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Media Access Control in Wireless Sensor Networks - II

Media Access Control in Wireless Sensor Networks - II

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What We Have Learned Last TimeWhat We Have Learned Last Time

B-MAC = ?

CSMA + LPL + Noise Floor Estimation + Explicit ACK

X-MAC = ?

B-MAC + Early ACK + Encoded preamble

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OutlineOutline

Overview

TDMA/CSMA

Advantages and disadvantages

S-MAC

Z-MAC

Design concepts, performance evaluation and issues

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Classification of Multiple Access ProtocolsClassification of Multiple Access Protocols

Multiple Access Protocols

Random Access Controlled Access

CSMA

TDMA (FDMA, CDMA)

ALOHA Static channel allocation

B-MAC

X-MACX-MAC

S-MACZ-MAC

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CSMACSMA

CSMA: listen before transmit:If channel sensed idle: transmit entire pkt

If channel sensed busy, defer transmission

Persistent CSMA: retry immediately with probability p when channel becomes idle (may cause instability)Non-persistent CSMA: retry after random interval

human analogy: don’t interrupt others!

I want to talk now

Me too

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TDMATDMA

TDMA: Time Division Multiple Access

Access to channel in "rounds"

Each station gets a fixed length slot (length = pkt Tx time) in each round

Unused slots go idle

Example: 6-station LAN, 1,3,4 have pkts, slots 2,5,6 idle

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Medium Access ParadigmsMedium Access Paradigms

Contention Based (CSMA)

Random-back off and carrier-sensingSimple, no time synch, and robust to network changesHigh idle listening and overhearing overheads• Solve this by duty cycling

TDMA Based (or Schedule based)

Nodes within interference range transmit during different times, so collision freeRequires time synch and not robust to changes.Low throughput and high latency even during low contention.Low idle listening and overhearing overheads• Wake up and listen only during its neighbor transmission

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TDMA vs. CSMA for Sensor NetworksTDMA vs. CSMA for Sensor Networks

Parameter TDMA CSMA

Energy for Synchronization

Bad Good

Throughput Good for multiple sources

Good for single source

complexity Bad Good

Fairness Good Bad

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TDMA vs. CSMA for Sensor Networks cont …TDMA vs. CSMA for Sensor Networks cont …

Parameter TDMA CSMAScalability Bad Good

Latency Bad/Good Good/Bad

Dealing with node failures, new node arrivals Bad Good

Energy for collision Avoidance

Good Bad

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How about combine TDMA and CSMA?How about combine TDMA and CSMA?

S-MAC: for the benefit of Energy Efficiency

Z-MAC: for the benefit of throughput

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Z-MAC: a Hybrid MAC for Wireless Sensor Networks

-Injong Rhee, Ajit Warrier, Mahesh Aia and Jeongki Min

Z-MAC: a Hybrid MAC for Wireless Sensor Networks

-Injong Rhee, Ajit Warrier, Mahesh Aia and Jeongki Min

Medium Access Control with Coordinated Adaptive Sleeping for Wireless Sensor Networks

Wei Ye, John Heidemann and Deborah Estrin

Medium Access Control with Coordinated Adaptive Sleeping for Wireless Sensor Networks

Wei Ye, John Heidemann and Deborah Estrin

Sensys 2005

TON 2004

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S-MAC: IntroductionS-MAC: Introduction

S-MAC stands for Sensor-MAC

Key Idea in SMAC:

Combine key advantages of scheduled (TDMA) and unscheduled (CSMA) protocols

S-MAC = lite-802.11 + Scheduling

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S-MAC: Energy savingsS-MAC: Energy savings

S-MAC tries to reduce wastage of energy from at least 3 sources of energy inefficiency:

Nodes periodically sleep to reduce energy consumption in listening to an idle channel

Resolve contention by using RTS and CTS

Avoid overhearing – S-MAC sets the radio to sleep during transmissions of other nodes

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Periodic sleepingPeriodic sleeping

Problem: Idle listening consumes significant energy

Solution: Periodic listen and sleep

Turn off radio when sleeping

Reduce duty cycle to ~ 10% (120ms on/1.2s off)

listen listensleep sleep

Difference between S-MAC toggling and B-MAC toggling?

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Deal with global synchronization Deal with global synchronization

Node 1

Node 2

sleeplisten listen sleep

sleeplisten listen sleep

Schedule 2

Schedule 1

Schedules can differ, prefer neighbouring nodes to have same schedule

Border nodes may have to maintain more than one schedule.

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Overhearing AvoidanceOverhearing Avoidance

Problem:

Receive packets destined to others

Solution: Sleep when neighbors talk

Who should sleep?• All immediate neighbors of sender and receiver

How long to sleep?• The duration field in each packet informs other nodes the sleep

interval

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SMAC: Pros and ConsSMAC: Pros and Cons

Pros

Well-designed, complete protocol that addresses deficiencies of 802.11 if applied to a sensor network.

Schedules sleep and transmit times to enable low-power data transfer with reasonable-latency.

Cons

SMAC incurs some drawbacks of TDMA schemes• Topology maintenance, need for synchronization, additional

complexity at border nodes between two schedules…

Monolithic system architecture similar to 802.11• Combines carrier sense, link-layer reliability, RTS/CTS and sleep

scheduling into MAC layer.

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Z-MAC Motivation: Throughput Z-MAC Motivation: Throughput

# of Contenders

Channel Utilization

IDEAL

CSMATDMA

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Z-MAC: Hybrid Contention ResolutionZ-MAC: Hybrid Contention Resolution

Z-MAC (Zebra MAC) – a Hybrid MAC protocol combines the strengths of both CSMA and TDMA at the same time discounting their weaknesses

Z-MAC uses a base TDMA schedule as a hint to schedule the transmissions of the nodes, and it differs from TDMA by allowing non-owners of slots to 'steal' the slot from owners if they are not transmitting

MAC Channel Utilization

CSMA

TDMA

Low Contention High Contention

High Low

Low High

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Z-MAC FeaturesZ-MAC Features

Adaptability to the level of contention in the network

Under low contention behaves like CSMA

Under high contention behaves like TDMA

# of Contenders

Channel Utilization

IDEAL

CSMATDMA

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Z-MAC DesignZ-MAC Design

Z-MAC has the setup phase in which the following operations are run in sequence:

1. Neighbor discovery

2. Time slot assignment (DRAND)

3. Local frame exchange

4. Time synchronization

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Neighbor discoveryNeighbor discovery

When a node starts up, it runs a neighbor discovery protocol

Periodically broadcasts a ping to its one-hop neighborsPing message contains the current list of its one-hop neighbors

Through this message, each node gathers neighbor information

Q: How many hops neighbor information is need to avoid interference?

1 Hop, 2 Hop, More than 2 Hop? What’s the reality?

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Timeslot AssignmentTimeslot Assignment

The two-hop neighbor list is used as an input to a time-slot assignment algorithm

Current implementation of Z-MAC uses DRAND – a distributed implementation of RAND to assign time slots to every node in the network

DRAND ensures no two nodes within a two-hop communication neighborhood are assigned to the same slot.

This assignment guarantees that no transmission by a node to any of its one-hop neighbors interferes with any transmission by its two-hop neighbors.

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DRAND slot assignmentDRAND slot assignment

C D

A

FB

C D

A E

B

E

F

Radio Interference Map

Input Graph

C D

A E

B FDRAND slot assignment

0

0

1

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1

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Z-MAC Transmission ControlZ-MAC Transmission Control

A node can be in one of two modes:

Low Contention Level (LCL) or

High Contention Level (HCL)

Node is in HCL only:

when it receives an explicit contention notification (ECN) message from a two-hop neighbor within the last tECN period.

Otherwise, the node is in LCL.

A node sends an ECN when it experiences high contention

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Z-MAC Transmission Control cont…Z-MAC Transmission Control cont…

In LCL, any node can compete to transmit in any slot

But in HCL, only the owners of the current slot and their one-hop neighbors are allowed to compete for the channel

In both modes, owners have higher priority over non-owners.

If a slot does not contain an owner or its owner does not have data to send, non-owners can steal the slot.

This feature achieves high channel utilization even under low contention as a node can transmit as soon as the channel is available.

Z-MAC implements LCL and HCL using the back off, CCA and LPL interfaces of B-MAC

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Transmission rule (for the owner)Transmission rule (for the owner)

Owner takes a random back off within a fixed time period To

When the back off timer expires, it runs CCA and if the channel is clear, transmits the data.

If the channel is not clear, then it waits until the channel is not busy and repeats the above process.

Busy Owner Accessing Channel

Random Back off within To (Contention Window)

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Transmission rule (non owner - LCL)Transmission rule (non owner - LCL)

Waits for To and then performs a random back off within a contention window [To, Tno]

When the back off timer expires, it runs CCA and if the channel is clear, then it starts transmission.

If the channel is not clear, it waits until the channel is clear, and repeats the above process.

Busy Non-owner Accessing ChannelTo

Random Back off within [To, Tno] (Contention Window)

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Z-MAC Transmission Control (Continued)Z-MAC Transmission Control (Continued)

A A B B BA A A A B B B

TDMA and Z-MAC under high contention (Two node example)

A A A A A A

TDMA under no contention (Two node example)

A A A A A AA A A A A A

Z-MAC under no contention (Two node example)

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Z-MAC – Performance EvaluationZ-MAC – Performance Evaluation

Setup:

Single-hop, Two-hop and Multi-hop topology experiments on Mica2 motes.

Comparisons with B-MAC (default MAC of Mica2), with different back-off window sizes

Metrics: Throughput & energy efficiency

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Experimental Setup – Single HopExperimental Setup – Single Hop

Single-Hop Experiments:

Mica2 motes equidistant from one node in the middle.

All nodes within one-hop transmission range.

Tests repeated 10 times and average/standard deviation errors reported.

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Z-MAC – Two-Hop ExperimentsZ-MAC – Two-Hop Experiments

Setup – Two-Hop

Dumbbell shaped topology

Transmission power varied between low (50) and high (150) to get two-hop situations.

Aim – See how Z-MAC works when Hidden Terminal Problem manifests itself

Sources SourcesSink

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Experimental Setup - Test bedExperimental Setup - Test bed

42 Mica2 sensor motes in a Lab.

Wall-powered and connected to the Internet via Ethernet ports.

Programs uploaded via the Internet, all mote interaction via wireless.

Links vary in quality, some have loss rates up to 30-40%.

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Multi Hop Results – ThroughputMulti Hop Results – Throughput

Why B-MAC is better than Z-MAC in low traffic ?

Why B-MAC is worse than Z-MAC in high traffic ?

Z-MAC

B-MAC

MULTI-HOP

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Multi Hop Results – Energy EfficiencyMulti Hop Results – Energy Efficiency

B-MAC

Z-MAC HCL

MULTI-HOP

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Summary of Z-MACSummary of Z-MAC

Hybrid MAC :Combines strengths of TDMA and CSMA

Uses the TDMA schedule created by DRAND as a 'hint' to schedule transmissions

The owner of a time-slot always has priority over the non-owners while accessing the medium.

Unlike TDMA, non-owners can 'steal' the time-slot when the owners do not have data to send.

This enables Z-MAC to switch between CSMA and TDMA depending on the level of contention.

Hence, under low contention, Z-MAC acts like CSMA (i.e. high channel utilization and low latency), while under high contention, Z-MAC acts like TDMA (i.e. high channel utilization, fairness and low contention overhead).

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DiscussionDiscussion

Limitation of Z-MAC

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Take Away MessagesTake Away Messages

Hybrid Synchronous MAC S-MAC = lite-802.11 + Scheduling

Z-MAC = CSMA within TDMA slots

Asynchronous MAC B-MAC = CSMA + LPL + Noise Floor Estimation + Explicit ACK

X-MAC = B-MAC + Early ACK + Encoded preamble

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MAC SummaryMAC Summary

Commercial MAC (802.X, Bluetooth) are suitable for wireless LAN with much more powerful devices. Energy is secondary concern compared with throughput.

Asynchronous MAC (B/X-MAC) is flexible and works well in low traffic scenario (why is so widely used!)

Hybrid synchronous MAC (S/Z-MAC) can achieve better performance in high traffic scenario.