## 5 Cross-Layer Cluster-Based Energy-Efficient Protocol
### Slot Allocation Phase  
In the slot allocation phase, CH synchronization is required between other CHs, CMs and the sink node after its election. Moreover, CMs will have an additional listening state, so that adjacent CMs can synchronize their activities. During each contention period, all CMs keep their radios turned on. Here, CH will use a short preamble (similar to X-MAC [12]) to synchronize the CMs. In other words, when a new CH is elected, it will broadcast a SYN message, which contains its ID and the cluster-ID. After receiving the SYN message, the CM will send ACK messages to the CH. After receiving **ACK** message, the CH starts allocating the time slots based on residual energy. In this paper, we propose channel allocation based on contention-free communication to reduce the energy consumption. CCBE logically divides the time into slots with slot size \(t_{s}\). A frame length \(T\) is composed of consecutive time slots. The activities of every node are then repeated with period \(T\). We consider that all CHs have the same frame length. When a CM is transmitting packets to a CH, some other CMs that are in the listening state will also consume energy. Therefore, to reduce the total energy consumption, the other CMs that are not transmitting to CHs will go to sleep. The number of time slots for CMs is equal to the maximum possible number of members in the cluster. The CHs build a TDMA schedule and broadcast it to all CMs within the cluster. The scheduling algorithm assigns time slot \(1\leq t\leq T\) to all CMs. The members can be in any of the four possible states: transmitting, receiving, listening and sleeping. Considering that all CMs are perfectly synchronized and using TDMA, then no CM needs to be in the listening state during the transmission of other CMs of the same cluster. Our objective is to schedule the activities of sensor nodes to minimize the state transitions (especially from the sleeping state to active states) in order to increase energy efficiency. Surprisingly, the schedule for channel access is designed in such a way that CMs needs to wake up only twice: once for receiving information from their CHs and another time for transmitting their data to their CH. Moreover, consider a situation in which if CMs are scheduled in a random order, some CMs may need to wake up multiple times in a scheduling period T. This will increase the energy cost for each CM. Based on the new CCBE transmission schedule, each member turns on or off its radio. If a member has packets to send, the radios of all other members can be turned off. If the schedule indicates that a member node is a recipient in a certain time slot, its radio needs to be turned on. TDMA is used by both CHs and CMs for accessing the wireless channel. The time for accessing the channel is divided into different slots. Moreover, a CH or CM has to achieve the right to use a slot before data transmission. In this protocol, the sink assigns time slots to CHs, and the CH assigns time slots to CMs. From Figure 3, the sink time frame is divided into CH time slots and sink down-links. The packet header of CM packets is shown in Figure 3.  
The frame of CH as a starter (CH1), forwarder (CH4) and CHs' next hop as the sink (CH7) are shown in Figure 3. CH1 represents the CH of the cluster at the extreme end. Moreover, CH1 is not required to listen to other CHs for forwarding information to the sink. The frame of CH1 is divided into five slots. These are for listening to the CM and sink transmission, transmitting data to its next hop CH and its CMs and the sleep state. The CHs assign the time slot to CMs based on their remaining energy. The CM with the least energy will be given the first slot to transmit to the CH. The CM goes to sleep after transmission and only wakes up to receive the data from the CH in the same frame. Then, next is the CH forwarding state to the next hop CH in the direction of the sink. The sleep states of each CH are varied depending on the location of the CHs.