The MIC has two main purposes. First, it is encrypted along with the rest of the frame and makes it much more difficult to tamper with a frame's data. Second, the MIC contains a frame counter. This prevents someone from launching a wireless replay attack.
Implementing WPA
To take advantage of WPA, you must have adequate hardware and software. From a hardware standpoint, this means only that your wireless access points and your wireless NICs must recognise the WPA standard. Unfortunately, most hardware manufacturers won't support WPA through a firmware upgrade, so you may find yourself forced to buy new wireless hardware if you want to use WPA.
From a software standpoint, none of the Windows operating systems will support WPA by themselves. Windows machines with WPA-compliant hardware can use WPA, but only after you have installed the WPA client. The WPA client will work only for machines running Windows Server 2003 and Windows XP. You can download the necessary client from Microsoft.
Mix and match
Obviously, switching wireless hardware and implementing WPA can be a big undertaking. Fortunately, it isn't something you have to do all at once. Wireless access points can support WPA and WEP at the same time. This allows for a gradual transition into WPA.
The only thing you need to know about mixing WEP and WPA is that doing so prevents the global encryption key from being automatically re-keyed. Remember that WEP clients do not support automatic re-keying. To prevent key recognition problems, automatic re-keying is initiated by the access point only when no clients are running WEP. However, all of the other WPA security measures will work during the transition period.
As you look ahead to future WLAN deployments, keep in mind that you will probably want to change your security methods to encompass WPA and/or the similar set of security standards that is forthcoming from the IEEE.
