## 2 Literature Review  
Numerous research was conducted on multi-channel MAC protocols that focus on the use of multiple channels to enhance network performance [25; 26; 27]. Few researchers have proposed an energy-efficient MMAC protocol [28; 29; 30; 31], whereas other proposed schemes ensure reliability and fairness [32; 33; 34]. In this section, we highlight the protocols that were used in this study. The basic contention mechanism of the IEEE 802.11 standard for WLANs has been used in many MMAC protocols. Therefore, we begin with the discussion of the contention mechanism in IEEE 802.11 that uses Carrier-sense multiple access with collision avoidance (CSMA/CA) [35; 36; 37]. The nodes always sense the medium before transmitting to avoid a collision. Each node selects a random backoff time that is based on uniform distribution between [0, CW) where CW denotes the current contention window. When the channel is found idle, the node decrements the backoff time, and upon reaching 0, transmits its frame. If it finds the channel busy, it pauses its backoff timer until it finds the medium idle again. Upon collision, CW is doubled until \(CW_{max}\) and attempts to transmit the frame until the retry limit is reached. Upon successful transmission, CW is set to \(CW_{min}\) and nodes wait for the DIFS (distributed coordination function interframe space) [38] time to sense the channel again. The receiver also acknowledges the (ACK frame) reception of data after waiting for the SIFS (short interframe space) time. The issue in this protocol is the time spent in backoff and contention that reduces the throughput of the network [39; 40]. MMAC improves the throughput significantly by having nodes transmitting on multiple channels [41]. All channels of the spectrum are orthogonal to each other, meaning transmission in one channel would not interfere with the transmission in another channel. In MMAC protocols, the contention is basically in the control channel and any two nodes winning the contention and deciding on the channel to use, switch to that channel for data transmission. The nodes contend using control frames in the control channel [16]. The first MMAC protocol considered in this work is a bidirectional multi-channel MAC (BiMMAC) [42; 43], which uses the IEEE 802.11 MAC protocol to contend in the control channel. Multiple channels are used in BiMMAC where one channel is exclusively used for the control channel and others are used for the data channel. The process of data transmission is divided into two phases, namely the control phase and the data exchange phase. In the control phase, control frames, such as request-to-send (RTS) and clear-to-send (CTS) [44], are exchanged to negotiate the channel that can be used for data transmission. Another control frame named channel reservation (CRN) is used by the source to announce data transmission to all neighbors of the source to avoid the multi-channel hidden problem. In the data exchange phase, both nodes switch to the negotiated channel for data transmission. After the data are successfully transmitted, the nodes switch back to the control channel. The main feature of BiMMAC is the transmission of two frames in the data channel with a single handshake, i.e., if both nodes have packets to send to one another then they will exchange data frames without the need to contend again. In case the receiver has no data to send, it can send a normal ACK and both nodes switch to the control channel. Due to the extra frame sent by the receiver without contention or control frame exchanges, control overhead is reduced and network throughput is increased. The drawback of this protocol is that the nodes suffer large delays due to the waiting time in exchanging two frames. The authors of [45] proposed a collision-free asynchronous MMAC for low-cost and more power-efficient nodes that have a single half-duplex transceiver. One common control channel is used and the rest are used as data channels.