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Since low-power Wi-Fi sensors are connected to the Internet, effective radio spectrum use is crucial for developing an efficient Medium Access Control (MAC) protocol for Wi-Fi sensor networks. A connectivity-based multipolling mechanism was employed for Access Points to grant uplink transmission opportunities to Wi-Fi nodes with a reduced number of multipolling frame transmissions.
- wireless connectivity
- multipolling
- MAC
1. Introduction
Many wireless sensors must be connected to the Internet, ensuring that the static information of products to which sensors are attached and the dynamic sensing information of the environment where sensors are deployed are delivered and utilized for efficient decision making to realize a ubiquitous computing society. An increasing number of wireless sensors are connected to the Internet for public security and health data monitoring because of the development of wireless LAN technologies, such as IEEE 802.11ah and IEEE 802.11s, and other non-802.11 technologies. Developing an efficient Medium Access Control (MAC) protocol that harnesses the radio resource of wireless sensor networks with a large number of sensors is a vital part of the success of wireless sensor networks in the future.
The connectivity-based multipolling MAC protocol was developed to improve the efficiency of the multipolling MAC protocol by reducing the multipolling frame transmissions [1][2][3][4]. Using connectivity and interference information among nodes, a simultaneous polling method was proposed to allow multiple direct data communication between nodes to be carried out [1]. The backtracking algorithm was introduced to derive the connected multipolling sequence [2]. Frame aggregation and connectivity-based multipolling techniques were combined to efficiently collect Radio Frequency Identification (RFID) tag information from nodes [3]. The combined clustering and sequencing method was proposed for a heuristic approach for deriving the connected multipolling sequence of RFID readers [4]. However, in some cases, a scheduling algorithm based on the Traveling Salesman Problem (TSP) model, which is employed by the connectivity-based multipolling MAC protocol, requires too much time to obtain the minimal number of serially connected multipolling sequences in cases where a large number (e.g., >100) of nodes exist in wireless LANs [1][2][3][4]. An efficient real-time implementable algorithm should be developed for deriving serially connected multipolling sequences of nodes when a large number of nodes exist in wireless LANs.
An efficient node insertion algorithm is proposed by modifying the existing scheduling algorithm based on the TSP model to construct serially connected multipolling sequences of nodes for the connectivity-based multipolling MAC protocol. The next node can be selected to be added to a serially connected multipolling sequence based on the connectivities from recently added multiple nodes. The nodes simultaneously connected from the most recently added nodes are not skipped. They are selected to be added to the serially connected multipolling sequences using the proposed node insertion algorithm. Therefore, a smaller number of serially connected multipolling sequences of nodes can be derived to cover all Wi-Fi sensor nodes in a wireless LAN. In densely populated wireless LANs, a single serially connected multipolling sequence can be derived for all Wi-Fi sensors, which will be shown by the simulation results. Although the previous scheduling algorithm may backtrack to obtain a smaller number of serially connected multipolling sequences, the proposed node-insertion algorithm does not backtrack to derive a near-optimal number of serially connected multipolling sequences in real time.
2. Multipolling MAC Protocol in Wi-Fi Sensor Networks
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) [5]. 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.
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. For efficient and reliable uplink data transfer by the connectivity-based multipolling MAC protocol, the bitmap technique can assign a bit to each recipient MAC address in the multipolling frames. The APs can indicate whether the previous uplink data transfer from the node of the corresponding MAC address was successful by setting the bit assigned to a recipient MAC address to zero or one.
This entry is adapted from the peer-reviewed paper 10.3390/app132111974
References
- Choi, W.-Y. Hybrid Polling Method for Direct Link Communication for IEEE 802.11 Wireless LANs. EURASIP J. Wirel. Commun. Netw. 2008, 2008, 598038.
- Choi, W.-Y. An Efficient Polling Scheme for Enhancing IEEE 802.11 PCF Protocol. Frequenz 2005, 59, 268–271.
- Choi, W.-Y. Combining Multipolling Method with Frame Aggregation for Collecting RFID Tag Information in IEEE 802.11 Wireless LANs. AEU-Int. J. Electron. Commun. 2011, 65, 345–348.
- Choi, W.-Y.; Chatterjee, M. Cluster-Based Multipolling Sequencing Algorithm for Collecting RFID Data in Wireless LANs. Frequenz 2015, 69, 141–147.
- IEEE Std 802.11; Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. IEEE Press: New York, NY, USA, 2007.
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