Medium Access Control Protocols Lecture 7 (Lecture material contributed by K. Langendoen(TUDelft) and W. Ye(USC/ISI)) September 23, 2004 EENG 460a / CPSC 436 / ENAS 960 Networked Embedded Systems

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Medium Access Control ProtocolsLecture 7
(Lecture material contributed by K.
Langendoen(TUDelft) and W. Ye(USC/ISI))September
23, 2004EENG 460a / CPSC 436 / ENAS 960
Networked Embedded Systems Sensor Networks

• Andreas Savvides
• andreas.savvides_at_yale.edu
• Office AKW 212
• Tel [phone]
• Course Website
• http//www.eng.yale.edu/enalab/courses/eeng460a

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Protocol stack
OSI
Network
Layer 3

• Data link layer
• mapping network packets ? radio frames
• transmission and reception of frames over the air
• error control
• security (encryption)

Data Link
Layer 2
Physical
Layer 1
3
Medium Access Control

• Control access to the shared medium (radio
  channel)
• avoid interference between transmissions
• mitigate effects of collisions (retransmit)
• History

4
Medium Access Control

• Control access to the shared medium (radio
  channel)
• avoid interference between transmissions
• mitigate effects of collisions (retransmit)
• Approaches
• contention-based no coordination
• schedule-based central authority (access point)

5
Collision-based MAC protocols

• ALOHA
• packet radio networks
• send when ready
• 18-35 channel utilization
• CSMA (Carrier Sense Multiple Access)
• listen before talk
• 50-80 channel utilization

6
Hidden terminal problem
cs
Time
cs
Carrier sense at sender may not prevent collision
at receiver
7
CSMA/CA Collision Avoidance

• MACA
• Request To Send
• Clear To Send
• DATA
• MACAW (Wireless)
• additional ACK

cs
Time
8
Exposed terminal problem
cs
Time
Parallel CSMA transfers are synchronized
by CSMA/CA Collision avoidance can be too
restrictive!

9
IEEE 802.11

• Operation
• infrastructure mode (access point)
• ad-hoc mode
• Power save mechanism not for multi-hop networks
• Protocol
• carrier sense
• collision avoidance (optional)

10
IEEE 802.11

• Network Allocation Vector (NAV)
• collision avoidance
• overhearing avoidance other nodes may sleep

11
Schedule-based MAC protocols

• Communication is scheduled in advance
• no contention
• no overhearing
• support for delay-bound traffic (voice)
• Time-Division Multiple Access
• time is divided into slotted frames
• access point broadcasts schedule
• coordination between cells required

12
TDMA

• Typical WLAN setup
• no direct communication between nodes
• access point broadcast Traffic Control (TC) map
• (new) nodes signal needs in Contention Period (CP)

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Requirements for Sensor Networks

• Handle scarce resources
• CPU 1 10 MHz
• memory 2 4 KB RAM
• radio 100 Kbps
• energy small batteries
• Unattended operation
• plug play, robustness
• long lifetime

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The battery crisis

• Limited capacity
• Slow increase of capacity
• 8 yearly increase (Wh/cm3)
• doubles every 9 years

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Sensor Node Energy Roadmap (DARPA)
10,000 1,000 100 10 1

• Deployed (5W)

• PAC/C Baseline (.5W)

Average Power (mW)

• (50 mW)

• (1mW)

software does it!
2000 2002 2004
Srivastava2002
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Energy consumption (mW)
25
20
15
10
5
0
LED
Light
5 MHz
1 MHz
Sleep
Standby
Receive
Transmit
Compass
Accelerometer
Hoesel2004
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Energy-Efficient MAC Design

• 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

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Energy-Efficient MAC Design

• Unicast example of PS mode in 802.11 DCF

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Communication patterns

• WSN applications
• local collaboration when detecting a physical
  phenomenon
• periodic reporting to sink
• Characteristics
• low data rates
• small messages
• fluctuations (in time and space)

lt1000 bps
25 bytes
Kulkarni2004
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Design guidelines

• switch radio off when possible (duty cycle)
• AND, minimize number of switches
• low complexity (memory footprint)
• trade off performance for energy
• optimize for traffic patterns

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Design guidelines

• switch radio off when possible (duty cycle)
• AND, minimize number of switches
• low complexity (memory footprint)
• trade off performance for energy
• optimize for traffic patterns

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Energy-efficient medium access control

• Performance/Cost trade-off
• latency
• throughput
• fairness
• energy consumption
• Organizational/Flexibility trade-off
• contention-based
• schedule-based

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Sources of overhead

• idle listening (to handle potentially incoming
  messages)
• collisions (wasted resources at sender and
  receivers)
• overhearing (communication between neighbors)
• protocol overhead (headers and signaling)
• traffic fluctuations (overprovisioning and/or
  collapse)
• scalability/mobility (additional provisions)

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Contention-based vs. Schedule-based
source of overhead performance (latency, throughput, fairness) cost (energy-efficiency)
idle listening C
collisions C C
overhearing C
protocol overhead C,S C,S
traffic fluctuations C,S C,S
scalability/mobility S S
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Energy-efficient MAC protocols

• WSN-specific protocols
• starting from 2000 (1 paper)
• exponential growth (2004, 16 papers)
• Classification (up to May 2004, 20 papers)
• the number of channels used
• the degree of organization between nodes
• the way in which a node is notified of an
  incoming msg

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Protocol classification
Protocol Channels Organization Notification
2000
SMACS 34 FDMA frames schedule
2001
PACT 28 single frames schedule
PicoRadio 10 CDMAtone random wakeup
2002
STEM 33 datactrl random wakeup
Preamble sampling 6 single random listening
Arisha 2 single frames schedule
S-MAC 36 single slots listening
PCM 18 single random listening
Low Power Listening 13 single random listening
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Protocol classification
2003
Sift 17 single random listening
EMACs 15 single frames schedule
T-MAC 5 single slots listening
TRAMA 30 single frames schedule
WiseMAC 7 single random listening
2004
BMA 24 single frames schedule
Miller 27 datatone random wakeuplist
DMAC 26 single slots listening
SS-TDMA 23 single frames schedule
LMAC 14 single frames listening
B-MAC 29 single random listening
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Case Study S-MAC

• S-MAC by Ye, Heidemann and Estrin
• Tradeoffs
• 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)

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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
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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

4
1
2
3
listen

• Reduce latency by at least half

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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 Dont 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

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Implementation on Testbed Nodes

• Platform
• Mica Motes (UC Berkeley)
• 8-bit CPU at 4MHz,
• 128KB flash, 4KB RAM
• 20Kbps radio at 433MHz
• TinyOS event-driven
• Configurable S-MAC options
• Low duty cycle with adaptive listen
• Low duty cycle without adaptive listen
• Fully active mode (no periodic sleeping)

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Experiments two-hop network

• Topology and measured energy consumption on
  source nodes

• 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

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Energy Consumption over Multi-Hops

• Ten-hop linear network at different traffic load

• 3 configurations of S-MAC

• At light traffic load, periodic sleeping has
  significant energy savings over fully active mode
• Adaptive listen saves more at heavy load by
  reducing latency

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Latency as Hops Increase

• Adaptive listen significantly reduces latency
  causes by periodic sleeping

Latency under highest traffic load
Latency under lowest traffic load
10 duty cycle without
adaptive listen
10 duty cycle without
adaptive listen
Average message latency (S)
Average message latency (S)
10 duty cycle with
adaptive listen
10 duty cycle with adaptive listen
No sleep cycles
No sleep cycles
Number of hops
Number of hops
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Throughput as Hops Increase

• Adaptive listen significantly increases throughput

Effective data throughput under highest traffic
load

• Using less time to pass the same amount of data

No sleep cycles
Effective data throughput (Byte/S)
10 duty cycle
with adaptive listen
10 duty cycle without adaptive listen
Number of hops
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Combined Energy and Throughput
Energy-time cost on passing 1-byte data from
source to sink

• Periodic sleeping provides excellent performance
  at light traffic load
• With adaptive listening, S-MAC achieves about the
  same performance as no-sleep mode at heavy load

No sleep cycles
Energy-time product per byte (JS/byte)
10 duty cycle without
adaptive listen
10 duty cycle with adaptive listen
Message inter-arrival period (S)
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IEEE 802.15.4 MAC Protocol

• Based on an IEEE standard for WPAN
• Goal Ultra-low cost, low power radios
• Support multiple configurations (e.g
  point-to-point, groups, ad-hoc etc)
• CSMA-CA based protocol
• Each packet can be individually acknowledged
• Key features
• Three types of node functionalities
• PAN Coordinator, Coordinator and Device
• Two device types
• FFD Full Function Device
• RFD Reduced Function Device

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Frequencies and Data Rates
BAND COVERAGE DATA
RATE OF CHANNEL(S) 2.4 GHz ISM
Worldwide 250 kbps
16 868 MHz Europe 20 kbps
1 915 MHz ISM Americas 40
kbps 10
See class website for more information about
Zigbee More abut MAC protocols on the next lecture
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Paper Reading

• Elson02 Fine-Grained Network Time
  Synchronization using Reference Broadcasts,
  Jeremy Elson, Lewis Girod and Deborah EstrinIn
  Proceedings of the Fifth Symposium on Operating
  Systems Design and Implementation (OSDI 2002),
  Boston, MA. December 2002. UCLA Technical Report
  020008. 
• You should all read this paper closely before
  lecture 9!