2/17/20051 Guest lecture for CS113, UCLA Medium Access Control in Wireless Sensor Networks Wei Ye...

2/17/20052/17/2005 11Guest lecture for CS113, UCLAGuest lecture for CS113, UCLA

Medium Access Control in Medium Access Control in Wireless Sensor NetworksWireless Sensor Networks

Wei YeWei Ye

USC Information Sciences InstituteUSC Information Sciences Institute

2/17/20052/17/2005 Guest lecture for CS113, UCLAGuest lecture for CS113, UCLA 22

OutlineOutline

• Introduction to MAC

• MAC attributes and trade-offs

• Scheduled MAC protocols

• Contention-based MAC protocols

• Case studies

• Summary


Introduction to MACIntroduction to MAC

• The role of medium access control (MAC)– Controls when and how each node can transmit

in the wireless channel

• Why do we need MAC?– Wireless channel is a shared medium

– Radios transmitting in the same frequency band interfere with each other – collisions

– Other shared medium examples: Ethernet


Where Is the MAC?Where Is the MAC?

• Network model from Internet

• A sublayer of the Link layer– Directly controls the radio– The MAC on each node only cares about its

neighborhood

Application layer

Transport layer

Network layer

Link/MAC layer

Physical layer

End-to-end reliability, congestion control

Routing

Per-hop reliability, flow control, multiple access

Packet transmission and reception


What’s New in Sensor Networks?What’s New in Sensor Networks?

• 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


Next…Next…





• Case studies

• Summary


Primary MAC AttributesPrimary MAC Attributes

• Collision avoidance– Basic task of a MAC protocol

• Energy efficiency– One of the most important attributes for sensor

networks, since most nodes are battery powered

• Scalability and adaptivity– Network size, node density and topology change


Other MAC AttributesOther MAC Attributes

• Channel utilization– How well is the channel used? Also called

bandwidth utilization or channel capacity

• Latency– Delay from sender to receiver; single hop or

multi-hop

• Throughput– The amount of data transferred from sender to

receiver in unit time

• Fairness– Can nodes share the channel equally?


Energy Efficiency in MAC DesignEnergy 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 apps• Idle listening consumes 50—100% of the power

for receiving (Stemm97, Kasten)

Dominant factor


Classification of MAC ProtocolsClassification of MAC Protocols

• Schedule-based protocols– Schedule nodes onto different sub-channels

– Examples: TDMA, FDMA, CDMA

• Contention-based protocols– Nodes compete in probabilistic coordination

– Examples: ALOHA (pure & slotted), CSMA


Next…Next…





• Case studies

• Summary


Scheduled Protocols: TDMAScheduled Protocols: TDMA

• Time division multiple access– Divide time into subchannels

– Advantages• No collisions• Energy efficient — easily support low duty cycles

– Disadvantages• Difficult to accommodate node changes• Requires strict time synchronization


• Master-slave configuration– The master node decides which slave can send

by polling the corresponding slave

– Only direct communication between the master and a slave

– A special TDMA without pre-assigned slots

– Examples• IEEE 802.11 infrastructure mode (CPF)• Bluetooth piconets

Scheduled Protocols: PollingScheduled Protocols: Polling


Scheduled Protocols: BluetoothScheduled Protocols: Bluetooth

• Wireless personal area network (WPAN)– Short range, moderate bandwidth, low latency– IEEE 802.15.1 (MAC + PHY) is based on

Bluetooth

• Nodes are clustered into piconet– Each piconet has a master and up to 7 active

slaves – scalability problem– The master polls each slave for transmission– CDMA among piconets– Multiple connected piconets form a scatternet

• Difficult to handle inter-cluster communications


• By Sohrabi and Pottie– Have a pool of independent channels

• Frequency band or spreading code• Potential interfering links select different channels

– Talk to neighbors in different time slots

– Sleep in unscheduled time slots

– Looks like TDMA, but actual multiple access is accomplished by FDMA or CDMA

• Any pair of two nodes can talk at the same time

– Low bandwidth utilization

Scheduled Protocols: Self-OrganizationScheduled Protocols: Self-Organization


Scheduled Protocols: LEACHScheduled Protocols: LEACH

• Low-Energy Adaptive Clustering Hierarchy — by Heinzelman, et al.– Similar to Bluetooth

– CDMA between clusters

– TDMA within each cluster• Static TDMA frame• Cluster head rotation• Node only talks to cluster head• Only cluster head talks to base station (long dist.)

– The same scalability problem


Next…Next…





• Case studies

• Summary


• ALOHA– Pure ALOHA: send when there is data

– Slotted ALOHA: send on next available slot

– Both rely on retransmission when there’s collision

• CSMA — Carrier Sense Multiple Access– Listening (carrier sense) before transmitting

– Send immediately if channel is idle

– Backoff if channel is busy• non-persistent, 1-persistent and p-persistent

Contention Protocols: ClassicsContention Protocols: Classics


• Hidden terminal problem

– CSMA is not enough for multi-hop networks (collision at receiver)

• CSMA/CA (CSMA with Collision Avoidance)– RTS/CTS handshake before send data

– Node c will backoff when it hears b’s CTS

Contention Protocols: CSMA/CAContention Protocols: CSMA/CA

a b cNode a is hidden from c’s carrier sense


Contention Protocols: MACA and MACAWContention Protocols: MACA and MACAW

• MACA — Multiple Access w/ Collision Avoidance– Based on CSMA/CA

– Add duration field in RTS/CTS informing other node about their backoff time

• MACAW– Improved over MACA

– RTS/CTS/DATA/ACK

– Fast error recovery at link layer


Contention Protocols: IEEE 802.11Contention Protocols: IEEE 802.11

• IEEE 802.11 ad hoc mode (DCF)– 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 (fragment + ACK)• First (fragment + ACK) reserve time for second…• Give up transmission when error happens


Contention Protocols: IEEE 802.11 (cont.)Contention Protocols: IEEE 802.11 (cont.)

• 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/received• Unicast ATIM needs acknowledgement• Broadcast ATIM wakes up all nodes — no ACK


Contention Protocols: IEEE 802.11 (cont.)Contention Protocols: IEEE 802.11 (cont.)

• Unicast example of PS mode in 802.11 DCF


Contention Protocols: Tx Rate ControlContention Protocols: Tx Rate Control

• By Woo and Culler– Based on a special network setup

• A base station tries to collect data equally from all sensors in the network

– CSMA + adaptive rate control

– Promote fair bandwidth allocation to all sensors• Nodes close to the base station forward more

traffic, and have less chances to send their own data

– Helps in congestion avoidance


Contention Protocols: Piconet Contention Protocols: Piconet

• By Bennett, Clarke, et al.– Not the same piconet in Bluetooth– Low duty-cycle operation — energy efficient

• Sleep for 30s, beacon, and listen for a while• Sending node needs to listen for receiver’s beacon

first, then• CSMA before sending data

– May wait for long time before sending


Contention Protocols: PAMASContention Protocols: PAMAS

• PAMAS: Power Aware Multi-Access with Signalling — by Singh and Raghavendra– Improve energy efficiency from MACA

– Avoid overhearing by putting node into sleep

– Use separate control and data channels• RTS, CTS, busy tone to avoid collision• Probe packets to find neighbors transmission time

– Increased hardware complexity• Two channels need to work simultaneously,

meaning two radio systems.


Contention Protocols: ZigBee Contention Protocols: ZigBee

• Based on IEEE 802.15.4 MAC and PHY– Three types devices

• Network Coordinator• Full Function Device (FFD)

– Can talk to any device, more computing power

• Reduced Function Device (RFD)– Can only talk to a FFD, simple for energy conservation

– CSMA/CA with optional ACKs on data packets

– Optional beacons with superframes

– Optional guaranteed time slots (GTS), which supports contention-free access


Contention Protocols: ZigBee (cont.)Contention Protocols: ZigBee (cont.)

• Low power, low rate (250kbps) radio

• MAC layer supports low duty cycle operation– Target node life time > 1 year


Next…Next…





• Case studies

• Summary


Case Study 1: S-MACCase Study 1: S-MAC

• By Ye, Heidemann and Estrin

• Tradeoffs

• Major components in S-MAC– Periodic listen and sleep

– Collision avoidance

– Overhearing avoidance

– Massage passing

Latency

FairnessEnergy


Coordinated SleepingCoordinated Sleeping

• Problem: Idle listening consumes significant energy

• Solution: Periodic listen and sleep

• Turn off radio when sleeping• Reduce duty cycle to ~ 10% (120ms

on/1.2s off)

sleeplisten listen sleep

Latency Energy



• Schedules can differ

• Prefer neighboring nodes have same schedule— easy broadcast & low control overhead

Border nodes: two schedules

or broadcast twice

Node 1

Node 2



Schedule 2

Schedule 1



• 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



• Adaptive listening– Reduce multi-hop latency due to periodic sleep

– Wake up for a short period of time at end of each transmission

41 2 3

CTS

RTS

CTS

Reduce latency by at least half

listen listenlisten

t1 t2


Collision AvoidanceCollision Avoidance

• S-MAC is based on contention

• 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


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


Message PassingMessage 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 immediately

• Other nodes sleep for whole message time

FairnessEnergy

Msg-level latency


Implementation and ExperimentsImplementation and Experiments

• Platform: Mica Motes• Topology: 10-hop

linear network

• S-MAC saved a lot of energy compared with a MAC without sleep

0 2 4 6 8 100

5

10

15

20

25

30

Message inter-arrival period (S)

En

erg

y co

nsu

mp

tion

(J)

10% duty cycle without adaptive listen

No sleep cycles

10% duty cycle with adaptive listen

Energy consumption at different traffic load


Case Study 2: B-MACCase Study 2: B-MAC

• Another low-power MAC for sensor networks

• B-MAC design considerations– Simplicity: based on simple CSMA

– Configurable options

– Minimize idle listening

– Based on model of periodic sensor data transfer

• B-MAC components– CSMA without RTS/CTS

– Optional Low-power listening (LPL)

– Optional ACK


Low-Power ListeningLow-Power Listening

• Determine channel status by quick sampling– Very low overhead on wake-up

Joe Polastre, et al., SenSys’04


Low Duty Cycle with LPLLow Duty Cycle with LPL

• Nodes periodically sleep and perform LPL

• Nodes do not synchronized on listen time

• Sender uses a long preamble before each packet to wake up the receiver

• Shift most burden to the sender


Comparison of S-MAC and B-MACComparison of S-MAC and B-MAC

S-MAC B-MAC

Collision avoidance CSMA/CA CSMA

ACK Yes Optional

Message passing Yes No

Overhearing avoidance Yes No

Listen period Pre-defined + adaptive listen Pre-defined

Listen interval Long Very short

Schedule synchronization Required Not required

Packet transmission Short preamble Long preamble

Code size 6.3KB 4.4KB (LPL & ACK)


Next…Next…





• Case studies

• Summary


MAC Design for Sensor NetworksMAC Design for Sensor Networks

• MAC protocols can be classified as scheduled and contention-based

• Major considerations– Energy efficiency

– Scalability and adaptivity to number of nodes

• Major ways to conserve energy– Low duty cycle to reduce idle listening

– Effective collision avoidance

– Overhearing avoidance

– Reducing control overhead


Scheduled vs. Contention ProtocolsScheduled vs. Contention Protocols

Scheduled Protocols

Contention Protocols

Collisions No Yes

Energy efficiency GoodNeed

improvement

Scalability and adaptivity

Bad Good

Multi-hop communication

Difficult Easy

Time synchronization

StrictLoose or not

required


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2/17/20051 Guest lecture for CS113, UCLA Medium Access Control in Wireless Sensor Networks Wei Ye...

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