Description: Assigns one of N allowed frequency pairs for the radio 
transmitter and reciever as described in zhoneRadio-TC.mib.

5.7Ghz family (sysObjId) systems:
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Writing to this value will change the current operating frequency.
The active channel is the default channel used on power up. 

The channel is switched at the near end, and if possible, also at
the far end, towards minimizing link down time.  

Every time the active channel number changes BITS WILL BE DROPPED.  
In poor conditions there is a risk that the new frequency assignment 
will fare more poorly.  The user should expect alarms where the facility 
is terminated until the nodes re-sync.

If a new active channel number is equal to the current value of
active channel, the command will have no effect and no error message will result. 


Allowed range of allowed values:   0 - 17

This value will read back the current active channel.

Note:  if this value is changed to a new active channel number,
and the system can not establish a radio link at this channel,
the system may automatically switch to the standby channel, if
wtnAutoChannelSwitchEnable is true(1), and the value of 
wtnStandbyChannelNumber is neither the same channel number nor zero.

23Ghz family (sysObjId) systems: 
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This variable is not used and so should not be presented to user.
Use direct frequency assignments instead.   
See wtnActiveTransmitFrequency.