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Sensor MAC Design

• Requirements: Energy efficiency Simple operations Working with a large number of sensors Fair share of the channel among competing sensor nodes

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Introduction 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

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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

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Key Questions

• How should Media Access Control (MAC) protocols be designed for sensor networks?

• What are the metrics for MAC in a multi-hop scenario?

• How can we achieve these metrics with local algorithms over limited resources?

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What’s New in Sensor Networks?

• A special multi-hop wireless network Large number of nodes Battery powered Topology and density change due to node’s sleeping or

failure 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

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Primary 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

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Other 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?

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Energy Efficiency in MAC Design

• Energy is primary concern in sensor networks• What causes energy waste?

1. Collisions

2. Control packet overhead

3. Overhearing unnecessary traffic

4. Long idle time1. bursty traffic in sensor-net apps

2. Idle listening consumes 50—100% of the power for receiving (Stemm97, Kasten) Dominant factor

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Classification of MAC Protocols

• Schedule-based protocols Schedule nodes onto different sub-channels Examples: TDMA, FDMA, CDMA

• Contention-based protocols (our focus) Nodes compete in probabilistic coordination Example: 802.11 DCF

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• 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: CSMA

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• Hidden terminal problem

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

• CSMA/CA (CSMA with Collision Avoidance) RTS/CTS handshake before sending data Node c will backoff when it hears b’s CTS

Contention Protocols: CSMA/CA

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

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Contention 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

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Case Study 1: S-MAC

• By Ye, Heidemann and Estrin• Tradeoffs

Latency

FairnessEnergy

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S-MAC Overview

• Design goals Energy efficiency Self-configuration and flexibility to node changes

• Approaches Contention-based MAC with various energy-conserving

features

• Major components in S-MAC Periodic listen and sleep Collision avoidance Overhearing avoidance Massage passing

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Coordinated 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

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Coordinated Sleeping

• 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

sleeplisten listen sleep

sleeplisten listen sleep

Schedule 2

Schedule 1

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Coordinated Sleeping

• 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

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Coordinated Sleeping

• 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

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Collision 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

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Overhearing 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

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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 immediately

• Other nodes sleep for whole message time

FairnessEnergy

Msg-level latency

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Implementation 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

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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

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Evaluation Metric: Aggregate Bandwidth

• Traditional MAC metric

• channel capacity is a precious resource

• Maximize total delivered bandwidth from every node in the network to the base station

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Evaluation Metric: Energy Efficiency

• Observation: Energy is the precious resource

• Goal: Minimize energy per unit of successful communication to base

station while sustaining reasonable channel utilization

Turn off radio whenever possible Avoid over commit the networkTotal energy spent in data propagation over a network

Total packets received by the base stationE =

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Limitations of SMAC

• Look at the assumptions made Broadcast assumes every one can hear the message Bursty data applications versus periodic applications Power consumption model In-band signaling?

• Let us revisit the design using the link measurement findings

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Fairness Challenge

Challenge:• want roughly equal data coverage

Observation:• originated traffic competes with route-thru traffic• at odd with energy efficiency and aggregate bandwidth

Goal: • minimize variance in bandwidth delivered to base station

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Case Study 2: Berkeley MAC Design

• Carrier Sense Multiple Access (CSMA) no extra control packets (energy efficient)

• Save energy: Shorten listening period as much as possible turn radio off during backoff

Trade bandwidth for battery life• Provide feedback to applications to desynchronize

Backoff should signal application to shift phase of sampling• Random delay before each transmission

break close synchronization

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Hidden Nodes inMulti-hop Networks

• Occurs between every other pair of levels• CSMA fails to detect• Traditionally addressed with contention-based protocols, but

Control packets (e.g. RTS/CTS/ACKs) induce high overhead given data packets are small

ACKs can be free in multi-hop networks By hearing your parent forwards your packets Data aggregation is application specific

Not adequate to solve hidden node problem in multi-hop case (Related Work: V. Bharghavan et al. MACAW)

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Avoid Hidden Node Corruption

A

B

C D

exploits application characteristics“A” refrains from sending for a packet time after parent transmits

Hidden node cases like this may be avoided withoutuse of control packets.

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Platform of Study

• Rene 4MHz, 8KB flash, 512B RAM 916MHz RF transceiver

10kbps 1 - 100 feet range

Sensors: temperature, light, magnetic field, acceleration

• Operating System: TinyOS tiny network stack and other communication support Small packets size (tens of bytes)

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Multi-hop Extensions

• Rate control module inserted between MAC and application

• Adapts data sampling rate to available bandwidth

• Balances demand for upstream bandwidth between local, originating traffic and route-thru traffic by adjusting transmission rate

Multihop Merging traffic flow

• Provides a mechanism for progressive feedback deep down into the network

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Rate Control Mechanism

• snoop on route-thru traffic to estimate children (n)• Open parameters

,• Apply for forwarding route-thru traffic

Progressive feedback deep down into the network Packet loss rate provides natural damping effect

x

+ /n R x p

if fails

if success

x + /n

x + /n

Estimate n based on route-thru traffic

Route-thru Traffic

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Other ARC Results

• Proposed CSMA with ARC scheme: Aggregate bandwidth:

~ 60% of proposed CSMA without ARC scheme

Energy efficiency: ~ 50% of proposed CSMA at a low

Fairness: 5 – 10 times lower variance

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Summary of B-MAC

• Sensor networks characteristics differ from traditional settings enough to require revisiting the basic protocols

• MAC design fairness and energy efficiency goals modified CSMA shown effective

• Transmission control local adaptive scheme on originating traffic effective Implemented and evaluated on simulation and real networked

sensors Each node achieves 20% of multi-hop channel capacity

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Summary for MAC Design in 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

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What is next: New Sensor Link Protocols

• Look at the experimental results ! These new findings are not addressed yet

• Learn from the topology With static sensor settings, link protocols can be more

efficient

• Application-aware link protocol Get hints from applications about traffic patterns, rate, etc. New forms of cross-layer design