# DCO-MAC: A Hybrid MAC Protocol for Data Collection in Underwater Acoustic Sensor Networks
## 2 Related Work
In a UASN with a low traffic load, the probability that no senders respond to the transmission initiated by the receiver is high, which results in the performance degradation. The performance of the contention-based and schedule-based MAC protocols are poor when the traffic load distribution in the network is non-uniform. In order to achieve a good performance in such a situation, some hybrid MAC protocols are proposed for the UASNs. In the adaptive multi-mode MAC protocol (MAC-AMM) [14], multiple protocols are incorporated. When the traffic load is low, a random-access MAC protocol is performed. As the traffic load increases, a contention-based or a reservation-based MAC protocol is chosen according to the network architecture. Underwater practical MAC protocol (UPMAC) [15] chooses suitable MAC protocols based on the traffic load. In UPMAC, ALOHA is performed when the network works in a low traffic load mode, in which the sensor nodes transmit the data packets to the receiver without channel reservation, and the number of data packets intended to transmit to the receiver is included in the data packets. The receiver extracts this information and performs a reservation-based MAC protocol if the number of data packets exceeds the threshold. The hybrid MAC protocols achieve a good performance in the single-hop underwater sensor networks. However, when these hybrid MAC protocols extended to a multi-hop underwater sensor network, a node may carry out several protocols simultaneously, and transmission collision may be caused. Several MAC protocols intended for multi-hop underwater sensor networks are also proposed. The cluster-based MAC protocol (CH-MAC) [16] and cluster-based data collection schemes with direct sink access (CDS-DSA) [19] are both designed for a clustered UASN. In both of the MAC protocols, the transmission is partitioned into two phases, namely the intra-cluster communication phase and the inter-cluster communication phase. The transmission of the two phases is executed periodically, in order to avoid the transmission collision. In the intra-cluster communication, time division multiple access (TDMA) is carried out in these two MAC protocols. In the inter-cluster communication, ALOHA and CSMA/CA are used in CDS-DSA and CH-MAC, respectively. CDS-DSA and CH-MAC are energy-efficient, and achieve a low end-to-end packet delay. However, if the traffic load of the inter-cluster communication is heavier than that of the intra-cluster communication, ALOHA and CSMA/CA will lead to high collision probability in the inter-cluster communication. In an underwater mobile network, the SN or sensor nodes can be a mobile node. CC-DCP [20] is a TDMA-based MAC protocol used in a UASN, in which the SN is a mobile node. The SN travels along a pre-scheduling path to collect the data packets of the static sensor nodes. In order to improve the performance, the SN communicates with multiple sensor nodes in one transmission, and schedules the transmission time slot of the sensor nodes to avoid collision. MC-MAC [18] is another MAC protocol that is used in a UASN, in which the sensor nodes are mobile. The sensor node, having data packets to be transmitted, moves to the SN to initiate transmission, and the CDMA technique is used to avoid the transmission collision.