CS 15-849E: Wireless Networks (Spring 2006)
MAC Layer
Discussion Leads:
Abhijit Deshmukh
Sai Vinayak
Instructor: Srinivasan Seshan
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
Outline
•
•
•
•
•
•
•
Motivation
MAC – Wireless Sensor Networks
Heterogenous Wireless MACs
MAC for Mesh Networks
Take Aways
Similarities and Differences
Q&A
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
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
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
Related Work
• PAMAS
• Avoid overhearing among neighbors
• Two independent radio channels
• Suffers from idle listening
• TDMA
• Natural Savings
• Scheduling
• Static
• Piconet
• Periodic Sleep
Sensor-MAC Protocol Design
• Periodic Listen and Sleep
• Message Passing
• Collision and Overhearing Avoidance
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
Periodic Listen and Sleep
• Choosing and Maintaining Schedules
• Schedule Table
• Synchronizer
• Follower
SYNC
Rebroadcast
Listen
Wait (random)
Wait (random)
Periodic Listen and Sleep
• Maintaining Synchronization
• SYNC packet
• Listen Interval
• SYNC + RTS
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
E
x
C
A
B
D
x
F
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
Implementation
•
•
•
•
•
Rene Motes + Tiny OS
Simplified IEEE 802.11
Message Passing (overhearing avoidance)
S-MAC (Message Passing + Periodic Sleep)
Topology used
Results
• Low performance for high loads?
•Synchronization overhead (SYNC packets)
•Latency
Heterogeneous Wireless MACs
• Basic Service Set (BSS)
• Careful Channel Assignment
• Wireless interference
• Limited orthogonal channels
Motivation
• Exposed Receiver – Hidden Sender
CTS / RTS ?
data
data
ACK
x
Blocked
S1  R1 ?
4-way Handshake?
• Hidden Receiver
• Exposed Sender
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
Intra-BSS Interference Mitigation
• When to use 4-way handshake?
• Client detecting data transmission vs.
Client’s data transmission being detected
• Access point to initiate channel access?
• BSS in center
• Less chance of interference from other BSS
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
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
MAC for Long Dist. 802.11 Mesh
• Motivation
• Extend 802.11 for long haul
• Challenges
• Use off-the shelf hardware
• Low cost
Overview
• Basic Principle
• SynRx & SynTx
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
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
2P Operation on Single Link
• Marker acts as a token
• Loose Synchrony
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 ?
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
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
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
Throughput
2P vs TCP
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?
Q&A