Distributed Scheduling of Broadcasts in a Radio Network

1. Distributed Scheduling of Broadcasts in a Radio Network Yang Yang WS70/08 Wireless Sensor Network

2. Outline • Motivation • Basic knowledge • Algorithm • Conclusion • Reference WS70/08 Wireless Sensor Network

3. Motivation • A Packet Radio Network (i.e. PRN) is a collection of geographically distributed, possibly mobile, users which share a common radio channel for exchanging messages among each other. • In a multi-hop packet radio network, a node’s transmission may be received by any other nodes within range. • Broadcasting is an important function in a communication network, such as • Updating distributed databases • Updating routing tables • It is important to have efficient algorithms to schedule these broadcasts. WS70/08 Wireless Sensor Network

4. Basic 4 5 2 6 1 7 3 8 Next Page WS70/08 Wireless Sensor Network

5. Two nodes can broadcast in same slot without conflicts if and only if they are more than one hops away from each other. • Maximum Broadcasting Set • Minimum Length Schedule WS70/08 Wireless Sensor Network

6. 4 5 2 6 1 7 3 8 Algorithm Description • Centralized Implementation In order (1,5,2,7,8,3,6,4) Each note only in one slot WS70/08 Wireless Sensor Network

7. 4 5 2 6 1 7 3 8 In order (1,5,2,7,8,3,6,4) Maximal broadcasting set WS70/08 Wireless Sensor Network

8. e g 5 6 f 11 10 7 9 8 s a b g 4 c 3 h 2 1 d i • Distributed Implementation • a TOKEN message is generated at a source node • depth first search (DFS), the path may be represented as a tree. • sends token to each of neighbors who have not received the token yet • if all its neighbors have received the token, it picks its transmission slot • post-order traversal: first leaf node reached by DFS is the first node pick a slot, the source node is the last node. • A node broadcasts its current schedule table along with the token, all its neighboring nodes received them, then update their schedule tables. WS70/08 Wireless Sensor Network

9. 1 2 5 3 4 • if there is a cycle in the graph of the network • a token be generated at node 1, and circulated to node 2,3,4,and 5 in that order • node 5 is the first to pick its slot, followed by node 4,3,2, and1. • two situations create conflicts: • node 2 can pick an incorrect slot, because it does not have any information about node1’s schedule table • node 1 does not have the current schedule table of node 5 WS70/08 Wireless Sensor Network

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11. Conclusion • It is a polynomial time algorithm which can be implemented efficiently in a distributed manner. • Disadvantage • It is not robust with regard to topology changes during its execution. • The second stage of the algorithm need not necessarily be executed. • No data transfer can take place during the execution of the algorithm. WS70/08 Wireless Sensor Network

12. references • Ramaswami R, Parhi K K. Distributed scheduling of broadcasts in a radio network[J]. INFOCOM 1989, 2:497-504. • Sayrafian-Pour, K.   Ephremides, A. Interference-free time-frequency broadcast scheduling in multi hop packet radio networks. WCNC 2000,1:106-111.   WS70/08 Wireless Sensor Network