# Receiver-Initiated Handshaking MAC Based on Traffic Estimation for Underwater Sensor Networks +
## 2 Applications, Motivations, and Challenges
### Motivations for Receiver-Initiated MAC  
**Enabling Data Aggregation at the Link Layer. ** In receiver-initiated MAC protocols, the receiving node requests and gathers data packets from surrounding neighbors at each round of communication. This feature enables data aggregation at the link layer which can significantly reduce the traffic overhead with the link layer packet fusion [17]. As assistance to the end-to-end data fusion, local information gathering is essential to support a fast response to the dynamics of a network and meanwhile, to extend the network's lifetime. In addition, receiver-initiated MAC protocols are preferable to the applications with network coding [18]. In order to successfully decode one data packet, the receiver may need multiple packets from different senders, since the coded packets generally diverge to the surrounding neighbor users. The scheme of one receiver invites multiple packets from different nodes in receiver-initiated protocols to fit the data gathering request of network coding applications. **Significantly Reducing the Overhead on Control Messages. ** For the purpose of achieving reliable communication, an acknowledgment (\(ACK\)) is needed for the receiver to inform the senders of successful data delivery. Since only one receiver is involved in parallel reservation with the receiver-initiated handshake, one \(ACK\) packet is enough to notify all the senders of the successful data reception in one round of communication. In contrast, in the sender-initiated-based protocols, each receiver in the parallel reservation is required to respond with an \(ACK\) message due to the one-to-many scheme, which results in extra energy consumption. When the overhead of control packets is comparable to the data packets in UWSNs owing to the long preamble problem [19], reducing the number of \(ACK\) messages could remarkably improve the energy efficiency of acoustic communications. Based on the length of control and data packets listed in Table 1[20], we summarized the control message overhead of the traditional sender-initiated handshaking without parallel reservation and the receiver-initiated method with parallel reservation, using the AquaSeNT OFDM (orthogonal frequency division multiplexing) Modem as an example. We assumed that five reservations were made on average with each round of parallel reservations. * single-sender to single-receiver \((RTS+CTS+ACK)/Data=0.66\times 3/1.15\approx 172\%\)
* single-sender to multiple-receiver \((RTS+5 CTS+5 ACK)/5 Data=0.66\times 11/1.15/5\approx 126\%\)
* multiple-sender to single-receiver \((RTR+5 ATS+ACK)/5 Data=0.66\times 7/1.15/5\approx 80\%\)
* Request-to-Send message \(CTS\) Clear-to-Send message \(ACK\) Acknowledgment message \(RTR\) Request-to-Receive message \(ATS\) Available-to-Send messageBesides energy saving with parallel reservation, the receiver-initiated communication scheme further reduced the network overhead roughly from 126% to 80%, illustrating a promising application for the energy-constrained UWSNs. **Enhancing Collision Avoidance Capacity. ** The interference area, marked as red in Figure 1, is the region within which the transmission of nodes will interfere with the reception process of the receiving node. In the communication with parallel reservation, either one sending node communicates with multiple receivers in sender-initiated-based methods, or one receiver requests data packets from multiple senders in receiver-initiated-based protocols. The interference area in Figure 1a changes while the sender communicates with different receivers in sender-initiated methods. In contrast, the receiver-initiated handshaking communication has a static interference area, as shown in Figure 1b, since only one receiver is involved. This static interference area enhances the collision avoidance for the time scheduling on the receiver side.