1-1 medium-access control. 1-2 medium access r radio communication: shared medium. m throughput,...
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Medium-Access Control
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Medium Access
Radio communication: shared medium. Throughput, delay, and fairness.
MAC for sensor networks: Must also be energy efficient.
Contention- versus scheduled-based access.
Energy efficiency through switching nodes to low-power, sleep mode.
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Hidden- and Exposed Terminals
A B C
Hidden Terminal Problem:a transmits to B; C does notsense A, and also transmits to B: collision!
A B C D
Exposed Terminal Problem: B is supposed to transmit to A and C to D; but C senses B’s transmission and defers: waste ofbandwidth.
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Contention- versus Scheduled-Based Access
Contention-based access performs well in low load scenarios.
Examples: Packet radio (1970’s):
Aloha, Slotted Aloha. CSMA, CSMA-CD
(Ethernet).
Collisions?
Scheduled-based access are collision free.
Example: TDMA. No nodes within 2 hops
of each other may use the same slot.
Lends itself to energy efficient approaches. Turn nodes off when
they are not sending/receiving.
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MAC in Sensor Networks Sensor networks are a special class of
multihop wireless networks.
Ad-hoc deployment.
Self-configuring.
Unattended.
Battery powered.
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Network Architecture
Sink Sink
•Thousands of nodes.
•Short radio range (~10m).
•Event-driven.
•Hierarchical deployment.
•Fault tolerance.
Information Processing
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MAC Protocols Regulate channel access in a shared
medium.
A
C
B Collision at B
X
No coordination (ALOHA)
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CSMA
Listen before transmittingStations sense the channel before transmittingdata packets.
S R H
XCollision at R
Hidden Terminal Problem
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CSMA with Collision Avoidance Stations carry out a handshake to
determine which one can send a data packet (e.g., MACA, FAMA, IEEE802.11, RIMA).
S R H
RTSCTS
Data
ACK Backoff due to CTS
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Idle Listening
Tx
Rx
Idle Listen
Idle Listen
Tx
Rx
Sleep
Sleep
Sleep Scheduling
Medium Access Energy-efficient channel access is important to prolong the life-time of sensor
nodes. Conventional media-access control protocols waste energy by collisions, and idle
listening. Radios have special sleep mode for energy conservation.
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Achieving energy efficiency
When a node is neither transmitting nor receiving, switch to low-power sleep mode.
Prevent collisions and retransmissions. Need to know Tx, Rx and when
transmission event occurs.
Scheduled-based(time-slotted) MAC Protocols
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Contention-based Channel Access Protocols: S-MAC & T-
MAC
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S-MAC [Ye etal] Features
Collision Avoidance Similar to 802.11 (RTS/CTS handshake).
Overhearing Avoidance All the immediate neighbors of the sender and
receiver goes to sleep. Message Passing
Long messages are broken down in to smaller packets and sent continuously once the channel is acquired by RTS/CTS handshake.
Increases the sleep time, but leads to fairness problems.
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S-MAC Overview
Time is divided in to cycles of listen and sleep intervals.
Schedules are established such that neighboring nodes have synchronous sleep and listen periods.
SYNC packets are exchanged periodically to maintain schedule synchronization.
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S-MAC Operation
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Schedule Establishment
Node listens for certain amount of time. If it does not hear a schedule, it chooses a
time to sleep and broadcast this information immediately.
This node is called the ‘Synchornizer’. If a node receives a schedule before
establishing its schedule, it just follows the received schedule.
If a node receives a different schedule, after it has established its schedule, it listens for both the schedules.
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S-MAC Illustration
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T-MAC[Dam etal]: S-MAC Adaptive Listen
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T-MAC: Early Sleeping Problem
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T-MAC/S-MAC Summary
Simple contention-based channel access with duty cycle-based sleeping.
Restricting channel contention to a smaller window negative effect on energy savings due to collisions.
Requires schedule co-ordination with one hop neighbors.
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Scheduling-based Channel Access Protocols: TRAMA and
FLAMA
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TRAMA: TRaffic-Adaptive Medium Access
Establish transmission schedules in a way that: is self adaptive to changes in traffic, node
state, or connectivity. prolongs the battery life of each node. is robust to wireless losses.
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TRAMA - Overview Single, time-slotted channel access. Transmission scheduling based on two-hop
neighborhood information and one-hop traffic information.
Random access period Used for signaling: synchronization and updating two-
hop neighbor information. Scheduled access period:
Used for contention free data exchange between nodes.
Supports unicast, multicast and broadcast communication.
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TRAMA Features
Distributed TDMA-based channel access.
Collision freedom by distributed election based on Neighborhood-Aware Contention Resolution (NCR).
Traffic-adaptive scheduling to increase the channel utilization.
Radio-mode control for energy efficiency.
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Time slot organization
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Each node maintains two-hop neighbor information.
Based on the time slot ID and node ID, node priorities are calculated using a random hash function.
A node with the highest two-hop priority is selected as the transmitter for the particular time slot.
Neighborhood-aware contention resolution (NCR)[Bao et al., Mobicom00]
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A
C
D
BE
F
3
2
15
8
9
11CWinner
G
H
I
13
3
14NCR does not elect receivers and hence, no support for radio-mode control.
NCR - Example
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TRAMA Components
Neighbor Protocol (NP). Gather 2-hop neighborhood information.
Schedule Exchange Protocol (SEP). Gather 1-hop traffic information.
Adaptive Election Algorithm (AEA). Elect transmitter, receiver and stand-by
nodes for each transmission slot. Remove nodes without traffic from election.
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Neighbor Protocol
Main Function: Gather two-hop neighborhood information by using signaling packets.
Incremental neighbor updates to keep the size of the signaling packet small.
Periodically operates during random access period.
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Packet Formats
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Schedule Exchange Protocol (SEP) Schedule consists of list of intended
receivers for future transmission slots. Schedules are established based on the
current traffic information at the node. Propagated to the neighbors periodically. SEP maintains consistent schedules for the
one-hop neighbors.
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Schedule Packet Format
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Adaptive Election Algorithm (AEA) Decides the node state as either
Transmit, Receive or Sleep. Uses the schedule information obtained
by SEP and a modified NCR to do the election.
Nodes without any data to send are removed from the election process, thereby improving the channel utilization.
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Simulation Results
Delivery Ratio
Synthetic broadcast traffic using Poisson arrivals.
50 nodes, 500x500 area.
512 byte data.
Average node density: 6
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Energy Savings
Percentage Sleep Time Average Length of sleep interval
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TRAMA Limitations
Complex election algorithm and data structure.
Overhead due to explicit schedule propagation.
Higher queueing delay.
Flow-aware, energy-efficient framework.
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TRAMA Summary
Significant improvement in delivery ratio in all scenarios when compared to contention-based protocols.
Significant energy savings compared to S-MAC (which incurs more switching).
Acceptable latency and traffic adaptive.
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Flow-aware Medium Access Framework Avoid explicit schedule propagation.
Take advantage of application. Simple election algorithm to suit systems
with low memory and processing power (e.g., 4KB ROM in Motes).
Incorporate time-synchronization, flow discovery and neighbor discovery during random-access period.
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Flow Information
Flow information characterizes application-specific traffic patterns.
Flows can be unicast, multicast or broadcast.
Characterized by source, destination, duration and rate.
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Example: Data Gathering Application
A
C
E
BD
Sink
Fb
Fc
Fd
Fe
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Flow Discovery Mechanism
Combined with neighbor discovery during random-access period.
Adapted based on the application. for data gathering application, flow
discovery is essentially establishing the data forwarding tree.
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Example
S
Sink initiates neighbor discovery, flow discovery and time synchronization.
Broadcasts periodic SYNC packets.
Potential children reply with SYNC_REQ.
Source reinforces with another SYNC packet.
Once associated with a parent, nodes start sending periodic SYNC broadcasts.
A
B
C
Sink
SYNC
SYNC_REQ
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Election Process
Weighted election to incorporate traffic adaptivity.
Nodes are assigned weights based on their incoming and outgoing flows.
Highest priority 2-hop node is elected as the transmitter.
A node listens if any of its children has the highest 1-hop priority. Can switch to sleep mode
if no transmission is started.
S
A
B
C
Sink ws=0
wc=1
wc=1
wa=3
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Simulation Results (16 nodes, 500x500 area, CC1000 radio, grid topology, edge sink)
Delivery Ratio Queueing Delay
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FLAMA Summary
Simple algorithm that can be implemented on a sensor platform.
Significant performance improvement by application awareness.