1 medium access control in sensor networks huaming li electrical and computer engineering michigan...
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Medium Access Control in Medium Access Control in Sensor NetworksSensor Networks
Huaming Li
Electrical and Computer Engineering
Michigan Technological University
Computer Engineering Seminar
Department of ECE
OutlineOutline
Overview S-MAC: an energy-efficient MAC protocol for
wireless sensor networks Other MAC Techniques References
Computer Engineering Seminar
Department of ECE
Medium Access Control in Sensor NetworksMedium Access Control in Sensor Networks
Sensor networks Consist of a set of sensor nodes; Each node is equipped with one or more sensors and is
normally battery operated; Nodes communicate with each other via wireless
connection. Medium Access Control (MAC)
Fundamental task is to avoid collisions so that two interfering nodes do not transmit at the same time
Computer Engineering Seminar
Department of ECE
Characteristics of Sensor NetworkCharacteristics of Sensor Network
A special wireless ad hoc network Large number of nodes Battery powered Topology and density change Nodes for a common task In-network data processing
Sensor-net applications Sensor-triggered bursty traffic Can often tolerate some delay
Speed of a moving object places a bound on network reaction time
Energy efficiency
Scalability & Self-configuration
Fairness not importantMsg-level
Latency
Trade for energy
Adaptivity
Adaptivity
Computer Engineering Seminar
Department of ECE
MAC Protocols ClassificationMAC Protocols Classification
Scheduling-Based MAC Protocols Contention-Based MAC Collision Free Real Time MAC Hybrid MAC
Computer Engineering Seminar
Department of ECE
Scheduling Based MACScheduling Based MAC
Time is divided into slots Each node knows when to transmit Schedule is predetermined TDMA Synchronization problems Adaptability problems
Computer Engineering Seminar
Department of ECE
Contention Based MACContention Based MAC
Carrier sensing & collision avoidance In-band, out-band handshaking Busy-tone multiple access (BTMA) Multiple access with collision avoidance
(MACA) High priority packets
Computer Engineering Seminar
Department of ECE
Common MAC Protocol RequirementsCommon MAC Protocol Requirements
Quality of service (QoS) Tolerate message loss Support real time guarantees
Decentralized Global information may not be available
Flexibility Diversity of applications
Computer Engineering Seminar
Department of ECE
MAC Requirements in Sensor NetworksMAC Requirements in Sensor Networks
Important requirements of MAC protocols Collision avoidance Energy efficiency Scalability & Adaptivity Latency Fairness Throughput Bandwidth utilization
Primary
Secondary
Computer Engineering Seminar
Department of ECE
Energy Efficiency in MAC DesignEnergy Efficiency in MAC Design
Energy is primary concern What causes energy waste on radio?
Long idle time Control packet overhead Overhearing unnecessary traffic Collisions
bursty traffic in sensor-net apps Idle listening consumes 50—100% of the power for receiving
(Stemm97, Kasten)
Dominant in sensor networks
Computer Engineering Seminar
Department of ECE
S-MAC Design OverviewS-MAC Design Overview
Tradeoffs
Major components in S-MAC• Periodic listen and sleep• Collision avoidance• Overhearing avoidance• Massage passing
Latency
FairnessEnergy
Computer Engineering Seminar
Department of ECE
Periodic Listen and SleepPeriodic Listen and Sleep
Reduce long idle time Reduce duty cycle to ~ 10% (120ms on/1.2s off)
Schedules can differ
Latency Energy
Node 1 sleeplisten listen sleep
Node 2 sleeplisten listen sleep
Prefer neighboring nodes have same schedule— easy broadcast & low control overhead
Computer Engineering Seminar
Department of ECE
Periodic Listen & SleepPeriodic Listen & Sleep
Nodes are in idle for a long time if no sensing event happens
Put nodes into periodic sleep mode i.e. in each second, sleep for half second and listen
for other half second
Computer Engineering Seminar
Department of ECE
Schedule 2Schedule 1
Coordinated SleepingCoordinated Sleeping
Nodes coordinate on sleep schedules Nodes periodically broadcast schedules New node tries to follow an existing schedule
Nodes on border of two schedules follow both Periodic neighbor discovery
Keep awake in a full sync interval over long time
12
Computer Engineering Seminar
Department of ECE
Choose & Maintain ScheduleChoose & Maintain Schedule
Each node maintains a schedule table that stores schedules of all its neighbors
Nodes exchange schedules by broadcasting them to its neighbors Try to synchronize neighboring nodes together
Computer Engineering Seminar
Department of ECE
Choose ScheduleChoose Schedule
If not hear a schedule from others, the node randomly chooses a schedule and broadcast the schedule
If receive a schedule, the node follows that schedule, wait for a random delay then rebroadcast this schedule
If receive a different schedule, the node adopt both, broadcast its own schedule
Computer Engineering Seminar
Department of ECE
Maintain SynchronizationMaintain Synchronization
Listen/sleep scheme requires synchronization among neighboring nodes
Looser synchronization (compared to TDMA) Listen period is significantly longer than clock error
or drift Use relative time rather than absolute Update schedule by sending SYNC packets
Computer Engineering Seminar
Department of ECE
Maintain Sync (contd.)Maintain Sync (contd.)
Divide listen time into two parts: For receiving SYNC packets For receiving data packets
Each part is further divided into many time slots for senders to perform carrier sense
Computer Engineering Seminar
Department of ECE
Maintain Sync (contd.)Maintain Sync (contd.)
CS: carrier sense
Computer Engineering Seminar
Department of ECE
Collision AvoidanceCollision Avoidance
Adopt IEEE 802.11 collision avoidance Virtual carrier sense
During field Network allocation vector (NAV)
Physical carrier sense RTS/CTS exchange (for hidden terminal problem)
Broadcast packets (SYNC) are sent without RTS/CTS Unicast packets (DATA) are sent with RTS/CTS
Computer Engineering Seminar
Department of ECE
Overhearing AvoidanceOverhearing Avoidance
Problem: Receive packets destined to others Solution: Sleep when neighbors talk
Basic idea from PAMAS (Singh, Raghavendra 1998) But we only use in-channel signaling
Who should sleep?• All immediate neighbors of sender and
receiver
How long to sleep?• The duration field in each packet informs
other nodes the sleep interval
Computer Engineering Seminar
Department of ECE
ExampleExample
Who should sleep when node A is transmitting to B?
All immediate neighbors of both sender & receiver should go to sleep
Computer Engineering Seminar
Department of ECE
Message PassingMessage Passing
How to efficiently transmit a long message? Single packet vs. fragmentations
Single packet: high cost of retransmission if only a few bits have been corrupted
Fragmentations: large control overhead (RTS & CTS for each fragment), longer delay
Problem: Sensor network in-network processing requires entire message
Computer Engineering Seminar
Department of ECE
Message PassingMessage Passing
Solution: Don’t interleave different messages Long message is fragmented & sent in burst RTS/CTS reserve medium for entire message Fragment-level error recovery — ACK
— extend Tx time and re-transmit immediately Other nodes sleep for whole message time
FairnessEnergy
Msg-level latency
Computer Engineering Seminar
Department of ECE
Implementation on Testbed NodesImplementation on Testbed Nodes
Configurable S-MAC optionsLow duty cycle with adaptive listenLow duty cycle without adaptive listenFully active mode (no periodic sleeping)
PlatformMica Motes (UC Berkeley)
8-bit CPU at 4MHz,128KB flash, 4KB RAM20Kbps radio at 433MHz
TinyOS: event-driven
Computer Engineering Seminar
Department of ECE
Implementation on Testbed NodesImplementation on Testbed Nodes
Layered model on Motes MAC layer: S-MAC Physical layer
Radio state control, Carrier sense CRC checking, Channel coding, Byte buffering
Nested headers Avoid memory copy across layers
ApplicationTransportRouting
MAC/LinkPhysical
Computer Engineering Seminar
Department of ECE
Test BedTest Bed
Three test MAC modules Simplified IEEE 802.11 DCF Message passing with overhearing avoidance Complete S-MAC
Topology in experiments
Source 1
Source 2
Sink 1
Sink 2
Computer Engineering Seminar
Department of ECE
Experiment ResultExperiment Result
Average source nodes energy consumption
• S-MAC consumes much less energy than 802.11-like protocol w/o sleeping
• At heavy load, overhearing avoidance is the major factor in energy savings
• At light load, periodic sleeping plays the key role
0 2 4 6 8 10
200
400
600
800
1000
1200
1400
1600
1800Average energy consumption in the source nodes
Message inter-arrival period (second)
Ene
rgy
cons
umpt
ion
(mJ)
802.11-like protocolwithout sleep
Overhearing avoidance
S-MAC w/o adaptive listen
Computer Engineering Seminar
Department of ECE
Experiment Result (contd.)Experiment Result (contd.)
Percentage of time source nodes in sleep
Computer Engineering Seminar
Department of ECE
Experiment Result (contd.)Experiment Result (contd.)
Energy consumption in the intermediate node
Computer Engineering Seminar
Department of ECE
S-MAC ConclusionsS-MAC Conclusions
Advantages: Periodically sleep reduces energy consumption in
idle listening Sleep during transmissions of other nodes Message passing reduces contention latency and
control packet overhead Disadvantages:
Reduction in both per-node fairness & latency
Computer Engineering Seminar
Department of ECE
Other MAC TechniquesOther MAC Techniques
Timeout-MAC (T-MAC) S-MAC has fixed duty cycle and not optimal Reduce idle listening by transmitting data in
bursts Sleep in between bursts to save power End the active time in an intuitive way Timeout on hearing nothing
Computer Engineering Seminar
Department of ECE
T-MACT-MAC
Every node periodically wakes up and communicates with its neighbors
A node will keep listening and potentially transmitting, as long as it is in active period
An active period ends when no activation event has occurred for time TA
Computer Engineering Seminar
Department of ECE
Activation eventActivation event
The firing of periodic timer The reception of any data on radio The sensing of communication on the radio The end of transmission of a node’s own data
packet The knowledge through prior RTS and CTS
packets
Computer Engineering Seminar
Department of ECE
T-MACT-MAC
A node will sleep if it is not in an active period TA determines the minimum amount of idle
listening per frame All communication occurs as a burst in the
beginning of the frame Buffer capacity determines the upper bound on
the maximum frame time
Computer Engineering Seminar
Department of ECE
EvolutionEvolution
CSMA/CD CSMA/CA SMAC TMAC
IEEE 802.11
Carrier Sense Multiple Access with Collision
Detection
IEEE 802.3
Carrier Sense Multiple Access with Collision
Avoidance
Fixed duty cycle
Adaptive duty cycle
ARC
Adaptive Rate
Control
DMAC/MMAC
Directional
Antennas
Computer Engineering Seminar
Department of ECE
Performance Analysis of 802.15.4 in WPANPerformance Analysis of 802.15.4 in WPANPerformance Analysis of 802.15.4 in WPANPerformance Analysis of 802.15.4 in WPAN
One promising kind of sensor network: Wireless Personal Area Network (WPAN)
Medical sensing and control
Wearable computing
Location awareness and identification
Implanted medical sensors (Focus)
Coronary care
Diabetes
Optical aids
Drug delivery
Computer Engineering Seminar
Department of ECE
Critical Metric : Battery LifeCritical Metric : Battery LifeCritical Metric : Battery LifeCritical Metric : Battery Life
Implanted medical sensors (Main concern)
Objective
Make Batteries work 10-15 years
Method
Ensure that all sensors are powered down or in sleep mode when not in active use
Tradeoff
Battery life VS. latency
Computer Engineering Seminar
Department of ECE
Protocol OptionsProtocol OptionsProtocol OptionsProtocol Options
Market Name
Standard
GPRS/GSM
1xRTT/CDMA
Wi-Fi™
802.11b
Bluetooth™
802.15.1
ZigBee™
802.15.4
Application Focus
Wide Area Voice & Data
Web, Email, Video
Cable Replacement
Monitoring & Control
System Resources
16MB+ 1MB+ 250KB+ 4KB - 32KB
Battery Life (days)
1-7 .5 - 5 1 - 7 100 - 1,000+
Network Size 1 32 7 255 / 65,000
Bandwidth (KB/s)
64 - 128+ 11,000+ 720 20 - 250
Transmission Range (meters)
1,000+ 1 - 100 1 - 10+ 1 - 100+
Success MetricsReach, Quality
Speed, Flexibility
Cost, Convenience
Reliability, Power, Cost
Possible options
Computer Engineering Seminar
Department of ECE
802.15.4 (LR-WPAN) 802.15.4 (LR-WPAN) 802.15.4 (LR-WPAN) 802.15.4 (LR-WPAN)
IEEE 802.15.4 MAC
Upper Layers
IEEE 802.2 LLC Other LLC
IEEE 802.15.4
2400 MHz
PHY
IEEE 802.15.4
868/915 MHz
PHY
Physical Medium
Computer Engineering Seminar
Department of ECE
802.15.4 (LR-WPAN) 802.15.4 (LR-WPAN) 802.15.4 (LR-WPAN) 802.15.4 (LR-WPAN) MAC Layer (prefer star topology)
Why star topology here?
Computer Engineering Seminar
Department of ECE
802.15.4 (LR-WPAN) 802.15.4 (LR-WPAN) 802.15.4 (LR-WPAN) 802.15.4 (LR-WPAN)
Coordinator is external to the body
PDA, mobile phone or bedside monitor station
Easy to replace of charge batteries
Easy to communicate with other networks
Coordinator defines the start and end of a superframe and is charge of the association and disassociation of the other nodes
Why star topology here?
Computer Engineering Seminar
Department of ECE
IEEE 802.15.4 superframe structure IEEE 802.15.4 superframe structure
Computer Engineering Seminar
Department of ECE
Two Communication methods Two Communication methods
Beacon mode
Pros: Coordinator can communicate at will
Cons: Listeners have to keep awake
Non-beacon mode
Pros: Nodes can sleep more
Cons: Communication latency
Computer Engineering Seminar
Department of ECE
Network scenarios and power analysisNetwork scenarios and power analysisNetwork scenarios and power analysisNetwork scenarios and power analysis
Sensor power consumption with beacon reception
Problem: The sensor devices within a beacon network have to wake up to receive the beacon from the coordinator (Power consuming)
Timebase Tolerances
Warm-up time
Computer Engineering Seminar
Department of ECE
Network scenarios and power analysisNetwork scenarios and power analysisNetwork scenarios and power analysisNetwork scenarios and power analysisData Transfer Mechanisms (Beacon)
Data transfer to a coordinator (upload)
Is the upload period
Computer Engineering Seminar
Department of ECE
Network scenarios and power analysisNetwork scenarios and power analysisNetwork scenarios and power analysisNetwork scenarios and power analysis
Data Transfer Mechanisms (Beacon)
Data transfer from a coordinator (download)
Is the download period
Computer Engineering Seminar
Department of ECE
ResultsResultsResultsResults
Average Back-off
With a small number of sensors that are effectively off most of the time, the probability of a channel being free is greater than 99 %. Therefore, for the relatively small number of sensors used in the WBAN networks explored here, it would be more economical to keep the CSMA/CA switched off.
This is to ensure that the automatic initial back-off is avoided.
Computer Engineering Seminar
Department of ECE
Node Lifetime in Beacon NetworksNode Lifetime in Beacon Networks
Computer Engineering Seminar
Department of ECE
Node Lifetime in Beacon NetworksNode Lifetime in Beacon Networks
Computer Engineering Seminar
Department of ECE
Node Lifetime in Beacon NetworksNode Lifetime in Beacon Networks
15-year lifetime may only be obtained for very low upload rates.
It is under very limited data rate conditions and a tight tolerance crystal, which typically must be better than 25 ppm.
Computer Engineering Seminar
Department of ECE
GTS OptionGTS Option
The main drawback of using
GTS is that the receiver in the
sensor remains on for the duration of
the timeslot regardless of the size of the data
packet.
Computer Engineering Seminar
Department of ECE
ConclusionConclusionConclusionConclusion
As a solution to the challenge of the personal area network, the IEEE 802.15.4 standard would provide a limited answer in its non-beacon form.
Sensors that do not have large amounts of data to transfer could be used, i.e., small packets of data several times per hour.
Computer Engineering Seminar
Department of ECE
Questions and CommentsQuestions and CommentsQuestions and CommentsQuestions and Comments