DSS usage

Arnaud Desmier adesmier at sequans.com
Thu Apr 10 07:54:44 PDT 2014

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 

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 :-)




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