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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.