CS 15-849E: Wireless Networks (Spring 2006)

MAC LayerDiscussion 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 levelCarrier 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 DifferencesSimilarities• MAC protocol implementations• Extend 802.11 for a specific environment• Others?Differences• Deployment scenarios• Energy Saving, Long haul, Heterogeneity• Writing Style• Others?

Q & A