A Spatio-temporal Access Network for Area Event Monitoring p.1

Transcription

1 A Spatio-temporal Access Network for Area Event Monitoring Jussi Haapola Centre for Wireless Communications (CWC) A Spatio-temporal Access Network for Area Event Monitoring p.1

2 Motivation Large sensor networks differ from traditional radio networks in a number of ways. A pure TDMA scheme for sensor networks with high number of nodes demands very high complexity for the sink nodes and the available data rate for individual sensor nodes falls rapidly close to zero. The longevity of sensor nodes is of paramount importance, so new ways for saving energy are needed. A Spatio-temporal Access Network for Area Event Monitoring p.2

3 Outline Introduction Spatio-Temporal MAC Sensing Area Classification Sensing Area Node Discovery Next-Hop-to-Sink Forwarding Application Layer Elements A Spatio-temporal Access Network for Area Event Monitoring p.3

4 Problem A Spatio-temporal Access Network for Area Event Monitoring p.4

5 Problem A Spatio-temporal Access Network for Area Event Monitoring p.4

6 Introduction The course work will describe a sensor network that communicates using a spatial and temporal division access method. We define a sensing area that consists of a set of sensors spatially in close proximity that are likely to sense a similar event at any given time. The sink node has spatial antennas, so that multiple sensor node messages can be received simultaneously if they are from different parts of the network. A Spatio-temporal Access Network for Area Event Monitoring p.5

7 Introduction (cont.) The measurement sensor nodes have omnidirectional antennas and lower transmission ranges. In addition, the course work will look at issues of data processing and forwarding in sensor networks, especially applied for use with this MAC technique. Data prevention can be applied globally so that nodes don t need to send unnecessary data. A Spatio-temporal Access Network for Area Event Monitoring p.6

8 Spatio-temporal MAC A Spatio-temporal Access Network for Area Event Monitoring p.7

9 Spatio-temporal MAC (cont.) A sink node has longer transmission range, sectored antenna with 4 sectors, network synchronisation functionality, extended battery life time, more processing power, and guaranteed channel access times compared to a sensor node. The sink periodically transmits a beacon frame to all sensor nodes. After the beacon there is a predetermined period in which the sensor nodes can communicate freely using any MAC protocol, for example nonpersitent CSMA. The free communication is limited to only paired or odd (0 belonging to paired) numbered areas at a time. Adjacent sectors operate in different phases: when one sector s paired areas are free to communicate, the sectors adjacent to it must have the odd areas free for communication. The communications flow is directed always towards the sink for the sensor nodes and the sink can directly transmit to any node in its area of influence. A Spatio-temporal Access Network for Area Event Monitoring p.8

10 Spatio-temporal MAC (cont.) The sink node periodically sends out a beacon signal to the nodes. beacon number, antenna sector ID, sink ID, beacon transmission power, next beacon time, and an optional packet of data for control or expected data values. The optional packet may include anything the sink wishes to inform the sensor nodes. All sensor nodes should listen to the beacon unless they are in deep sleep mode. A Spatio-temporal Access Network for Area Event Monitoring p.9

11 Sensing Area Classification The sink indicates in the beacon the transmission power TX pow (in db) it uses for transmitting the beacon. The sensor nodes listen a number of consecutive beacons and with a received power of TX pow 10 db classify themselves to belong to area 0, TX pow 25 db belong to area 1, TX pow 45 db belong to area 2, TX pow 70 db belong to area 3, etc. A Spatio-temporal Access Network for Area Event Monitoring p.10

12 Sensing Area Node Discovery Whenever a sensor node needs to transmit data, it also sends a frame header consisting of the following fields beacon number, antenna sector ID, sink ID, area number that it belongs to, and data. A Spatio-temporal Access Network for Area Event Monitoring p.11

13 Sensing Area Node Discovery (cont.) Because of the random access MAC protocol and spatio-temporal allocation, a sensor node will receive data frames from four different sources a node associated to different sink, the wrong sector, the area one hop further from the sink node, and own area. It is up to the APL to define individual node addresses because only the sinks need distinct IDs to communicate with each other. A Spatio-temporal Access Network for Area Event Monitoring p.12

14 Next-Hop-to-Sink Forwarding The MAC protocol within the spatio-temporal framework does not have many limitations. Good candidates are CSMA, CSMA/CA, and MACAW type protocols. Independent of the choice of the MAC protocol communication will be multicast, always. The source address is {area n, sector x, sink y}. the nodes in area n 2 who receive data from area n contend for the channel and the winner responds to area n. A Spatio-temporal Access Network for Area Event Monitoring p.13

15 Next-Hop-to-Sink Forwarding (cont.) Energy consumption equalisation: use function f(lower) for probability of reception contention whenever receive a message. Increase the probability by function f(raise) after period T raise. Depending on buffer size the same methodology can be applied to the transmitter. Nodes closer to a sink have to receive and transmit more data than nodes further from the sink. Compensation: the APL does data aggregation/prevention and progressive deepening. A Spatio-temporal Access Network for Area Event Monitoring p.14

16 Application Layer Elements Global aggregation A technique for tracking expected values globally in a sink node in order to prevent unneeded data updates. Neighbour data processing Within a sensing area duplicate values may be of no use to the sink, depending on the application. The application can then make the decision of whether it has overheard a duplicate value. If a duplicate or close enough value discard from queue. Else insert packet to MAC queue and resume contention Forwarding Aggregate the data with nodes own data, or forward without aggregation. A Spatio-temporal Access Network for Area Event Monitoring p.15

17 Thank You Questions? Jussi Haapola, A Spatio-temporal Access Network for Area Event Monitoring p.16