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

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CS 15-849E Wireless Networks (Spring 2006) MAC
LayerDiscussion Leads Abhijit Deshmukh
Sai VinayakInstructor Srinivasan Seshan
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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

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Outline

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

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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

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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

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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

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

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Related Work

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

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Sensor-MAC Protocol Design

• Periodic Listen and Sleep
• Message Passing
• Collision and Overhearing Avoidance

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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

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Periodic Listen and Sleep

• Choosing and Maintaining Schedules
• Schedule Table
• Synchronizer
• Follower

Rebroadcast
SYNC
Listen
Wait (random)
Wait (random)
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Periodic Listen and Sleep

• Maintaining Synchronization
• SYNC packet
• Listen Interval
• SYNC RTS

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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
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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

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Implementation

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

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Results

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

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Heterogeneous Wireless MACs

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

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Motivation

• Exposed Receiver Hidden Sender

CTS / RTS ?
data
data
x
Blocked
S1 ? R1 ?
ACK
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4-way Handshake?

• Hidden Receiver

• Exposed Sender

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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

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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

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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

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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

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MAC for Long Dist. 802.11 Mesh

• Motivation
• Extend 802.11 for long haul
• Challenges
• Use off-the shelf hardware
• Low cost

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Overview

• Basic Principle
• SynRx SynTx

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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

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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

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2P Operation on Single Link

• Marker acts as a token
• Loose Synchrony

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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 ?

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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

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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

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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

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Throughput
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2P vs TCP
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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?

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• Q A