aleksander Morgado <aleksander lanedo com>]

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.

> 
> 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 :-/

-- 
Aleksander