Wireless NIC

Wireless antennas are designed for specific ranges of frequencies. This allows the antenna to only pass the RF energy for the frequencies that they are designed to operate on. In the case of 802.11b/g, we are referring to radio waves in the 2.4 GHz ISM bands. The antenna is designed specifically to focus on this frequency. Here again, there are some pretty complex mathematics around antenna design, but for the sake of our discussion, these antennas try to block radio waves except 2.4 GHz. Blocking all radio waves from other frequencies is not possible; however, the design of the antennas helps to at least reduce the amount of signal received on other frequencies. 

To try to isolate the 802.11b/g signals from the other types of 2.4 GHz signals, the developers of the wireless NICs came up with a second line of defense. This second line of defense is designed to filter out the unwanted RF. To understand this, we refer to our encoding schemes. Since the 10/100/1000 Ethernet encoding schemes are copper dependent, unfortunately they could not be used for wireless communications. Instead, the engineers designed different robust, complex protocols that are capable of discerning 0s from 1s out of RF energy. These encoding schemes are those that we mentioned previously—BPSK, QPSK, 16-QAM, and 64-QAM, like in the ever-increasing and ever more complex world of Ethernet. 

As with Ethernet encoding schemes, 802.11 encoding schemes have also become more complex. For Ethernet to move from 10 Mbps to 100 Mbps to 1000 Mbps, the encoding systems increased in complexity. As 802.11 speeds increase, a similar transition occurred with the wireless encoding schemes. Over the years, 802.11 wireless networks have increased the speeds of data transmissions by moving from BPSK used in 1 and 2 Mbps transmissions to QPSK used in 5.5 and 11 Mbps transmissions finally on to the even higher 54 Mbps transmissions supported by OFDM. As the radio signal is processed by this filter, which is based on the encoding systems supported by the wireless NIC, we can now finally see the bits. Just like in the wired NIC, the bits are strung together into a string of 0s and 1s, and in the format of preamble, header, frame body, and FCS. Again, just like the wired NIC, the preamble is discarded, the header is processed to see whether the frame is targeted for the wireless device, and finally the FCS is calculated to ensure that all the included bits were accurate. At this point, the data payload is sent up the protocol stack to the OS as a designated and approved frame.

All of these tasks are just like the ones performed by a wired NIC. However, there are a few differences between the processes performed by the wired and wireless NICs. First, the wireless NIC must use its antenna and encoding filter to keep out all unwanted RF signals and thus unwanted bits as well. There is another unique difference between the way wireless NICs and wired NICs process the incoming data. The wireless NIC will use some of the specific information gleaned from the RF to bit transition process to actually add information to the wireless frame. This additional information is added at the receiving station and is in addition to the bits sent from the source. This added information is called the Radiotap Header as the shown the following picture. It includes date and time stamps, channel stamp, signal stamp, and a noise stamp. The date and time stamps are obvious. The channel stamp is based on the frequency that the NIC was on while it received this bit stream. 

With this data resulting from the Radiotap Header information, a wireless NIC can learn about the environment around it by scanning and listening to the different channels available. Many Wi-Fi tools use this technique to learn of the RF environment, such as NetStumbler and inSSIDer. Some vendors also use this same technique of listening in on channels to determine data points to help in their automatic channelizing and power balancing systems. However, none of these devices can see raw ambient RF; they only see what is received in the form of bits or modulated RF encoded by one of our protocols.