[ns] need help regarding Mac/802.11Ext

Hi,
I need guidance for using Mac/802.11Ext.

1) How to set different data rates while using Mac/802.11Ext?

2) What does the following parameter mean?
* Phy/WirelessPhyExt set noise_floor_ 2.512e-13
;#-96 dBm for 10MHz bandwidth*

does it change with different modulation schemes?

Thanks in advance

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Regards