tdma schleduling in wireless sensor network

TDMA Scheduling Algorithm for Wireless SensorNetworks

Neha AgarwalReg No.– 2011CS03

Motilal Nehru National Institute of Technology Allahabad

March 27, 2012

Neha Agarwal Reg No.– 2011CS03 Motilal Nehru National Institute of Technology Allahabad

TDMA Scheduling Algorithm for Wireless Sensor Networks

Outline

I Motivation for TDMA Scheduling

I Introduction

I TDMA Scheduling

I Node-based Scheduling Algorithm

I Level-based Scheduling Algorithm

I Scalability and Distributed Implementation

I Conclusion



Motivation for TDMA Scheduling

I More power consumption in IEEE 802.11 MAC protocol

1. Idle listening2. Wastage of energy in collision

I No delay guarantee



Introduction

I TDMA protocols are more energy efficientI Nodes can enter inactive states until their alloted time slotsI TDMA eliminate collisions and bound the delay

I e.g. TDMA protocol for a traffic monitoring network has alifetime of 1200 days compared with 10 days using the IEEE802.11 protocol



TDMA Scheduling

I Allocation of time slots depending upon the topology and thenode packet generation rates

I One-hop or multi-hop scheduling

I One-hop: Allocation of time slots is done depending on theallocation request and deadline of the nodes

I Multi-hop: More than one node can transmit at the same timeslot if their receivers are in non-conflicting parts of the network

I Two types of conflicts:

1. Primary conflict2. Secondary conflict

I Find the smallest length conflict free assignment of slots



Network and Transmission Model

I Network is represented by graph G=(V,E),where V is a set ofnodes including the access point AP and E are thetransmission links to be scheduled

I Interference graph C=(V,I)is known where I ,subset of VXV,is the set of edges such that (u,v) belongs to I if either u or vcan hear each other or one of them can interfere with a signalintended for the other

I If u is transmitting ,v should not be scheduled to receive fromother node at the same time

I Conflict graph GC=(V,EC) corresponding to graph G=(V,E)and C=(V,I) EC comprises of edges between node pair in Gthat should not transmit at the same time,and is generated bytaking into account primary and secondary conflicts



Scheduling Problem

I Given the interference graph , the scheduling problem is tofind a minimum frame during which all nodes can send theirpackets to AP

I So we reduce the problem of finding the chromatic number ofa graph to the scheduling problem



Node-based Scheduling Algorithm

I Adapted from classical multi-hop scheduling algorithm withthe idea of scheduling as many non-conflicting set of nodes aspossible in each time slot

I Algorithm has two parts:

1. Coloring the network: Nodes are ordered in non-increasingorder of degree,the algorithm then assign smallest color to thenodes in such a way that none of the nodes of the same colorhave an edge in the conflict graph

2. Scheduling the network: A superslot is collection ofconsecutive time slots such that each node with at least onepacket at the beginning of the superslot transmits at least onepacket during the superslot



Example:Schedule for Node-based Scheduling Algorithm



Level-based Scheduling Algorithm

I Algorithm has three parts:

1. Obtain a linear network: If the original tree network hasdepth N, the linear network GL = (VL,EL) has nodes VL =v1,..., vN with node vl corresponding to all nodes at level l inthe original network and edges . The interference graph CL =(VL, IL) includes edge (vj, vl) . The resulting conflict graphGCL = (VL, ECL) includes edge (vj, vl) if the transmissions ofa node at level j and a node at level l conflict in the originalnetwork.

2. Color this linear network3. Schedule the original network:If nodes vi, vj in the linear

network are assigned the same color, they do not interfere. Byconstruction of the linear network any two nodes in theoriginal network, one chosen from level i and the other fromlevel j, can transmit at the same time



contd..

I A superslot is a collection of consecutive time slots such thateach level of the tree with at least one packet at thebeginning of the superslot forwards at least one packet to thelower level during the superslot.

I Number of slots in a superslot is at most equal to the totalnumber of colors used for coloring the linear network



Example: Schedule for Level-based Scheduling Algorithm



Scalability and Distributed Implementation

I Clustering the nodesI The central controller corresponding to each cluster should

take into account the interferers that are outside their rangewhile generating schedules

I Central controllers are assigned colors so that no twoconflicting controllers are assigned the same color

I Distributed algorithmsI Schedules of the nodes are generated based on the local

topology information of the nodesI Color assignment is performed in two stagesI Each node picks one slot for transmission in the order of the

traversal DFS of the graph GI DFS is repeated and now each node picks as many of the

remaining colors as it can for transmission



Scalability and Distributed Implementation

I The DFS traversal starts with a token message generated atthe AP

I Upon receipt of the token, the node performs the colorassignment and then sends this information to its one-hop andtwo-hop neighbors. It then sends the token to each of itsneighbors in G who have not received the token yet.

I Once it finds that all its neighbors have received the token, itsends the token back to its parent

I At the end of the traversal, the token carries the informationof the number of colors used in the network



Example: Schedule for Distributed Scheduling Algorithm



Example 2



Conclusion

I Scheduling problem is to determine the smallest lengthconflict-free assignment of slots

I Two centralized algorithms has been proposed

I In node based scheduling the schedule is obtained based onthe coloring of the original network

I In level-based scheduling firstly linear network is obtained thenschedule is obtained based on the coloring of the linearnetwork



References

I TDMA scheduling algorithms for wireless sensor networks bySinem Coleri Ergen Pravin Varaiya

I Ephremides, A., Truong, T. V. (1990). Scheduling broadcastsin multihop radio networks. IEEE Transactions onCommunica- tions, 38(4), 456460.

I Tavli, B., Heinzelman, W. B. (2004). MH-TRACE: Multihoptime reservation using adaptive control for energyefficiency.IEEE Journal on Selected Areas in Communications,22(5), 942 953.



Thank You

