CS 15-849E: Wireless Networks (Spring 2006) MAC Layer Discussion Leads: Abhijit Deshmukh Sai Vinayak Instructor: Srinivasan Seshan - PowerPoint PPT Presentation

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CS 15-849E: Wireless Networks (Spring 2006) MAC Layer Discussion Leads: Abhijit Deshmukh Sai Vinayak Instructor: Srinivasan Seshan

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MAC Layer Discussion Leads: Abhijit Deshmukh Sai Vinayak Instructor: Srinivasan Seshan Papers An Energy-Efficient MAC Protocol for Wireless Sensor Networks Wei ... – PowerPoint PPT presentation

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Title: CS 15-849E: Wireless Networks (Spring 2006) MAC Layer Discussion Leads: Abhijit Deshmukh Sai Vinayak Instructor: Srinivasan Seshan


1
CS 15-849E Wireless Networks (Spring 2006) MAC
Layer Discussion Leads Abhijit Deshmukh
Sai Vinayak Instructor Srinivasan Seshan
2
Papers
  • An Energy-Efficient MAC Protocol for Wireless
    Sensor Networks
  • Wei Ye, John Heidemann, Deborah Estrin
  • The Case for Heterogenous Wireless MACs
  • Chung-cheng Chen, Haiyun Luo
  • Design and Evaluation of a new MAC Protocol for
  • Long-Distance 802.11 Mesh Networks
  • Wei Ye, John Heidemann, Deborah Estrin

3
Outline
  • Motivation
  • MAC Wireless Sensor Networks
  • Heterogenous Wireless MACs
  • MAC for Mesh Networks
  • Take Aways
  • Similarities and Differences
  • Q A

4
Motivation
  • Last Lecture
  • MACAW, Carrier Sense, Idle Sense
  • Basic Terms, Algorithms
  • Major Focus on Fairness
  • Very Generic
  • Special Requirements for
  • Sensor Networks
  • Heterogeneous
  • Mesh Networks

5
MAC for Sensor Networks
  • Sensor Networks
  • Sensors, Embedded processor, Radio, Battery
  • Ad hoc fashion
  • Proximity, short-range multi-hop communication
  • Committed to One or few applications
  • MAC Protocol
  • Energy Efficiency
  • Scalability
  • Accommodate network changes
  • Fairness, Latency, Throughput and Bandwidth

6
Sensor Networks
  • Sources of Energy Waste ?
  • Collision
  • Overhearing
  • Control packet overhead
  • Idle Listening
  • Tradeoff of fixing these
  • Reduction in per-hop fairness and latency. How?
  • Message Passing, Fragment long message
  • Why not a big concern in Sensor Networks?
  • Application-level performance

7
  • Energy Saving turn off radio . Issues?
  • Latency
  • In-network processing. Power consumption?

8
Related Work
  • PAMAS
  • Avoid overhearing among neighbors
  • Two independent radio channels
  • Suffers from idle listening
  • TDMA
  • Natural Savings
  • Scheduling
  • Static
  • Piconet
  • Periodic Sleep

9
Sensor-MAC Protocol Design
  • Periodic Listen and Sleep
  • Message Passing
  • Collision and Overhearing Avoidance

10
Periodic Listen and Sleep
  • Basic Scheme
  • Turn off Radio, set timer to wake up, sleep
  • Clock Drift
  • Sync using relative timestamps
  • Long listen period
  • Reduce Control Overhead
  • Sync with neighbors, exchange schedules
  • Advantage over TDMA ?
  • Looser Synchronization
  • Disadvantage?
  • Latency due to switching, RTS/CTS

11
Periodic Listen and Sleep
  • Choosing and Maintaining Schedules
  • Schedule Table
  • Synchronizer
  • Follower

Rebroadcast
SYNC
Listen
Wait (random)
Wait (random)
12
Periodic Listen and Sleep
  • Maintaining Synchronization
  • SYNC packet
  • Listen Interval
  • SYNC RTS

13
Collision Overhearing Avoidance
  • Collision Avoidance
  • NAV
  • Virtual vs. Physical Carrier Sense
  • Overhearing Avoidance
  • Listening to all transmissions
  • Who all should sleep?
  • All neighbors of sender and receiver

x
x
E
C
A
B
D
F
14
Message Passing
  • Long vs. Short Message Length
  • Stream of Fragments, single RTS-CTS
  • Problem?
  • No Fairness
  • 802.11 Methodology?
  • Why send ACK after each fragment?
  • Prevent hidden terminal problem

15
Implementation
  • Rene Motes Tiny OS
  • Simplified IEEE 802.11
  • Message Passing (overhearing avoidance)
  • S-MAC (Message Passing Periodic Sleep)
  • Topology used

16
Results
  • Low performance for high loads?
  • Synchronization overhead (SYNC packets)
  • Latency

17
Heterogeneous Wireless MACs
  • Basic Service Set (BSS)
  • Careful Channel Assignment
  • Wireless interference
  • Limited orthogonal channels

18
Motivation
  • Exposed Receiver Hidden Sender

CTS / RTS ?
data
data
x
Blocked
S1 ? R1 ?
ACK
19
4-way Handshake?
  • Hidden Receiver
  • Exposed Sender

20
Incomplete vs. Inconsistent
  • Channel status at sender
  • Incomplete estimate of receiver
  • Inconsistent at multiple competing senders
  • Incomplete channel status high packet loss
  • Inconsistent channel status unfair channel
    sharing

21
Intra-BSS Interference Mitigation
  • When to use 4-way handshake?
  • Client detecting data transmission vs.
  • Clients data transmission being detected
  • Access point to initiate channel access?
  • BSS in center
  • Less chance of interference from other BSS

22
Inter-BSS Interference Mitigation
  • RTR (Request to receive)
  • RTR-DATA vs. RTS-CTS-DATA
  • ACK in form of next RTR
  • Stateless Approach
  • Alternating between MAC protocols
  • Simple Design and Implementation
  • Low Channel Utilization

23
Fairness
  • Why is flow 2?3 getting unfair treatment?
  • Client 3 is exposed receiver
  • Receiver 1 is not interfered by 2?3
  • How to solve it ?
  • Switch to receiver initiated protocol
  • Increase power levels of CTS/RTS

24
MAC for Long Dist. 802.11 Mesh
  • Motivation
  • Extend 802.11 for long haul
  • Challenges
  • Use off-the shelf hardware
  • Low cost

25
Overview
  • Basic Principle
  • SynRx SynTx

26
Design and Implementation
  • Design decisions driven by
  • Low cost considerations
  • Usage of off-the-shelf 802.11 hardware
  • Achieving SynOp
  • Get rid of immediate ACKs
  • Get rid of carrier sense backoffs

27
Design and Implementation (contd.)
  • Immediate Acks
  • Use IBSS mode of operation
  • Convert IP unicast to MAC broadcast
  • No ACKs for broadcast packets in IBSS mode
  • Broadcast Unicast since link is 1-1
  • ACKs can be implemented at the driver level
  • Carrier Sensed Backoffs
  • Make use of feature provided by Intersil Prism
    chipsets

28
2P Operation on Single Link
  • Marker acts as a token
  • Loose Synchrony

29
2P Operation on Single Link (contd.)
  • Need to handle 2 scenarios
  • Temporary loss of synchrony (loss of marker)
  • Link recovery after failure
  • 2P handles both using timeouts
  • Advantages
  • Link-resync process is quick
  • CRC errors do not cause timeout (other than
    marker) . Why ?

30
2P Operation on Single Link (contd.)
  • Two ends of a link get out of synchrony at the
    same time and timeout together . So?
  • They would not hear each others marker packets
    since both SynTx coincides So?
  • Repeated Timeouts !!! Solution ?
  • Staggered timeouts ? Bumping

31
Topology Formation
  • What are the topologies in which 2P?
  • Bipartite ?
  • A tree is trivially bipartite
  • Bad in terms of fault tolerance
  • Add redundancy but turn on only one tree at a
    time (Morphing)
  • 3 Heuristics
  • Reduce length of links used
  • Avoid short angles between links
  • Reduce hop-count

32
Evaluation
  • Goal is threefold
  • Measure impact of step by step link establishment
  • Study effect of 2P in a large topology
  • Study performance of TCP over 2P
  • Link Establishment
  • 12.9 ms for first case (delay due to bumping)
  • 4.9 afterwards

33
Throughput
34
2P vs TCP
35
Similarities and Differences
  • Similarities
  • MAC protocol implementations
  • Extend 802.11 for a specific environment
  • Others?
  • Differences
  • Deployment scenarios
  • Energy Saving, Long haul, Heterogeneity
  • Writing Style
  • Others?

36
  • Q A
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