## 3 Connectivity-Based Multipolling MAC Protocol  
This section will briefly explain the mechanism of the connectivity-based multipolling MAC protocol [1; 2; 3; 4]. The connectivity-based multipolling MAC protocol is based on the Point Coordination Function (PCF) [39]. According to the PCF, APs transmit a separate polling frame to each node to grant uplink transmission opportunity. However, when a large number of nodes exist in wireless LANs, and the uplink MAC frames have a small payload, which is the case for wireless LANs with a large number of Wi-Fi sensors having relatively a small amount of static and dynamic sensing information, the MAC overhead of the separate polling transmissions for all nodes becomes significant. The connectivity-based multipolling MAC protocol was proposed to resolve this problem. The connectivity-based multipolling MAC protocol mainly focuses on optimizing the general uplink data transmission procedure in Wi-Fi sensor networks with multipolling frame transmissions but not on the query-based information search for a specific sensor or a group of sensors. \begin{table}
\begin{tabular}{c c c c} \hline  & & & **MAC Method** \\ \cline{3-4}  & & **Contention-Based** & **Contention-Free or Reservation-Based** \\ \hline \multirow{2}{*}{Infrastructure Network} & Wi-Fi Network & Most & Little \\  & Non-802.11 Network & Fair & Fair \\ \hline \end{tabular}
\end{table} 
Table 1: Trend of research on wireless sensor networks. Initially, when APs have not yet collected connectivity information among nodes, the connectivity-based multipolling MAC protocol is operated according to the PCF. The connectivity information among nodes indicates whether each node can successfully hear and decode the transmission signals from other nodes. Each node \(i\) responds to APs by piggybacking on its uplink data or null frame whether it can hear the transmission signals from each other node \(j\), that is, node \(i\) is connected from node \(j\), overhearing the transmission signals from node \(j\). From the collected connectivity information among nodes, APs construct serially connected multipolling sequences of nodes that cover all nodes that are associated with themselves. When APs multicast a multipolling frame to the nodes in a serially connected multipolling sequence, in which the recipient MAC addresses are specified in the MAC header of the multipolling frame, each recipient node knows its order in the sequence. The first recipient transmits its data or null frame in a short interframe space (SIFS) period after the reception of the multipolling frame, and other nodes in the sequence transmit their data or null frames in an SIFS period after receiving the transmission signals from the node of the previous order in the sequence. Note that each node, besides the first recipient, can hear the transmission signals from the previous recipient in a serially connected multipolling sequence. Each node responds to APs with updated connectivity information during the connectivity-based multipolling process, which is the change in the set of other nodes from which it is connected. APs should update the serially connected multipolling sequences based on the updated connectivity information among nodes. Based on the mechanism of each sensor reporting to APs, the change in connectivity information, and APs updating the serially connected multipolling sequences based on the updated connectivity information among nodes, the dynamic nature of Wi-Fi sensor networks can be reflected in the connectivity-based multipolling MAC protocol. When a node does not respond to a multipolling frame, the APs should retransmit the multipolling frame for error recovery, where the multipolling sequence should include only the partial sequence of the original sequence after the failed node. When the failed node is not the first node in the multipolling sequence, the APs consider the connectivity from the previous node of the failed node to the failed node to be broken.