# Receiver-Initiated Handshaking MAC Based on Traffic Estimation for Underwater Sensor Networks +
## 6 Adaptive Data Polling for Teri-Mac
### How Much Data to Poll  
The second challenge of the data polling problem is to decide how much data to request from each sender. Since the receiver has no prior knowledge regarding the number of data packets cumulated on the intended senders, the receiver has trouble determining how many packets to request from senders without any extra communication. By assigning more channel resources to the sender, a higher packet delivery coverage and thus, a shorter communication delay can be achieved at the cost of lower efficiencies for both energy and channel utilization. The receiver can improve the energy and channel efficiency by assigning few transmission slots to senders, such that the assigned slots have less chance of being wasted, which, however, may result in an extra queuing delay when the cumulated packets cannot be sent out in a timely manner. Therefore, a good estimation of the probability distribution of the senders' traffic is especially important. With the assistance of a good traffic predictor, the receiver is able to guarantee an overall packet delivery ratio. More information about the traffic estimation is introduced in Section 7.  
In an underwater network, deployed as Figure 2 for example, the receiver node initiates the handshaking by requesting data from its neighbors. The traffic from different senders may vary since it includes both the self-generated data and the packets forwarded for other nodes, according to the network topology and the routing protocol. The traffic in node \(Q_{i},i\in\{1,\ldots,N\}\) would follow the binominal distribution, and we have  
\[TRA_{Q_{i}}\sim\sum_{j=1}^{N_{Q_{i}}}B(T_{j},p_{gl}) \tag{6}\]  
where \(N_{Q_{i}}\) represents the number of nodes that have packets to travel through node \(Q_{i}\) which may vary with time and depends on the network topology and routing algorithm. \(T_{j}\) is the time interval between two successive rounds of data requests, and \(p_{gl}\) is the self-generated average data rate.  
* **The delivery percentage \(P_{del}\):** When the number of packets in the polled node follows a given distribution \(F(x)\), there is a certain probability \(P_{del}\) that the assigned slots \(T_{S}\) will be enough to transmit all of the queued packets: \[P_{del}=\int_{0}^{T_{S}}F(x)dx.\] (7) Since the number of packets is a random process, there is no way for the receiver to predict an exact number without error. However, based on traffic distribution knowledge, the receiver can assign time slots for each sender in order to guarantee a predetermined delivery percentage \(P_{del}\). By adjusting percentage \(P_{del}\), we can achieve a tradeoff between channel resource utilization and communication delay. A high \(P_{del}\) increases the chance that a packet will delivered in the current round of communication; however, it also increases the probability that the channel resource be wasted because of the overassigned time slots. A low \(P_{del}\), on the contrary, leads to a high average delay, which is not profitable for the network communications. Choosing an appropriate \(P_{del}\) is important to TERI-MAC, as will be discussed later in Section 8.1.