DSS usage

Dmytro Milinevskyy dmilinevskyy at sequans.com
Thu Apr 10 08:10:30 PDT 2014

I would like to add few cents to Arnaud's email. 
Is smth preventing modem manager to keep wwan interface up if DSS channels are in use? I expect that modem manager is aware of their presence. 
In my opinion the link state of wwan interface should always be "up". If there's a need to disable wwan interface the modem manager can flush IP address(es) assigned to it and clean the routing table(which normally should be done automatically). 
This will eliminate any change to existing kernel drivers. 

Best regards, 
~ Dmytro 

----- Original Message -----

> Hi all,

> Using DSS we got 2 issues due to VLAN mapping of DSS channels:
> 1. If your interface come down you can not anymore communicate with
> your DSS, but DSS should not be related to wwan intarface status.
> 2. DSS are not IP based, but as kernel saw them as network interface
> it forward ip broadcast and multicast to it. Even when we set VLAN
> interface flags to only UP we got some unknown packets forwarded to
> it (Wireshark is not able to decode them).

> After some discussions with Bjørn Mork to understand how mbim driver
> is working for DSS channels we concluded that currently DSS channels
> are fully dependent of wwan interface state. It means that even if
> we switch to TTY we will get the same behavior except for point 2
> that should be solved.

> MBIM specification says that we have 2 bulk endpoints for data
> communications (IP based and DSS), one is used for TX and the second
> for RX. But those endpoints are handled by usbnet driver that don't
> know about DSS.

> One solution could be to alter usbnet driver to add a new flag
> preventing endpoints to be down when wwan interface come down and at
> least one DSS channel is up.

> Does anyone already encounter this kind of problem?

> Windows is handling DSS channels as we could expect, It's a bad news
> if Windows can do better than Linux :-)

> Regards,

> Arnaud


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