Sensor and energy-efficient networking

Oregon Graduate Institute 1

Sensor and energy-efficient networking

CSE 525: Advanced NetworkingComputer Science and Engineering Department

Winter 2004


Energy efficient MAC

An energy-efficient MAC protocol for wireless sensor networks by W. Ye, J. Heidemann, and D. Estrin

An Energy Efficient MAC Protocol for Wireless LANs by E. Jung and N. Vaidya

Energy efficient communications in ad hoc networks using directional antennas by A. Spyropoulos and C. Raghavendra


Motivation

Challenged Networks are normally battery operated, hence power limited

Other goals: Self-configuration good scalability collision avoidance

Fairness and latency are NOT important


Conventional MAC Layer An access mechanism for nodes that

ensure that NO two nodes have access to the communication channel concurrently

If air is clear, send; for receiving, wait

How do we know when to receive? Listen always!

Usually designed to allow for maximum throughput

Hence not energy-efficient


Sources of energy inefficiency

Collision Overhearing Control packet overhead Idle listening


#1: S(sensor)-MAC It is contention based Tries to reduce wastage of energy from

all four sources of energy inefficiency Collision – by using RTS and CTS Overhearing – by switching the radio off

when the transmission is not meant for that node

Control overhead – by Message Passing Idle listening – by Periodic Sleep


There is no FREE dinner!

In exchange there is some reduction in both per-hop fairness and latency

But this does not necessarily result in lower end-to-end application fairness and latency – Which acceptable in Challenged Network


Periodic Listen and Sleep

Each node go into periodic sleep mode during which it switches the radio off and sets a timer to awake later

To reduce control overhead, neighboring nodes are synchronized (i.e. Listen and sleep together)


Periodic Listen and Sleep Broadcasting schedule to all its

immediate neighbor neighbor can have different schedules.

If multiple neighbor want to talk to a node, use 802.11 (RTS/CTS) like contention scheme

After they start data transmission, they do not follow their sleep schedules until they finish transmission.


Choosing and Maintaining Schedules

Each node maintains a schedule table that stores schedules of all its known neighbors.

To establish the initial schedule (at the startup) following steps are followed: A node first listens for a certain amount of

time. If it does not hear a schedule from another

node, it randomly chooses a schedule and broadcast its schedule immediately.

This node is called a SYNCHRONIZER.


If a node receives a schedule from a neighbor before choosing its own schedule, it just follows this neighbor’s schedule.

This node is called a FOLLOWER and it waits for a random delay and broadcasts its schedule.

Choosing and Maintaining Schedules


Border Nodes

If a node receives a different schedule after it selects and broadcasts its own schedule, it adopts both schedules

Border nodes consume more energy


Maintaining Synchronization

Timer synchronization among neighbors are needed to prevent the clock drift.

Synchronizer needs to periodically send SYNC to its followers.

If a follower has a neighbor that has a different schedule with it, it also needs update that neighbor.


Maintaining Synchronization

Time of next sleep is relative to the moment that the sender finishes transmitting the SYNC packet

Receivers will adjust their timer counters immediately after they receive the SYNC packet

Listen interval is divided into two parts: one for receiving SYNC and other for receiving RTS


Timing Relationship of Possible Situations


Collision/Overhearing Avoidance Collision Avoidance: 802.11

RTS/CTS NAV: virtual carrier sense Physical carrier sense

Overhearing Avoidance Go to sleep if overhear RTS/CTS packet

NAV Avoid overhearing “long” data packet

All immediate neighbors of both sender and receiver go to sleep


Message Passing Short packets are used in wireless networks

More robust Overhead of control packets (RTS/CTS) In-network processing requires a complete msg

Divide the long message into small fragments and transmit them in a burst. RTS/CTS/Data1/Ack1/Data2/Ack2/…/DataN/AckN If a packet is lost, extend the duration and

immediately retransmit it Data and Acknowledge include a field of duration


Energy Savings vs. Increased Latency

Delay includes: Carrier sense Back off Transmission Propagation Processing Queuing Sleeping


Conclusions and Future work

S-MAC has good energy conserving properties comparing to IEEE 802.11

Future work Analytical study on the energy

consumption and latency Analyze the effect of topology

changes


#2: EEM for WLAN Optimize PSM in the DCF in IEEE 802.11

Standard Dynamic PSM (DPSM)

What is PSM? PSM for DCF, divide time into intervals called

beacon intervals, each node in power save mode periodically wakes up at the beginning of beacon interval for a duration called ATIM [Ad-hoc Traffic Indication Message] window to exchange control information.


What’s wrong?

Fixed ATIM window does not perform good in all situation Adaptive mechanism to dynamically

adjust ATIM window Synchronization of beacon interval

of initially partitioned network Not addressed; it assumes, there is a

way to synchronize


Dynamic - PSM Each node chose its own ATIM window

size based on network traffic condition Allow to increase and decrease the

ATIM window dynamically; ATIMmin is defined as minimum level

Move into doze state, after completing packet transmission, if remaining time is not “too-small” – Save Energy


Dynamic - PSM Piggyback own window size on all

transmitted packets Packet marking use to adjust ATIM window


Dynamic - PSM Rules to increase ATIM

Pending packet can’t be announce in current window time

Based on piggyback information Receiving an ATIM frame after ATIM window Received a marked packet

Rules to decrease ATIM If node successfully announce ATIM frame

and none of the above rule satisfied


Result

Simulation result shows that DPSM improves energy consumption without degrading performance Only when energy gain from doze

state is more then energy loss [overhead of beacon, ATIM and ATIM-ACK frame]

Energy save in doze mode is 96% compare to idle mode.


#3: Directional Antenna Energy efficient routing and scheduling

algorithm in ad hoc network where each node has single directional antenna.

Using topology consisting of all the possible link in the network, find shortest cost path to be energy efficient.

Calculate the amount of traffic that has to go over each link and find the maximum amount of time each link can be up.

Schedule node’s transmissions, trying to minimize the total time it takes for all possible Tx-Rx pairs to communicate with each other.


Conclusion and future work Benefits of using directional antennas in

ad hoc networks. Energy efficient algorithm for routing. Scheduling

45% improvement in network life time that is achieved by using energy-aware routing.

Future work Multicasting and broadcasting in ad hoc

networks with directional antennas Scheduling algorithm in this context


Related work TDMA Based

Naturally have a duty cycle It is not easy to change the slot assignment

dynamically, hence scalability is not as good as contention based

Requires nodes to form real communication clusters and managing inter-cluster communication is difficult

Out-of band solutions [PAMAS]: Requires extra band for signaling


Questions?

Sensor and energy-efficient networking

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