1 Types of Power-Saving MACs Types of Power-Saving MACs (cont.) - - PDF document

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Media Access Control

Greg Hackmann CSE467S Spring 2009

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What Is Media Access Control?

• Media Access Control (MAC) determines who

gets access to the shared medium, and when

• In wired settings, usually just tries to avoid

contention

• In wireless settings, can also significantly reduce

energy consumption

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Carrier Sense Multiple Access, Collision Avoidance (CSMA/CA)

CS CS

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Hidden Terminal Problem

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Virtual Carrier Sense: RTS/CTS

RTS CTS Payload NAV 6

Power-Saving MAC

• Many applications’ expected lifetime: months or

years

• Actual lifetime:



AA batteries: 2000 mAh (if you’re lucky)



CC2420 radio: 19.7 mA when idle but awake (RX mode)



2000 mAh / 19.7 mA = 101.5 h ≈ 6 days

• This is a problem!
• Solution: keep the radio asleep most of the time



Duty cycles on the order of 0.1% – 1%

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Types of Power-Saving MACs

• Scheduled contention: nodes periodically wake up in

unison, contend for access to channel, then go back to sleep



S-MAC [Ye 2002], T-MAC [van Dam 2003]

• Channel polling: nodes independently wake up to

sample radio channel



B-MAC [Polastre 2004], X-MAC [Buettner 2006]

• Time division multiple access (TDMA): nodes maintain

schedule of when to wake and when they’re allowed to transmit



802.15.4 [IEEE 2003], DRAND [Rhee 2006]

Types of Power-Saving MACs (cont.)

• Different trade-offs for latency, power consumption,

runtime overhead, etc.

• Motivation for hybrid protocols: SCP [Ye 2006], Z-

MAC [Rhee 2005], Funneling MAC [Ahn 2006]

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The Sensor MAC (S-MAC)

• Nodes stay asleep most of

the time

• Periodically wake for short

intervals (0.5 s) to see if anyone’s sending

• Very low energy

consumption when traffic is low

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S-MAC: Boot Phase

5 s 4.5 s 3 s 3 s Waking up in 3 s 5 s Waking up in 5 s Waking up in 4.5 s 4.5 s

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S-MAC: Sending a Packet

• Time awake divided

into two parts: SYNC and RTS

• Node periodically

send SYNC packet to keep clocks in sync

• CSMA/CA used to

contend for access to wireless channel

CS CS

Receiver Wants to SYNC Wants to SYNC & send data Wants to send data

SYNC RTS

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S-MAC: Sending a Packet

• RTS section used for

transmitting data

• CSMA/CA again, followed

by RTS/CTS

CS CS CS CS

Receiver Wants to SYNC Wants to SYNC & send data Wants to send data

SYNC RTS

3

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S-MAC: Sending a Packet

• CTS for someone else
• > go to sleep



Overhearing avoidance

• Sender does one RTS/

CTS then sends data for rest of frame

• All data packets are

ACKed



Packet fragmentation = higher reliability

Receiver RTS Winner

RTS

RTS CTS DATA DATA ACK ACK

Sleep Sleep

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S-MAC: Evaluation

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S-MAC in Summary

• Power savings over standard 802.11 MAC
• Long listening period is expensive



Everyone stays awake unless somebody transmits

• Time synchronization overhead even when

network is idle

• RTS/CTS and ACK overhead when sending data
• Complex to implement

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Berkeley MAC (B-MAC)

t ≥ t

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B-MAC: Throughput

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B-MAC: Power Consumption

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B-MAC in Summary

• Low overhead when network is idle
• Simple to implement
• Better power savings, latency, and throughput than S-MAC
• Lower duty cycle -> longer preambles:



Higher average latency



Higher cost to send



Higher cost to overhear



More contention

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X-MAC: Room for Improvement

Sender Recipient Neighbor Payload Header 21

X-MAC: Overhearing Avoidance

Sender Recipient Neighbor Payload Header Destination 22

X-MAC: Preamble ACKing

Sender Recipient Neighbor Payload Header Destination ACK 23

X-MAC: Current Draw

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X-MAC: Latency

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X-MAC in Summary

• Better latency, throughput, and power consumption than

B-MAC for unicast traffic

• Little energy consumed by overhearing
• Still simple to implement
• No improvements over B-MAC for broadcast traffic
• On average, cuts preamble by half -> sending packets is

still expensive

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TDMA

A Transmits B Transmits C Transmits Sleep Sleep Time Sync Frame Slot

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TDMA in Summary

• Predictable latency, throughput, and duty cycle
• Low packet loss due to contention
• Schedules must be recreated when nodes leave/

enter neighborhood

• Time synchronization overhead
• Slots wasted when scheduled node has nothing

to send

• Often complex to implement

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SCP: Scheduled Contention + Channel Polling

t t

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SCP: Adaptive Channel Polling

• Add N sub-intervals whenever packet received



Reduces latency of bursty data

• Continue adding sub-intervals as long as they’re needed,

and as long as there’s room

Receiver Sender 30

SCP: Multi-Hop Pipelining

• Sender “gives up” first regular check after sending multi-

hop traffic



Whole network quickly moves into adaptive polling mode

Hop 1 Sender Hop 2

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SCP: Energy Consumption

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SCP: Multi-Hop Latency

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SCP in Summary

• Channel polling: low overhead when idle
• Scheduled contention: low cost to send
• Low latency for multi-hop traffic
• Complex to implement
• Experimental radio data required
• Overhead due to time synchronization



Reduced by piggybacking on data packets

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Zebra MAC (Z-MAC): TDMA + Channel Polling

A Transmits B Transmits C Transmits Sleep Sleep Time Sync

A B C 35

Z-MAC in Summary

• Reduces waste from unused slots



Higher throughput and lower latency

• Throughput, latency, and duty cycle no worse

than pure TDMA

• Nodes still stay awake if no one transmits
• Still overhead from time sync
• Complex to implement

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So Why Are They Rarely Used?

• MAC protocols have additional radio-dependent

requirements beyond normal application code



Turn radio on and off, low latency I/O, carrier sense, etc.

• Typically implemented by forking radio stacks



Hard to implement



Must be maintained as original radio stack changes

• MAC layers supported by TinyOS today:



CC1000: “experimental” B-MAC



CC2420: BoX-MAC (X-MAC-like + pieces of B-MAC)

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A Better Approach: MLA

• MAC Layer Architecture (MLA) [Klues 07]



Low-level abstractions of radio functionality



High-level implementations of common MAC logic

• Implemented on TinyOS 2.0.2
• Used to create 5 platform-independent MAC

layers



B-MAC, X-MAC, SCP, TDMA, variant of Z-MAC

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References

• W. Ye, J. Heidemann and D. Estrin, Medium Access Control with

Coordinated, Adaptive Sleeping for Wireless Sensor Networks, IEEE/ACM Transactions on Networking, June 2004.

• J. Polastre, J. Hill, and D. Culler, Versatile Low Power Media Access for

Wireless Sensor Networks, SenSys, 2004.

• I. Rhee, A. Warrier, M. Aia, and J. Min, Z-MAC: a Hybrid MAC for Wireless

Sensor Networks, SenSys, 2005.

• M. Buettner, G. Yee, E. Anderson, R. Han, X-MAC: A Short Preamble MAC

Protocol for Duty-Cycled Wireless Sensor Networks, SenSys, 2006.

• W. Ye, F. Silva, and J. Heidemann, Ultra-Low Duty Cycle MAC with

Scheduled Channel Polling, SenSys, 2006.

• K. Klues, G. Hackmann, O. Chipara and C. Lu, A Component-Based

Architecture for Power-Efficient Media Access Control in Wireless Sensor Networks, SenSys 2007.