Etiquette protocol for Ultra Low Power Operation in Sensor Networks

Transcription

1 Etiquette protocol for Ultra Low Power Operation in Sensor Networks Samir Goel and Tomasz Imielinski {gsamir, DataMan Lab, Department of Computer Science Acknowledgement: Prof. Roy Yates helped in shaping some of the core ideas

2 Problem Sensor nodes operate on battery with limited energy (Life: 48 hrs with Energizer CR2450 battery[hill00]) Numerous applications require network lifetime of months/years Natural habitat monitoring (Great Duck Island Project, Redwood Forest Project) Structural monitoring (Golden Gate Bridge) Emergency networks (detecting forest fires) Intrusion detection, etc Lifetime of sensor nodes determine the cost of operating the network How does one extend the life of sensor nodes?

3 Target applications: Characteristics Channel bandwidth much higher than applications needs Motes: Evolution:: RENE Motes (2000) 10 Kbps (typical) MICA Motes (2002) 19.2 Kbps (effective) MICAz Motes (2004) 250 Kbps Example: In Great Duck Island project, sensor nodes were configured to transmit once every 5 mins It would take motes to produce an average load of 19.2 kbps (pkt size = 30 bytes) Tolerance for higher delay (100s msecs) Example: In Great Duck Island project, tolerance of a few seconds per-hop communication delay would be acceptable Requires multi-hop sensor network

4 Problem: Idle Listening Radio is the major consumer of energy in a sensor node Mica Motes [Ye02]: Transmit Power : W Receive Power : W Idle listening (radio is ON but IDLE) : W Example: Mote transmitting 1 pkt/min: Idle listening: 99.94% Pkt Size: 50 bytes, b/w: 19.2 Kbps Radio Always ON % energy spent in idle listening Energy consumed in idle listening dominates all other costs # transmissions/minute

5 Related Work IEEE like protocol Power Saving Mode only works for one-hop network IEEE Defines PHY and MAC layer for low rate Personal Area Network Power saving mode not defined for peer-to-peer communication topology S-MAC (Ye02) Active Sleep Day and night protocol: Synchronized Active and Sleep states Fixed duty cycle (solved in T-MAC (Dam03)) Synchronization messages constitute significant overhead Traffic concentrated in the active part Active Sleep Active Sleep

6 Etiquette Protocol

7 Basic Idea:: Teaching Assistant Analogy Office hr Office hr Office Hour Announcement Time Nodes hold office hours at regular intervals Accept request for appointments from neighbors Nodes select their office hours independently

8 Basic Idea :: Teaching Assistant Analogy, cont d Appointment Office hr Appt Request Grant-Request Time Communicating data: During office hours, neighbor requests an appointment Node responds with Grant-request (or, Deny-request) Perform communication at appointed time For a receiver, radio needs to be ON only during office hours and appointment

9 How to determine Office Hours of a node? Office hours Period (T off-period ) Office hr duration Office Hour Announcement Time Network property (invariant):: T off-period T off-max (T off-max is a pre-defined constant) Office Hour Announcement specifies: Duration of office hours Period Determining office hours involves scanning the channel for at most a known maximum period, T off-max

10 Nodes holding office hours Node A Office hr Office Hour Announcement Node B Time

11 Interaction Found office hours of B Node A Channel Scan Office hr Has packet for B Node B Time

12 Interaction Send Appt Request Node A Channel Scan Appointment Office hr Has packet for B Node B Time Send Grant-Request

13 Why do we expect to save energy? Relative size of various overheads: (Office hour period: in secs) Office hour duration : in 100s of msecs Appointment request Pkt : in 10s of msecs Appointment grant Pkt : in 10s of msecs Small fraction of Office hour period Radio can be turned OFF rest of the time Reduction in idle listening more than compensates for the overhead

14 Why do we expect to save energy? Relative size of various overheads: (Office hour period: in secs) Office hour duration : in 100s of msecs Appointment request Pkt : in 10s of msecs Appointment grant Pkt : in 10s of msecs Small fraction of Office hour period Channel Scanning Time (T scan ) : E[T scan ] = T off-period /2 Significant energy drain

15 How to reduce energy consumed in scanning? Node sends blurbs at regular intervals Blurb: Small message indicating the next start time for office hours

16 Using blurbs to reduce channel scanning time Node A Channel Scan Found office hours of B Office hr Has packet for B Node B Blurb Time

17 Node A Using blurbs to reduce channel scanning time Channel Scan Send Appt Request Appointment Office hr Has packet for B Node B Blurb Time Send Grant-Request

18 Scanning Cost: Analysis Cost of transmitting a blurb is very small Radio time of ~10 msecs Savings: In general, n blurbs can reduce E[T scan ] by (n+1) As n becomes large, the cost of transmitting blurbs may out-weigh the savings Optimal n : KToff J recv n* = 1 2J blurb We show that a node can estimate n* using local info

19 Important Characteristics Energy versus latency tradeoff Energy 1/office hour period Per-hop latency office hour period Etiquette: A Link-layer protocol: Controls radio ON/OFF schedule at macro-level MAC layer still handles the micro-level issues (e.g., channel contention) Onus of communication is on the sender Transport Network Etiquette MAC PHY Energy consumption proportional to number of packet transmissions Fine clock synchronization not required All times are relative Guard-bands are used to take care of clock-drifts Typical clock drift: ±0.2 msecs per sec [Ye02]

20 Performance Analysis

21 Simulation Setup Protocols Compared: S-MAC: duty cycle: 5% to 50% Etiquette: office hour period: 10 sec and 20 sec IDEAL : IEEE DCF with cost of idle-listening set to 0 Scenario: Homogeneous Traffic [Rajendran03, Dam03]:: A node sends each packet to a randomly selected neighbor Network Topology [Rajendran03, Dam03] 50 node network Dense (avg node degree: 6, avg # two-hop neighbors: 17)

22 Average Packet delivery ratio S-MAC: duty cycle max packet load Etiquette Performance for two different office hour periods is identical Graceful performance degradation At low load, packet delivery ratio suffers because of higher traffic

23 Energy Efficiency Graph truncated when corresponding pkt delivery ratio falls below 70% Characteristics: Etiquette performs much better than S- MAC at low load Energy/bit reduces with increase in office hours period

24 Average Queuing Delay Etiquette: Queuing delay in Etiquette stays constant Function of office hour period S-MAC: Queuing delay skyrockets when capacity is reached

25 Etiquette: Recap Simple, intuitive protocol Energy consumption proportional to the number of packet transmissions Ability to handle fluctuations in traffic load Allows trading latency for energy

26 Comments/Questions? More info:

27 Additional Slides

28 S-MAC Synchronized listen/sleep state Node A Listen Sleep Listen Sleep Listen Node B Listen Sleep Listen Sleep Listen Listen is split into two parts: SYNC, RTS/CTS Listen Receiver Sender 1 Sender 2 for SYNC for RTS for CTS Tx SYNC CS Tx RTS Got CTS CS Sleep Sleep Send data

29 T-MAC Improves S-MAC Adaptive duty cycle: (L fixed at 600 msecs) Active Active Active L Sleep Sleep TA TA TA A node is in active mode until no activation event occurs for time TA Periodic frame timer event, receive, carrier sense, send-done, knowledge about end of other transmissions Communication: S-MAC/ like Frame schedule maintenance: S-MAC-like

30 Low Power Listening [Hill02] Sleep Sleep Sleep Sleep Cordless-phone protocol: Receiver wakes up every few milliseconds to check for preamble Every transmission wakes up all neighbors Inefficient in dense networks

31 Can we increase latency in S-MAC? c L Listen Sleep Listen Sleep Listen Listen for SYNC for RTS for CTS Sleep Given a certain duty cycle, d: L is adjusted such that c/l = d c is kept constant (~68 msecs) For the same duty cycle, L cannot be changed

32 What happens when latency in Etiquette is reduced? Time Toff period Office hours Period ( ) Office hr ( ) T dur min T dur : minimum duration of office hours required to process at least one appointment request Function of channel bandwidth Toff period T min Tdur off period As reduces, the ratio increases

33 Average radio ON time Reason for bad performance: Traffic concentration in S-MAC Overhead of synchronization messages

34 Breakdown of Transmitter Energy Cost MAC-layer feedback reduces the cost of node-appt

35 Breakdown of Receiver Energy Cost