## 3 Effects of Interference on the PHY and MAC Layers of an IEEE 802.11 Network  
IEEE 802.11 networks operate at frequency bands of 2.4 and 5 GHz, which offer a maximum of 14 or 24 channels, respectively, though the number of usable channels depends on the region [31]. These frequency bands fall within the unlicensed ISM band, which is also used for non-IEEE 802.11 communications. IEEE 802.11 devices exchange frames on the MAC layer. Three types of frames are used: (1) management frames, which are used for joining and leaving the IEEE 802.11 network; (2) control frames, which are used to acknowledge received frames; and (3) data frames, which contain the data. Frames are broadcast wirelessly using half-duplex communication.  
To minimize the probability of collision on the network during such communication, IEEE 802.11-based devices check whether the channel is busy or idle before transmission. They perform this check using the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) algorithm [31; 32]. The device periodically samples the channel for the presence of radiofrequency energy, while a 'carrier sense' algorithm detects and encodes/decodes the IEEE 802.11 frames [32]. If the channel is idle, the first frame in the queue is transmitted. If the channel is busy, the device will wait for the current transmission to end and then \"contention\" starts: the device waits for a randomly determined time period or for a number of fixed-duration time slots (expressed in \(\upmu\)s), then re-checks the channel to see whether it is now idle [33]. If so, the device sends the frame. The device with the shortest random time wins and sends its frame [33; 34; 35]. In this way, the IEEE 802.11 network uses a \"stop-and-go\" mechanism to provide all devices with equal probability of accessing the channel. In other words, the next frame in the queue is transmitted only if the current frame is acknowledged appropriately by the receiving device within the acknowledgment control frame (ACK) [33; 34; 35]. This entire process is illustrated in Figure 1.  
This process can proceed quite differently in the presence of sporadic or continuous interference [4; 35; 36]. Such interference can be due to other IEEE 802.11 networks in the vicinity, or to Wi-Fi or non-Wi-Fi devices in the vicinity [22], such as cordless phones, Bluetooth handsets, audio and video transmitters, and microwave ovens. If the interferer transmits sporadically (Figure 2), an IEEE 802.11 device may receive signals both from the source IEEE 802.11 device and from the interferer. This may cause frame collision and corruption, which prevents the receiving device from acknowledging delivery of the frame (NO ACK) [34; 35]. This necessitates retransmission of the corrupted frame as soon as the channel becomes idle again. Repeatedly unsuccessful delivery of data frames and retransmissions decrease network throughput. This problem is even worse if interference is severe and continuous (Figure 3). In this case, IEEE 802.11 devices consider the medium busy most of the time and do not transmit [34; 35].  
Figure 1: Illustration of Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) involving three devices on one channel of an IEEE 802.11 network without interference. ACK, Acknowledgment Control Frame; Dest, Destination.