Adaptive Self-Configuring Sensor Network Topologies

ns-2 simulation & performance analysis

Zhenghua FuBen GreensteinPetros Zerfos

Sensor Networks Advances in micro-sensor and

radio technology will enable deployment of sensors for a range of environmental monitoring applications

Due to the low cost per node, networks of sensors may be densely distributed

Challenge Unattended sensor nets with

limited energy often must be long-lived

The network should be able to adaptively self-configure to maximize energy efficiency, while still achieving spatial coverage and robustness

ASCENTAdaptive Self-Configuring sEnsor Networks Topologies

Answers how to form a multi-hop topology Developed by Alberto Cerpa, UCLA Protocol assumes dense distribution of nodes ASCENT leverages the redundancy imposed by

high node density Each node assesses its connectivity and

adapts its participation in the multi-hop network topology

Network membership determined in a distributed fashion using measurements and calculations performed locally

What is ASCENT? It is NOT a routing or data

dissemination protocol ASCENT simply decides which

nodes should join the routing infrastructure In this respect, routing protocols are

complementary to ASCENT

Why not use a central configuration node? Scaling and robustness

considerations Nodes would need to communicate

detailed connectivity state information to this central node

Network Assumptions Ad-hoc deployment Energy constraints Unattended operation under

dynamics

In many such contexts, it will be easier to deploy large numbers of nodes initially rather than to

deploy additional nodes or energy reserves at a later date.

Effects of Node Density Too few nodes

Larger inter-node distance Higher packet loss rate

Too many nodes At best, unnecessary energy expenditure At worst, interfering nodes congest the

channel Equilibrium

Approximated using self-configuration

ASCENT Initially, only

nodes A and B are alive

All other nodes are passively listening, but are not part of the network

A

B

ASCENT A sends data to B Due to signal

attenuation and random shadowing B detects high message loss

Notion of “high” is application dependent

A

B

A

B

B attempts to remove this “communication hole”

B requests additional nodes in the region to join the network to serve to relay messages from A to B

ASCENT

A

B

Additional node(s) join the network

Alternatively, while passively listening, node C determines whether it would be “helpful” to join the multi-hop routing infrastructure

C

ASCENT

Neighbor Discovery Phase A neighbor is defined as a node

from which a node receives a certain percentage of messages over time

The number of neighbors can greatly increase the energy consumption in contention for resources

Neighbor Discovery Phase Entered at time of node

initialization Using local measurements, each

node obtains an estimate of the number of neighbors actively transmitting messages

As the neighbor count increases, so too should the neighbor’s message loss threshold

Join Decision Phase A node decides whether to join the

multi-hop diffusion sensor network A node may temporarily join and

test whether it contributes to improved connectivity

The decision is based on message loss percentage and number of neighbors

Active Phase A node enters the Active Phase

from the Join Decision Phase when it decides to join the network for a long time

Starts sending routing control and data messages

Adaptive Phase When a node decides NOT to join

the network A node has the option of either

powering down for a period of time or reducing its transmission range

Previous ASCENT Implementation PC-104 nodes RPC Radiometrix Radio Linux LECS barebones CSMA MAC Diffusion Routing ASCENT written on top of Diffusion ASCENT uses Diffusion for routing of its

control messages

ASCENT for NS Rewrote ASCENT for NS Placed the ASCENT code in the NS Link

Layer Removed calls to Diffusion to route

control packets Modified 802.11 and Link Layer code so

that nodes could “play dead” Removed retransmit functionality of

802.11

Motivation ASCENT has been tested on only 25

nodes. With NS it can be tested on hundreds

Verify that NS models the real world well

Confirm that ASCENT works

Simulation Setup Communication within 1-hop distance 90% message loss at 1100m. One source (sending frequencies of 1p/sec

and 10p/sec – CBR-like), one sink Densities of : 3, 4, 5, 10, 30, 50, 75 and 100

nodes, randomly distributed in a fixed area (800m x 800m)

MAC layer: IEEE 802.11 with no retransmissions - messages sent in broadcast

Propagation model : Shadowing (probabilistic)

Metrics Message Loss

End-to-end percentage of data packets created by source that were correctly received by sink

Event Delivery Ratio Percentage of packets that could have been

received that actually were received (at each node)

Delay (end-to-end) Overhead

Percentage of packets received (at each node) that were control packets

Message Loss vs. Density

Event Delivery vs. Density

Delay vs. Density

Overhead vs. Density

Message Loss vs. Density

Event Delivery vs. Density

Delay vs. Density

Overhead vs. Density

Conclusions Our experiments (2Mbps), Previous

experiments (13Kbps) Under-utilized channel Small vulnerable Period

Propagation effects examined better than collision effects

ASCENT performs well As # nodes increases (scalability) As sending frequency increases our

overhead remains low