# Receiver-Initiated Handshaking MAC Based on Traffic Estimation for Underwater Sensor Networks +
## 5 TERI-MAC Design
### Description of TERI-MAC
The senders send one or multiple data packets in a packet train according to the arrangement of the receiver. The time in the data transmission phase is divided into mini slots, which are assigned to the invited senders by the receiver in Phase 1, corresponding to \(N_{S_{i}}\) in the \(RTR\) message. The length of time slot is equal to the transmission time of the \(DATA\) packets. Each sender sends out its \(DATA\) packets in the scheduled time if it is able to transmit. Since the slot allocation is based on the traffic estimation, which may not be accurate, the slots may be wasted if the invited sender has fewer packets to send or if some of the packets are not be transmitted in time if insufficient slots are assigned and are therefore queued at the sender. For this reason, TERI-MAC is more preferred in delay-tolerant applications. However, as long as we can get a good approximation of the traffic distribution of the senders, which is stationary, at least in the short term, TERI-MAC can achieve a desirable energy efficiency and network latency based on the adaptive data polling scheme which will be discussed in Section 6. **Phase 4:** In the last phase of TERI-MAC, the receiver confirms the successful data reception by replying to an integrated acknowledgment (\(ACK\)) message to all senders, instead of individual \(ACK\)s from each receiver in the sender-initiated protocols. Considering the long preamble in acoustic modems, having fewer \(ACK\) packets will significantly reduce the power consumption and thus extend the network's lifetime. If the \(ACK\) message is lost on a sender, the node will not know whether the data reception is successful or not. The sender can either retransmit all involved data packets in the next round of data requests from the same receiver or dump those packets at the risk of potential data loss. In the simulation, we implemented the retransmission scheme to handle the occasional \(ACK\) loss to guarantee the delivery of data packets. It is worth noting that, the guard time is generally required in most of the MAC protocols that utilize parallel transmissions to prevent unexpected collisions caused by an inaccurate distance measurement, a synchronization error or the drifting of acoustic nodes with an ocean current. In order to prevent collisions among \(ATS\) messages in Phase 2 and \(DATA\) packets in Phase 3, a guard time of 1 ms is added in the implementation of TERI-MAC. Compared to the sender-initiated algorithms, one of the significant advantages of TERI-MAC is its energy efficiency. TERI-MAC conserves energy in three ways. Firstly, the receiver-initiated  
Figure 6: TERI-MAC transmission scheduling. Squares filled with and without oblique lines represent transmissions and receptions, respectively. The red cross indicates the occasional packet loss. Figure 7: Format of the Available-to-Send (\(ATS\)) message. reservation is more effective at preventing data packet collisions since all the potentially interfering nodes of the receiver are no longer hidden and are informed at the very beginning of the handshaking process. Secondly, TERI-MAC employs parallel reservation and a packet train to reduce the overhead of handshaking control messages. Furthermore, the number of \(ACK\) packets is reduced dramatically compared to the sender-oriented protocols.