Sorry, can't reproduce on current ns-3-dev. Patch according to your
description attached below. Please check that bug still exist.  

(In reply to comment #0)
> I am using the latest dev version of ns-3. 
> 
> I added the following lines into third.cc to see if I can figure out 
> how to use ChannelNumber: 
> insert at line 86 (above the phy.SetChannel (channel.Create ()); line: 
> YansWifiPhy testCN; 
> testCN.SetChannelNumber (1000); 
> 
> 
> When I do this I get an error: 
> "assert failed. file=../src/devices/wifi/wifi-phy-state-helper.cc, 
> line=379, cond="IsStateSwitching()" 
> line 379 is NS_ASSERT(IsStateSwitching()); 
> 
> 
> I checked in yans-wifi-phy.cc for the same assert, and on line 311 it 
> is NS_ASSERT(!IsStateSwitching()); 
> 
> 
> When I added the ! to line 379 of wifi-phy-state-helper.cc so that it 
> would be the same as line 311 of yans-wifi-phy.cc, the error went 
> away. 
> 
> 
> I'm not sure if I did something wrong with my attempt at using 
> ChannelNumber, or if the ! got lost.

Created an attachment (id=716) [details]
Can't reproduce

I think this may have been a case of trying to change channels when the sender
was sending.  As far as I can tell, that will still cause an assert error in
the code.  I'm not sure if there is a way to allow channel switching to be
scheduled even when a node is sending, even if you let the node finish sending
and then switch the channel.

Michael