Novatel 551L + qmi

[Dan Williams]

On Tue, 2014-09-16 at 12:39 -0700, Gopakumar Choorakkot Edakkunni wrote:
> Hi Dan,
> 
> Thanks a lot for the response. So I guess my firmware is old, I will try
> upgrading to see if it work with just QMI without needing any $NWQMICONNECT.
> 
> >> If eHRPD is not available on the network or is not enabled in the modem,
> then you'll have to disconnect to move between 3G and 4G.
> 
> When you say "you'll have to disconnect", do you mean to say user
> intervention is required - should the user click "reconnect" on the modem
> manager etc.. ? Or will the modem firmware do that internally without any
> user intervention required (yes, the user might see a connection getting
> reset and re-established, but thats still better than user having to click
> "reconnect")

Again, this is all if eHRPD is not available in the network, or somehow
not enabled/supported in the modem.  eHRPD is the protocol that allows
roaming between 3GPP2 (CDMA/EVDO) and 3GPP (LTE) RANs.

So, if eHRPD is not available/working:

If you are connected to EVDO, and LTE becomes available, the modem will
*not* hand the data connection from EVDO to LTE.  You must terminate the
data connection manually, wait for the modem to up-register to LTE, and
then manually re-start the data connection.

If you are connected to LTE and run out of LTE coverage but EVDO is
available, the data connection will simply terminate, the modem will
eventually register on EVDO, and you will need to manually re-start the
data connection.

Verizon had this hand-off issue for quite a while after deploying their
LTE network in 2011, though they seem to have largely solved it by 2013.
However, some of these initial launch devices (of which the USB551L is
one) may not have firmware updates that fix all eHRPD issues, because
they are long since out of production and unfortunately the
manufacturers don't really care about old hardware.

On the user side, a connection manager/modem manager would usually
handle the reconnection without user interaction if that policy was
desired.

> Also when in 3G mode, eventually the packet encapsulation is ppp / slip I
> guess - so does the firmware inside the modem initiate the ppp/slip when in
> 3G mode, because from linux side all we see is a wwan0 interface.

AFAIK yes, that's basically what happens in the firmware.  It
decapsulates the IP packets from the over-the-air protocol (PPP for
CDMA/EVDO, SNDCP for GPRS, IP for LTE) and re-encapsulates them into
ethernet packets for the wwan0 interface.

Dan

> Rgds,
> Gopa.
> 
> 
> On Tue, Sep 16, 2014 at 11:47 AM, Dan Williams <dcbw at redhat.com> wrote:
> 
> > On Mon, 2014-09-15 at 19:29 -0700, Gopakumar Choorakkot Edakkunni wrote:
> > > Hi,
> > >
> > > First of all I tried that modem on my Ubuntu 14.04 with the modemmanager
> > > GUI, I ask the GUI to scan for networks and it fails all the time. But
> > > thats ok - my real requirement was to get it running on my openwrt box.
> > > Plugged it in, saw that the option1 driver takes control of ttyUSB[0-3]
> > and
> > > qmi_wwan creates a wwan0 interface. After that running a dhcp client over
> > > wwan0 dint come back with any IP, so reading through the modemmanager
> > code,
> > > I found that we have to send an AT+$NWQMICONNECT over ttyUSB0. I did that
> > > and presto!, wwan0 gets an IP (thanks modemmanager developers !).  A
> > couple
> > > of questions
> >
> > The device runs optimally with QMI, but does work with $NWQMICONNECT as
> > a fallback.  However, QMI works much better after the firmware update to
> > 1.41.
> >
> > $NWQMICONNECT likely just calls the QMI WDS data session functions
> > anyway.  What you are probably missing is using qmicli or ModemManager
> > to start the WDS data session with the QMI protocol, and *then* you can
> > run DHCP on the network port.  You cannot just run DHCP without first
> > starting the data session via QMI or $NWQMICONNECT.
> >
> > > 1. Does anyone know "internally" what is the modem doing on getting an
> > > AT+$NWQMICONNECT ? Does it start a ppp session if the available band is
> > > only 3G ? Once started in 3G and later a 4G-LTE connection is available,
> > > will the modem internally drop the ppp session and switch the IP on
> > 4G-LTE
> > > (and vice versa) ?
> >
> > Handoff between 3G and 4G is kinda dicey with these first-gen
> > Qualcomm-based Verizon devices (USB551L, Pantech UML290).  Part of the
> > issue is eHRPD which is required for handoff between 3G and 4G.  If
> > eHRPD is not available on the network or is not enabled in the modem,
> > then you'll have to disconnect to move between 3G and 4G.
> >
> > > 2. Other than the AT+$NWQMICONNECT, do we need to send anything else to
> > the
> > > modem to make it work using all available bands based on availability
> > (3G /
> > > 4G) or do we have to specify anything to force the modem to work in
> > "these
> > > specific modes" ? I dint find anything like that in the modem manager
> > code,
> > > just thought of asking anyways.
> >
> > If you've left the technology preference, system selection preference,
> > and band preferences alone, then the modem will automatically search for
> > and connect to the best available technology (eg, LTE) and network.  But
> > the device only supports one Verizon LTE band (eg, Band 13), so you're
> > pretty limited on the LTE side.
> >
> > Dan
> >
> >
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