Outline Medium Access Control With Introduction to MAC Coordinated - - PDF document

1 Medium Access Control With Coordinated Adaptive Sleeping for Wireless Sensor Networks

Presented by: Arik Brooks

Washington University Department of Computer Science and Engineering

Outline

 Introduction to MAC  S-MAC Overview  S-MAC Evaluation  Critique  Comparison to MACAW

MAC (Medium Access Control)

 All shared-medium networks need an

effective MAC protocol

 Controls access to the shared medium  Ensures no two nodes interfere with each

• ther’s transmissions

C A B

A sends a message C sends a message B receives ???

General MAC Requirements

 Ensure reliable communication  Ensure fair access to shared physical

transmission medium for all contending streams

 Minimize delay for sending/receiving

messages

 Maximize bandwidth utilization

Typical MAC protocols

 Contention-based

 CSMA (Carrier Sense Multiple Access)  MACA/MACAW (Virtual Sensing RTS/CTS)  IEEE 802.11 (Physical & Virtual Sensing)  PAMAS

 Schedule-based

 TDMA (Time-Division Multiple Access)

 Others

 CDMA (Code-Division Multiple Access)

Typical MAC protocols

 Contention-based

 Pros – Avoids collisions well, no synchronization  Cons – Control overhead, idle listening

 Schedule-based

 Pros – Conserves energy  Cons – Doesn’t scale well, synchronization

difficult

2

MAC Requirements for Wireless Sensor Networks

 Energy Efficiency  Scalability/Adaptability to network size,

density, and topology

 Less emphasis on fairness, latency,

throughput, and bandwidth utilization

S-MAC: Design

 Uses elements of contention-based and

schedule-based MAC protocols

 Tries to reduce energy waste from all major

sources

 Collisions, Overhearing, Control packet overhead,

Idle Listening

 Trades performance for energy efficiency

 Increases per-hop delay  Reduces fairness

Sources of Energy Waste

 Collisions

 Re-transmitting packets takes a lot of energy

 Overhearing

 Receiving packets destined to other nodes

 Control packet overhead

 Sending/Receiving control packets

 Idle Listening

 An idle radio consumes almost as much power as

during reception

S-MAC: Low Duty-Cycle Operation

 Low duty-cycle operation

 Reduces idle-listening by putting nodes to sleep

most of the time

 However, increases latency, which can

accumulate on each hop

Listen Listen L… Sleep Sleep

S-MAC: Coordinated Sleeping

 Uses Coordinated Sleeping to reduce control

• verhead and latency

 Nodes adopt their neighbors sleep schedule  If conflicting schedules, nodes can adopt both or

just remember the second for transmit

 Only initiate communication during awake time,

during which neighbors should also be awake

 Periodically broadcast SYNC message to

maintain sync

 Periodic neighbor discovery (listens to whole sync

period)

S-MAC: Listen Schedule

 Listen broken into slots for SYNC, RTS, CTS

SYNC Receiver RTS CTS Listen Transmitter

Tx SYNC Tx RTS Rx CTS CS CS Tx Data Rx Data/Sleep

…

3 S-MAC: Collision Avoidance

 Collision and Overhearing Avoidance

 Uses physical AND virtual carrier sense  RTS/CTS/Data/Ack  All nodes that hear a RTS or CTS sleep for the

specified TX time to avoid overhearing/collision

 Inspired by PAMAS

C A B F D E

Sender Receiver

S-MAC: Message Passing

 Sending Long Messages is costly in terms of

energy and latency

 Re-transmission of a long message is costly  Transmitting many small fragments requires extra

• verhead

 Message Passing used to minimize costs of

sending a long message

 Use a single RTS/CTS for all fragments, reducing

control packet overhead

 Re-transmission limited to corrupted fragments

S-MAC: Adaptive Listening

 Uses Adaptive Listening to minimize delay  Nodes that hear RTS/CTS wake up and

listen for a short time immediately after the previous transmission should have ended

 Not as good as it sounds because RTS/CTS

packets exchanged during adaptive wakeup period are not during the next packets wake time

Added Delay

S-MAC: Example w/o Adaptive Listening

A

A B C D

B C D

Tx Data Rx Data Schedule Wake

CTS CTS RTS

Schedule Wake

RTS CTS CTS ACK CS CS

Added Delay

S-MAC: Example with Adaptive Listening

A

A B C D

B C D

Tx Data Rx Schedule Wake

CTS CTS RTS

Adaptive Wake Not Seen

RTS CTS CTS

Tx Data Rx Adaptive Wake

RTS

Not Seen Schedule Wake

CTS RTS ACK ACK CS CS CS CS

S-MAC: Energy Performance

 Two-hop network  Savings due to avoiding

• verhearing and

efficiently transmitting long messages

A B C E D

Source 1 Source 2 Sink 2 Sink 1

4 S-MAC: Energy Performance

1 2 3 10 11

Source Sink

… S-MAC: Latency Performance

1 2 3 10 11

Source Sink

…

Lowest Traffic Load Highest Traffic Load

S-MAC: Throughput Performance

1 2 3 10 11

Source Sink

…

Highest Traffic Load Another Traffic Load

S-MAC: Energy-Time Cost Performance

1 2 3 10 11

Source Sink

… Key Ideas

 Combines advantages of contention-based

(good collision avoidance) and schedule- based (energy efficiency)

 Low-duty cycle with coordinated sleeping  RTS/CTS/Data/Ack when awake  Overhearing avoidance based on info in

RTS/CTS packets

 Adaptive sleeping used to reduce latency

compared to other schedule-based protocols

Critique/Questions

 The authors dismiss fairness as an important part of

the MAC protocol because of an unsubstantiated assumption about the nature of wireless sensor network applications

 How does S-MAC compare to other MAC protocols

designed for energy efficiency (TDMA, etc…)?

 Do schedules ever combine or get reformed to

synchronize more groups of nodes? This doesn’t seem to meet the requirement of being flexible to network changes

5 Comparison to MACAW

 MACAW

 Designed to improve fairness, throughput, latency, and

reliability

 RTS-CTS-DS-DATA-ACK  Dynamic backoff algorithm (lower if contention level is low)

 S-MAC

 Designed to improve energy efficiency  Adds coordinated sleep for energy conservation  Backoff algorithm is random sleep  Uses fragmentation to reduce control overhead

Where to learn more

 http://www.isi.edu/scadds/projects/smac/

 General info and links related to S-MAC

 http://www.isi.edu/ilense/software/smac/

 Download source code  FAQ

 http://www.isi.edu/~weiye/pub/commstack.pdf

 Detailed description of the modified comm stack

used to implement S-MAC on TinyOS