cs 15-849e: wireless networks (spring 2006) mac layer discussion leads: abhijit deshmukh sai vinayak...
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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
Listen
Wait (random)
SYNC
Wait (random)
Rebroadcast
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 C A B D Fxx
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
data
ACK S1 R1 ?
data
Blocked
x
CTS / RTS ?
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 23 getting unfair treatment?• Client 3 is exposed receiver• Receiver 1 is not interfered by 23• 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