WLANs typically operate within three different frequency ranges—2.4 GHz, 3.6 GHz, and 4.9/5.0 GHz. Not all Wi-Fi cards support all these ranges and associated bands. For instance, older Alfa cards only support IEEE 802.11b/g. This would mean that these cards cannot operate in 802.11a/n. The key here is to sniff or inject packets in a particular band; your Wi-Fi card will need to support it.
Another interesting aspect of Wi-Fi is that, in each of these bands, there are multiple channels. It is important to note that your Wi-Fi card can only be on one channel at any given moment. It is not possible to tune into multiple channels at the same time. The best analogy I can give you is your car radio. You can tune it to only one of the available channels at any given time. If you want to hear something else, you will have to change the channel. The same principle applies to WLAN sniffing. This brings us to an important conclusion—we cannot sniff all...