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TDMA Scheduling in Wireless Sensor Networks. Presented by: Ang Tashi Lama Sherpa Department of Communications and Networking Aalto University School of Science and Technolgoy. Presentation Outline. Introduction Wireless sensor network (WSN) Issues in WSNs Medium Access Control (MAC)
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TDMA Scheduling in Wireless Sensor Networks Presented by: Ang Tashi Lama Sherpa Department of Communications and Networking Aalto University School of Science and Technolgoy
Presentation Outline • Introduction • Wireless sensor network (WSN) • Issues in WSNs • Medium Access Control (MAC) • TDMA Scheduling • Wireless Sensor Network Model • Contributions • Centralized link scheduling and channel assignment with balance tree formation algorthim • Distributed Broadcast TDMA (DB-TDMA) scheduling algorithm • Extension to DB-TDMA • Performance Metrics • Performance Evaluation • Conclusion and Future Work
Wireless Sensor Network (WSN) • Wireless sensor network is a collection of sensor nodes, ranging from tens to thousands, that are spatially distributed in a geographical area to cooperatively monitor and gather data about the environment. • Each sensor node in a WSN is a battery powered device consisting of sensors such as temperature and light, a small microcontroller, and a transreceiver for wireless communication.
Issues in WSNs • Jennifer Yick et.al [1] has grouped the issues in WSNs into three groups. • System: • In WSNs, each sensor node is an individual system and to support various application software, there is a need to develop new platforms, operating systems, and storage schemes. • Communication protocols: • Communication protocols enable communication between the application and sensors, and also between the sensor nodes. • Services: • Services are developed to enhance the application, and improve system performance and network efficiency.
Medium Access Control (MAC) • MAC protocol plays an important role in determining channel capacity utilization, network delays, and power consumption. • Time division multiple access (TDMA) MAC is a potential candidate for WSNs. • TDMA conserves energy by eliminating collisions, avoiding idle listening, and entering into inactive states until their allocated time slots. • TDMA can bound the delay of packets and guarantees reliable communication. One of the major challenge in adopting TDMA MAC is finding efficient time-schedule.
TDMA Scheduling • TDMA scheduling can be defined as the process of allocating time slots to the nodes or links between each pair of neighboring nodes, to ensure collision free channel access. • S. Ramanathan et.al. [2] has classified TDMA scheduling into two types: • Broadcast Scheduling: The stations themselves are scheduled. The transmission of a station must be received collision-free by all its one-hop neighbors • Link Scheduling: The links between stations are scheduled. The transmission of a station must be received collision-free by one particular neighbor. A proper TDMA scheduling should avoid collision and also minimize the number of time slots in a frame to reduce latency in the network.
TDMA Scheduling • The objective of TDMA Scheduling is to get rid of primary and secondary conflict. • Primary conflict: Occurs when one node transmits and receives at the same time slot or receivs more than one transmission destined to it at same time slot. • Secondary conflict: Occurs when an intended receiver of particular transmission is also within the transmission range of another transmission intended for other nodes
Wireless Sensor Network Model • Static sensor network: All nodes remain in the same position or has limited mobility throughout their lifetime in the network. • Connectivity: The network is assumed to be connected. • Single radio sensor node: It is assumed that the sensor nodes have only one radio transceiver. • Each node is assumed to have a unique node ID. • Global synchronization: All the nodes in the network are assumed to be synchronized with each other. • Bidirectional radio link: The radio link between two nodes is assumed to be bidirectional, i.e, if node v can hear node u, then node u can hear node v.
Centralized link scheduling and Channel assignment with balanced tree formation algorithm • A centralized link scheduling and channel assignment with balanced tree formation algorithm is designed to provide joint routing and multi-channel TDMA link scheduling for wireless sensor networks. • Balanced tree formation: The algorithm generates a balanced tree structured network topology that evenly distributes data traffic generated by sensor nodes across the different branches of the routing tree. • Channel allocation (CA): The channel allocation is receiver based, i.e., the node selects the receiving channel for itself. • Time Allocation: CA is then followed by time slot allocation, which consists of allocating time slots to the links between the nodes for uplink and downlink.
Distributed Broadcast TDMA Scheduling Algorithm (DB-TDMA) • A distributed broadcast TDMA scheduling algorithm (DB-TDMA) that allows each node to assign itself a time slot based on the two-hop neighborhood information. • The main objective of the proposed algorithm is to reduce the number of message transaction taking place between the nodes without compromising the convergence time of the algorithm. • The algorithm provides an option of having global time frame or local time frame in the network. • In global time frame, all the nodes have the same time frame. • In local time frame, the time frame of a node depends on its number of two hop neightbours(Fi), which is calculated as,
Extension to DB-TDMA • An extension to distributed broadcast scheduling algorithm is also proposed. • This extension proposed for DB-TDMA when local time framing is utilized. • The extension allows nodes with the largest one-hop neighorhood size among its one-hop neighbors to select multiple time slots for itself. We term these nodes as priority nodes.
Example of DB-TDMA and its extension with Local Time Framing
Performance Metrics • Schedule length: The schedule length is defined as the maximum number of allocated time slots after scheduling. • Message complexity: The message complexity is the average number of transmitted messages by each node, to decide on the time slots for all executions of the algorithm. • Running time: The running time denotes the number of rounds required for all the nodes in the network to decide on their time slots. Each round is a period of time during which, a node can: (a) send a request message, (b) receive messages from all its one hop neighbors, (c) select minimum time slot available, and (d) send one hop broadcast of the selected time slot.
Performance Evaluation • In order to evaluate the algorithms we use a simulation based approach using Matlab. • The network topology is generated by randomly deploying N nodes in the area of √N × √N unit2, so that when N varies, the node density within the transmission range is kept constant. The transmission range of the nodes are then adjusted to control the density of deployment keeping the number of nodes in the network constant.
Centralized link scheduling and channel assignment with balanced tree formation algorithm • Here, 100 nodes are deployed randomly in area of 10 × 10 unit2 and transmission range is varied from 1.5 to 2 units. • It is evident that there is a decrease in schedule length with increase in available channels in the network. • This decrease is observed to be less significant when number of available channels is more than two. • This behaviour is observed because the interference limitation is eliminated with two channels and beyond this point, the connectivity constraint, i.e., the number of children connected to a node, limits the performance
Scalability of DB-TDMA and its extension • Here, number of nodes is varied from 100 to 500 with constant transmission range of 1.5 units. • It is evident from the tables that the proposed distributed algorithms are scalable making them suitable for high density sensor networks.
Schedule Length of DB-TDMA and its extension • Here, 100 nodes are deployed randomly in area of 10 × 10 unit2 and transmission range is varied from 1 to 2 units. • The algorithms utilize local time framing. • The schedule length of both the algorithms is similar.
Message Complexity and Running Time of DB-TDMA and its extension • The message complexity and running time of extended DB-TDMA is observed to be higher than that of DB-TDMA. • The main reason behind this is that additional message transaction are required to ensure multiple slots to priority nodes.
Comparision of DB-TDMA with DRAND and DD-TDMA • Here, 100 nodes are deployed randomly in area of 10 × 10 unit2 and transmission range is varied from 1 to 2 units. • Global time framing is utilized for all the three algorithms. • DB-TDMA, DRAND, and DD-TDMA achieves similar schedule length.
Comparision of DB-TDMA with DRAND and DD-TDMA • The message complexity of DB-TDMA is observed to be lower than both DRAND and DD-TDMA.
Comparision of DB-TDMA with DRAND and DD-TDMA • The running time of DB-TDMA is observed to be similar to that of DD-TDMA and lower than that of DRAND.
Conclusion and Future Work • We have proposed two solution to the TDMA scheduling problem: centralized link scheduling and broadcast scheduling algorithm. • Implement the algorithms on real-life sensor network test beds. • Make the algorithms suitable for mobile wireless sensor networks.
References [1]J. Yick, B. Mukherjee, and D. Ghosal, Wireless sensor network survey, The International Journal of Computer and Telecommunications Networking, 2008 [2] S.Ramanathan, and E.L.Lloyd, Scheduling algorithms for multihop radio networks, IEEE/ACM Transactions on Networking (TON), vol.1, no.2, pp. 166-177, 1993
