Mac protocols sensor_20071105_slideshare
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Transcript of Mac protocols sensor_20071105_slideshare
6
Introduction● Why do we need MAC?
● Wireless channel is a shared medium ● Radios transmitting in the same frequency band
interfere with each other !
● The role of Medium Access Control ● Controls when and how each node can transmit in
the wireless channel ● Solves the contention and collision
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Introduction● MAC Protocols for WSNs ● Primary Concern
● Energy Efficiency ● Reduction of significance
● Contention/Collision (basic task) ● Scalability ● Latency ● Communication Patterns ● Adaptability to changes ● Fairness ● Throughput
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Introduction● Major Sources of Energy Wastes ● Control Packet Overhead
● E.g., RTS/CTS ● Collision
● Retransmission ● Overhearing
● The receiver of a packet is not the intended receiver of that packet
● Idle Listing ● Listening to possible traffic that is not sent
ab
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Classification of MAC Protocols● Contention-based protocols
● Nodes compete in probabilistic coordination ● Examples: ALOHA (pure & slotted), CSMA ● Simple, no time synch, and robust to network
changes ● High idle listening and overhearing overheads
● Solve this by duty cycling
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Classification of MAC Protocols● Schedule-based protocols
● Schedule nodes onto different sub-channels ● Examples: TDMA, FDMA, CDMA ● Collision-Free ● Requires time synch and not robust to changes. ● Low throughput and high latency even during low
contention. ● Low idle listening and overhearing overheads
● Wake up and listen only during its neighbor transmission
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S-MAC
W. Ye, J. Heidemann, and D. Estrin, “An Energy-efficient MAC Protocol for Wireless Sensor Networks,” in Proc. of IEEE Infocom, Jun. 2002, pp.1567–1576.
● Basic Idea ● Trades energy efficiency for lower throughput and
higher latency ● Main Components ● Periodic Listen and Sleep ● Collision Avoidance ● Overhearing Avoidance ● Message Passing
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Periodic Listen and Sleep● Nodes periodically sleep
● Turn off radio when sleeping ● Reduce duty cycle to ~10% (150ms on vs. 1.5s off)
● Trades energy efficiency for lower throughput and higher latency
Listen Sleep tSleepListen
Listen Sleep Listen t
Both the synchronization issue and the schedule maintenance issue can be found in the paper.
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Collision Avoidance● Similar to IEEE 802.11 ● Virtual Carrier Sense
● NAV (Network Allocation Vector) ● Physical Carrier Sense ● Four-way handshake
● RTS/CTS/DATA/ACK
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Overhearing Avoidance● Basic Idea ● A node can go to sleep whenever its neighbor is
talking with another node ● Who should sleep? ● The immediate neighbors of sender and receiver
● How to they know when to sleep? ● By overhearing RTS or CTS
● Hog long should they sleep? ● NAV
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Message Passing● How to transmit a long message? ● Transmit it as a single long packet
● Easy to be corrupted ● Transmit as many independent packets
● Higher Control Overhead & Longer Delay ● Divide into fragments, but transmit all in burst !!!
● RTS/CTS reserve medium for the entire sequence
RTSCTS
DataCTS
DataCTS
DataCTS
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B-MAC (Overview)● B-MAC is implemented in TinyOS
!● Major Feature: reconfigurable ● Above B-MAC, one can implement an RTS-CTS
scheme or a TDMS-like scheduling protocol !
● A small core of media access functionality ● arbitration, reliability, low power communication
J. Polastre, J. Hill, and D. Culler, “Versatile low power media access for wireless sensor networks,” in SenSys, Nov. 2004, pp. 95–107.
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B-MAC (Overview)● Channel Arbitration ● Clear Channel Assessment (CCA) & Backoffs !
● Reliability ● Link-layer acknowledgment !
● Low Power Communication ● Low Power Listening (LPL)
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Clear Channel Assessment● This paper proposes a way to estimate noise
floor using signal-processing techniques ● Noise floor can be viewed as a threshold !
● Where a node intend to transmit a packet ● 802.15.4: takes a single sample and compares it to
the noise floor ● B-MAC: outlier detection algorithm
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Clear Channel Assessment● MacControl
● command result_t EnableCCA(); ● command result_t DisableCCA(); !
● By disabling CCA, a scheduling protocol may be implemented above B-MAC !
● If CCA is enabled ● event uint16_t initialBackoff(void* msg); ● event uint16_t congestionBackoff(void* msg);
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Link-layer Acknowledgment● Similar to traditional MAC protocols !
● MacControl ● command result_t EnableAck(); ● command result_t DisableAck(); !
● RTS/CTS/DATA/ACK ?
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Low Power Listening● Low Power Listening
● Sender (Transmit mode) ● Send preamble before transmit the data ● Preamble Length !
● Receiver (Listening mode) ● Check Interval !
● To reliably receive data, the preamble length is matched to the interval that the channel is checked for activity ● If the channel is checked every 100 ms, the preamble must
be at least 100 ms long
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Low Power Listening
Sender
Receiver 1
Receiver 2
Backoff& CCA Preamble
wakeup
Data
wakeupCheck Interval
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Schedule-based MAC Protocols for WSNs● TDMA MAC Protocols ● TRAMA ● Convergecast in Tree-based Wireless Sensor
Networks
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Traditional TDMA MAC Protocols● Divide time into cycles
● A cycle consists of several slots !!!
● Advantages ● Collision-Free, Low Idle Listing and Overhearing Overheads
● Disadvantages ● Synchronization, Low Channel Utilization !
● An important Issue ● Slot Assignment Strategy
1 2 3 4 5 6 7 1 2 3
Cycle
3 4 5 cc 1 2 3 4 5 6 7
Cycle
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Slot Assignment Strategy● Basic Slot Assignment Strategy ● A node should own a slot different from its one-
hop and two-hop neighbors
R
S
S
Collision
One-hop
R
R
S S
Collision
Collision
Two-hop
RS R S
Three-hop
Acknowledgement?
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TRAMA
● TRAMA (TRaffic-Adaptive Medium Access) ● Avoid assigning slots to nodes with no traffic ● Allow nodes to determine when they can switch off
!● Main Components ● Neighbor Protocol ● Schedule Exchange Protocol ● Adaptive Election Algorithm
V. Rajendran, K. Obraczka, and J. Garcia-Luna-Aceves, “Energyefficient, collision-free medium access control for wireless sensor networks,” Wireless Networks, vol. 12, no. 1, pp. 63–78, Feb. 2006.
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Neighbor Protocol
To accommodate topology dynamics: 1. Dynamic: interval between random access periods could be larger 2. Static: interval could be smaller
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Schedule Exchange Protocol
Random Access
SCHEDULE_INTERVAL
SCHEDULE_INTERVAL
Winning Slot(Intended Receiver: a, b, c)
Winning Slot(Intended Receiver : d, e, f)
Winning Slot(Intended Receiver : φ)
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Convergecast in Tree-based Wireless Sensor Networks
6
4 54
1 2 3 1
0 1 0
3
MAXSLOT=7
2
Level 0
L1
L2
L3
1. Construct a BFS tree T !
2. Traverse the nodes of T in a bottom-up manner
a) if the node v is a leaf node, assign a min slot s(v) which is not interference with its interference neighbors
b) if v is the in-tree node, choose the min slot s(v) which is larger than the children’s slot and do not interference with others
!3. Re-assign the slot in a top-down
mannerY.-C. Tseng and M.-S. Pan, “Quick Convergecast in ZigBee/IEEE 802.15.4 Tree-Based Wireless Sensor Networks”, ACM MobiWac, 2006.
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Z-MAC● Z-MAC (Zebra MAC) ● A hybrid TDMA and CSMA MAC scheme ● The main feature is its adaptability to the level of
contention in the network ● under low contention, it behaves like CSMA, and ● under high contention, like TDMA
I. Rhee, A. Warrier, M. Aia, and J. Min, “Z-MAC: a hybrid MAC for wireless sensor networks,” in SenSys, Nov. 2005, pp. 90–101.
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Z-MAC● Design of Z-MAC ● Neighbor Discovery and Slot Assignment ● Local Framing ● Transmission Control of ZMAC ● Explicit Contention Notification ● Receiving Schedule of ZMAC ● Local Time Synchronization
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Funneling-MAC● Funneling effect in sensor networks
G.-S. Ahn, E. Miluzzo, A. T. Campbell, S. G. Hong, and F. Cuomo “Funneling-MAC: A localized, sink-oriented MAC for boosting fidelity in sensor networks,” in SenSys ’06
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Funneling-MAC● The authors propose a localized, sink-
oriented funneling-MAC !
● Localized ● Using local TDMA scheduling in the intensity
region only ● Sink-oriented ● Manage TDMA scheduling ● Compute and maintain the depth of the intensity
region