Help connecting to two networks (Adding a virtual interface)

[Bill McGonigle]

On 09/01/2009 05:49 PM, Ben Scott wrote:
>> IIRC, many wireless AP's don't support 802.1q in 802.11.
> 
>   Why would the AP care?  VLAN frames are just regular Ethernet
> frames, but with a particular frame type that VLAN-aware nodes
> recognize as the VLAN tag.  Equipment that isn't VLAN-aware should
> just see it as an unknown frame type.  VLANs even eat up a few bytes
> of the regular Ethernet MTU.

You can't just dump a regular Ethernet frame on wireless, they get
re-packaged because the frames are designed slightly differently.  For
instance, 802.11 doesn't do frame extension, which 'just' makes a long
ethernet packet in most wired 802.1q cases, but in 802.11 you need to
eat into your payload size to add the tag.  There is a spec for 802.1q
in 802.11 - I don't remember the details but you wind up with a subframe
inside a subframe inside a frame, or something like that.

Add to that that a lot of wireless work is done by the hardware (/its
firmware) and you can wind up with radios that don't support many of the
802.11 options and can't be easily enhanced from the OS.

>> Then there's hardware/driver support ...
> 
>   If we're talking strictly Linux, VLANs are implemented in the
> generic link-layer, above the NIC drivers.

yeah, that should be OK, but listening to multiple SSID's on a single
radio - I think that needs to be handled by the driver and supported by
the hardware.  I'm not even sure if iwconfig can handle that - I've
never tried but don't recall a help text that would seem appropriate.

>  It's not like 'doze where
> the NIC drivers are actually responsible for VLAN implementation.

oh, for pete's sake...  you're putting us on, right?

>   Probably not with the typical SOHO stuff, i.e., LinkSys, NetGear,
> etc.  I've heard of (but not looked into) "enterprise grade" WAPs
> which can do QoS for VoIP applications.  I have no idea how well they
> work myself, although I've heard others -- who weren't selling them,
> AFAIK -- recommend them for the purpose.  YMMV, etc.

The ones I've seen dedicate a SSID/radio/channel to the task.  I'm
trying to think about how the RTS/CTS handshaking works in a mixed QoS
environment if there's not 802.11e support among all devices.  Not that
there's anything wrong with dedicating resources to such a task.  This
must all subtly different in 802.11n - I seem to recall they've
addressed the hidden node problem more efficiently there.

-Bill

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