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1
MAC Layer Design for Wireless Sensor Networks
Wei YeUSC Information Sciences Institute
2
Introduction
Wireless sensor network Large number of densely distributed nodes Battery powered Multi-hop ad hoc wireless network Node positions and topology dynamically
change Self-organization
Sensor-net applications Nodes cooperate for a common task In-network data processing
3
Introduction
Important attributes of MAC protocols Collision avoidance
Basic task — medium access control Energy efficiency Scalability and adaptivity
Number of nodes changes overtime Latency Fairness Throughput Bandwidth utilization
4
Contention-based protocols CSMA — Carrier Sense Multiple Access
EthernetNot enough for wireless (collision at receiver)
MACA — Multiple Access w/ Collision AvoidanceRTS/CTS for hidden terminal problemRTS/CTS/DATA
Overview of MAC protocols
A B C
Hidden terminal: A is hidden from C’s CS
5
Overview of MAC Protocols
Contention-based protocols (contd.) MACAW — improved over MACA
RTS/CTS/DATA/ACKFast error recovery at link layer
IEEE 802.11 Distributed Coordination FunctionLargely based on MACAW
Protocols from voice communication area TDMA — low duty cycle, energy efficient FDMA — each channel has different frequency CDMA — frequency hopping or direct
sequence
6
Example: IEEE 802.11 DCF
Distributed coordinate function: ad hoc mode Virtual and physical carrier sense (CS)
Network allocation vector (NAV), duration field Binary exponential backoff RTS/CTS/DATA/ACK for unicast packets Broadcast packets are directly sent after CS Fragmentation support
RTS/CTS reserve time for first (frag + ACK)First (frag + ACK) reserve time for second…Give up tx when error happens
7
Example: IEEE 802.11 DCF
Timing relationship
8
Energy Efficiency in MAC Design
Energy is primary concern in sensor networks
What causes energy waste? Collisions Control packet overhead Overhearing unnecessary traffic Long idle time
bursty traffic in sensor-net appsIdle listening consumes 50—100% of the
power for receiving (Stemm97, Kasten)
Dominant in sensor nets
9
Energy Efficiency in MAC Design
TDMA vs. contention-based protocols TDMA can easily avoid or reduce energy
waste from all above sources Contention protocols needs to work hard in
all directions TDMA has limited scalability and adaptivity
Hard to dynamically change frame size or slot assignment when new nodes join
Restrict direct communication within a cluster Contention protocols easily accommodate
node changes and support multi-hop communications
10
TDMA Protocols
Bluetooth Clustering (piconet) FH-CDMA between clusters TDMA within each cluster
Centralized control by cluster headOnly master-slave communicationMaster polling slave and schedule tx
At most 8 active nodes can be in a cluster — scalability problem
11
TDMA Protocols
LEACH: Low-Energy Adaptive Clustering Hierarchy — by Heinzelman, et al. Similar to Bluetooth CDMA between clusters TDMA within each cluster
Static TDMA frameCluster head rotationNode only talks to cluster headOnly cluster head talks to base station (long
dist.) The same scalability problem
12
Self-Organaiztion — by Sohrabi and Pottie Have a pool of independent channels
Frequency band or spreading codePotential interfering links select different
channels Talk to each neighbor in different time slots Sleep during unscheduled time Example
6 different channelsEach node has its own scheduled time slots —
superframe
TDMA Protocols
1 3
42
13
TDMA Protocols
Self-Organaiztion (Contd.) Result (if a node has n neighbors)
Uses n different channels to talk to each of them
Schedules n time slots to send to each of them and n time slots to receive from each of them
Looks like TDMA, but actually FDMA or CDMAAny pair of two nodes can talk at the same
time Low bandwidth utilization
14
Contention-Based Protocols
PAMAS: Power Aware Multi-Access with Signalling — by Singh and Raghavendra Improve energy efficiency from MACA Avoid overhearing by putting node into sleep Control channel separates from data channel
RTS, CTS, busy tone to avoid collisionProbe packets to find neighbors transmission
time Increased hardware complexity
Two channels need to work simultaneously, meaning two radio systems.
15
Contention-Based Protocols
Piconet — by Bennett, Clarke, et al. Low duty-cycle operation — energy efficient
Sleep for 30s, beacon, and listen for a whileSending node needs to listen for receiver’s
beacon first, thenCarrier sense before sending data
May wait for long time before sendingTx rate control — by Woo and Culler
Carrier sense + adaptive rate control Promote fair bandwidth allocation Helps for congestion avoidance
16
Contention-Based Protocols
Power save (PS) mode in IEEE 802.11 DCF Assumption: all nodes are synchronized and can
hear each other (single hop) Nodes in PS mode periodically listen for beacons
& ATIMs (ad hoc traffic indication messages) Beacon: timing and physical layer parameters
All nodes participate in periodic beacon generation ATIM: tell nodes in PS mode to stay awake for Rx
ATIM follows a beacon sent/receivedUnicast ATIM needs acknowledgementBroadcast ATIM wakes up all nodes — no ACK
17
Contention-Based Protocols
Example of PS mode in IEEE 802.11 DCF
18
Energy Efficiency: Contention
Extend 802.11 PS mode for Multi-hops— By Tseng, et al. at IEEE Infocom 2002 Nodes do not synchronize with each other Designed 3 sleep patterns — ensure nodes
listen intervals overlap, example:Periodically fully-awake interval: similar to S-
MAC
Problem on broadcast — wake up each neighbor
19
S-MAC: Introduction
S-MAC — by Ye, Heidemann and EstrinTradeoffs
Major components in S-MAC• Periodic listen and sleep• Collision avoidance• Overhearing avoidance• Massage passing
Latency
FairnessEnergy
20
Periodic Listen and Sleep
Problem: Idle listening consumes significant energy
Solution: Periodic listen and sleep
• Turn off radio when sleeping• Reduce duty cycle to ~ 10% (150ms
on/1.5s off)
sleeplisten listen sleep
Latency Energy
21
Periodic Listen and Sleep
Schedules can differ
• Prefer neighboring nodes have same schedule— easy broadcast & low control overhead
Border nodes: two schedules broadcast twice
Node 1
Node 2
sleeplisten listen sleep
sleeplisten listen sleep
Schedule 2
Schedule 1
22
Periodic Listen and Sleep
Schedule Synchronization New node tries to follow an existing
schedule Remember neighbors’ schedules
— to know when to send to them Each node broadcasts its schedule every
few periods of sleeping and listening Re-sync when receiving a schedule update
Periodic neighbor discovery Keep awake in a full sync interval over long
periods
23
Collision Avoidance
Problem: Multiple senders want to talkOptions: Contention vs. TDMASolution: Similar to IEEE 802.11 ad hoc
mode (DCF) Physical and virtual carrier sense Randomized backoff time RTS/CTS for hidden terminal problem RTS/CTS/DATA/ACK sequence
24
Overhearing Avoidance
Problem: Receive packets destined to othersSolution: 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
25
Message Passing
Problem: Sensor net in-network processing requires entire message
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 immediatelyOther nodes sleep for whole message time
FairnessEnergy
Msg-level latency
26
Msg Passing vs. 802.11 fragmentation
S-MAC message passing
RTS 21 ......
Data 19ACK 18CTS 20
Data 17ACK 16
Data 1ACK 0
RTS 3 ......
Data 3ACK 2CTS 2
Data 3ACK 2
Data 1ACK 0
Fragmentation in IEEE 802.11 • No indication of entire time — other nodes keep
listening• If ACK is not received, give up Tx — fairness
27
Implementation on Testbed Nodes
Compared MAC modules1. IEEE 802.11-like protocol w/o sleeping2. Message passing with overhearing
avoidance3. S-MAC (2 + periodic listen/sleep)
PlatformMica Motes (UC Berkeley)
8-bit CPU at 4MHz,128KB flash, 4KB RAM433MHz radio
TinyOS: event-driven
28
Experiments: two-hop network
Topology and measured energy consumption on source nodes
Source 1
Source 2
Sink 1
Sink 2
• Each source node sends 10 messages
— Each message has 400B in 10 fragments
• Measure total energy over time to send all messages
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 protocol w/o sleep Overhearing avoidanceS-MAC
29
Recent Progress: Adaptive Listen
Reduces latency due to periodic sleepAt end of each transmission, nodes wake
up for a short timeNext transmission can start right awayExample: data flow ABC
A sends RTS, B replies CTS, C overhears CTS C will wake up when AB is done BC can start right away
A B C
30
Recent Progress: 10-hop Experiments
Compare S-MAC in 3 different modes10-hop linear network with 11 nodes
0 2 4 6 8 100
5
10
15
20
25
30Energy consumption on radios in the entire network
Message inter-arrival period (S)
En
erg
y co
nsu
mp
tion
(J)
10% duty cycle without adaptive listen10% duty cycle with adaptive listen No periodic sleep
0 2 4 6 8 100
2
4
6
8
10
12Average message latency under the lowest traffic load
Number of hops
La
ten
cy (
S)
10% duty cycle without adaptive listen10% duty cycle with adaptive listen No periodic sleep
31
S-MAC Information
URL: http://www.isi.edu/scadds/Released S-MAC source code (for
TinyOS 0.6.1)Currently porting to nesC environment
(TinyOS 1.0)