Client cards and access points compliant with the 802.11n standard are backward compatible with the common 802.11 versions, such as 802.11a, 802.11b, and 802.11g. In addition, 802.11ac is backward compatible with 802.11n (5 GHz) and 802.11a. You will likely not have interoperability issues with the basic 802.11 functions, such as association and data transfer, especially if all the devices on your network have undergone successful Wi-Fi interoperability testing. An 802.11g client radio, for example, can associate with an 802.11n access point and enable a user to browse the Internet.
Even with standards, however, you still face interoperability issues. For example, proprietary enhancements generally do not work across multiple vendor devices. Vendor-specific enhancements to 802.11-compliant products often make interoperability questionable. For example, a vendor might include a special performance enhancement in its access points, but it will work only if you use the same vendor’s client radios. To ensure interoperability with WLANs, it is best to implement client radios and access points (if possible) from the same vendor. You can implement multivendor WLANs successfully, but that reduces the WLAN features to the lowest common denominator, which is what the 802.11 standard specifies.