# DCO-MAC: A Hybrid MAC Protocol for Data Collection in Underwater Acoustic Sensor Networks
## 3 System Model  
In this paper, we consider a two-tier UASN for data collection, all of the sensor nodes are in the two-hop transmission range of SN, and the network topology is shown in Figure 1.  
Sensor nodes are classified into three types, namely, the sink, level-1 nodes, and level-2 nodes. The level-1 nodes are in the one-hop transmission range of the sink, such as A, B, C and D in Figure 1. Level-2 nodes are in the two-hop transmission range of the sink, such as A\({}_{1}\), A\({}_{2}\), and B\({}_{3}\), in Figure 1. In addition, the mobile nodes are included in the system model, such as D and B\({}_{3}\), in Figure 1. The mobile nodes join in the network only when they have data packet(s) to be transmitted. Because of the low movement speed of the mobile nodes, the change of their locations is small in the period of one data transmission. Hence, we assume that the position change of the mobile nodes will not influence the performance of the MAC protocols. Hence, the position change of the mobile nodes is neglected in our proposed MAC protocols.  
Because of the sparse distribution of the sensor nodes in the UASN, we assume that the level-1 nodes are not in the transmission range of the other level-1 nodes. The mobile nodes move along the designed path to collect data and transmit data packets to a level-1 node or the sink. All of the other sensor nodes float slightly with the ocean current around the fixed position.  
The data packet transmission path of each node is determined after the network is deployed, and each sensor node has one corresponding receiver. After being deployed, the sink broadcasts a path formation massage. The sensor nodes receiving the path formation massage from the sink are regarded as level-1 nodes, and their common receiver is the sink. Then, the level-1 nodes broadcast path formation messages. The sensor nodes receiving path formation messages from the level-1 nodes are regarded as level-2 nodes, and determine their corresponding receivers. If a level-2 node receives more than one path formation message, it selects the level-1 node, whose path formation message arrives earlier, to be its receiver. In this way, the data transmission path from each sensor node to the sink is determined.  
The DCO-UASN is partitioned into two kinds of sub-networks [21]. The level-1 sub-network consists of the sink and several level-1 nodes, such as A, B, C, and D in Figure 1. The level-2 sub-network consists of one level-1 node and several level-2 nodes. For example, a level-2 sub-network includes nodes A, A\({}_{1}\), A\({}_{2}\), A\({}_{3}\), and A\({}_{4}\) in Figure 1.  
In the DCO-UASN, the data packets generated by the sensor nodes are transmitted from the sensor nodes to the SN. From Figure 1, the level-1 nodes not only transmit the generated data packets, but also forward the data packets received from the corresponding level-2 nodes to the sink. Obviously, the traffic load in the level-1 sub-network is heavier than that in the level-2 sub-network. However, as most of the existing MAC protocols are designed for underwater sensor networks with a uniform distribution of the traffic load, they cannot be utilized to the DCO-UASN directly. Therefore, it is essential to design a suitable MAC protocol in the DCO-UASN. In this paper, we propose a hybrid MAC protocol for data collection in the UASN, where different sub-networks perform different MAC protocols according to their traffic load.