mac for wireless sensor networks

Present by: Yang LiuAICIP ResearchSep. 7, 2004

MAC for Wireless Sensor Networks

Multiple Access Problem

Example:Cocktail party – many people gather together in a large room. broadcast medium – air

Human rules: Give everyone a chance to speak.Don’t speak until you are spoken to.Don’t interrupt when someone is speakingRaise your hand if you have a questionDon’t fall asleep when someone else is talking…

Problem Statement

The same thing in Sensor Networks?Multiple sensor nodes want to communicate in a common geographic area using shared communication medium. Such as RF.

Problem: How should we coordinate and schedule the access of multiple sending and receiving nodes to a single shared broadcasting channel?How should we allocate our resources so that as many nodes as possible can communicate simultaneously?

5

7

1

2

4

36

Physical

MAC

Routing

Application

Motivation

What are the design goals?High throughput or good channel utilization;

Low latency or even plus real time support;

Fairness;

Efficient;

Simple and decentralized;

Wireless Medium Access Outline

Wireless Multiple Access Protocols

Scheduled Based Protocols

Contention Based Protocols

Other Protcols

FDMA

TDMA

CDMA

ALOPHA/SLOTTED ALOPHA

CSMA

CSMA/CA

Polling

MACA/MACAW IEEE 802.11

IEEE 802.15.4

TDMA/FDMA

TDMA:Divide time into frames Further divide each frame into N slots (number of competing nodes )Each node will have R/N rate. R is the total bandwidth.

FDMA:Divide the R bps channel into different frequency bands.

Assign each frequency to one of N nodes.

Cons:A node is limited to a bandwidth R/N, even when it is the only node with packets to send

Centralized Scheduling and synchronization is needed

Link Bandwidth

FDMA

SubBandwidth

SubBandwidth

1 2 3 N 1 2 3 N 1 2 3 N

Frame Slot

TDMA

CDMA

CDMA (code Division Multiple Access)

unique “code” assigned to each user.

All users share the same frequency, but each user has own “chipping” sequence (ie, code)

encoded signal = (original signal) X (chipping sequence)

decoding: innerproduct of encoded signal and chipping sequence

chipping sequences must be chosen orthogonal to each other





Other Protcols

FDMA

TDMA

CDMA


CSMA

CSMA/CA

Polling


IEEE 802.15.4

ALOHA/Slotted ALOHA

Pure ALOHA:Transmit a packet when it is generated

If the data transmission succeed, receiver responses with a ACK.

In case of collision, sender retransmit after a random period.

Slotted ALOHA:Time is divided into slots and nodes only transmit at the beginning of slots.

The length of a time slot equals to the length of the packet duration.

No partial overlap between packets.

Pros and ConsSimplePoor use of the channel capacity: 18% for Pure ALOHA and 36% for Slotted ALOHA.

A

B

C

D

E

F

Pure ALOHA

Slotted ALOHA

CSMA

Carrier Sense Multiple Access Key issues: it should not send if someone else is sending.

Strategies: Persistent and non-persistent CSMA

Persistent Strategy: reduces chance of collisions and also reduces the efficiency

Non-persistent Strategy: increases the chance for collisions.

Sense Channel

Free?

Wait

Send out the frame

Yes

No

Sense Channel

Free?

Send out the frame with Probability p

Yes

No

Non-persistant P-Persistant

DATA

CSMA/CA

Carrier Sense Multiple Access/Collision Avoidance Hidden Terminal and Exposed Terminal Problems:

RTS/CTS/DATA: Proposed in MACA

RTS/CTS/DATA/ACK: Proposed in MACAW

B CA D B CA DX

A B

RTS

CTSDATA

Hidden Terminal Exposed Terminal

A B

RTS

CTS

MACA

MACAW

ACK

A B

RTS

CTSDATADATA ACKDS

DCF ( Distributed Coordination Function) with Virtual Carrier Sense

Backoff Time ( Exponential Backoff Algorithm ) Backoff Time = rand() X SlotTimewhere rand() = [0,CW] CWmin <= CW <= CWmaxSlotTime = The value of the corresponding PHY characteristicCWnew = ( CWold + 1) x PF - 1 where PF =2

IEEE 802.11

Sender

Receiver

Other Nodes

RTSDIFS

CTS

SIFS

DATA

SIFS

ACK

SIFS DIFS

NAV(RTS)

NAV(CTS)

Contention Window

DIFS: Distributed Inter Frame Space 802.11 FH PHY 128 microseconds

SIFS: Short Inter Frame Space 28 micorseconds

NAV: Network Allocation Vector

PCF ( Point Coordination Function)a wireless channel has a superframe structure (contention Free Repetition Interval) that consists of a contention free period (CFP) and contention period (CP).

IEEE 802.11

Super Frame (Contention Free Repetition Interval)

Contention Free Period Contention Period

Becon Poll1 Data + Poll2 ACK END RTS DATA Becon

Becon

Becon

Becon

Becon

Poll1

Data + Poll2 ACK

CTS ACK

NAV

NAV

Point Coordinator

Station 1

Station 2

DIFS

SIFS

PIFSSIFSPIFS

SFIS<PIFS<DIFS

Null Frame





Other Protcols

FDMA

TDMA

CDMA


CSMA

CSMA/CA

Polling


IEEE 802.15.4

Polling

All data exchanges made through the primary deviceprimary device controls the channel and is initiator of the session

polling delaychannel becomes inoperative if master device failsExample: 802.15.4

IEEE 802.15.4 ( ZigBee )

Industry standard through application profiles running over IEEE 802.15.4 radios

Target applications are sensors networks, interactive toys, smart badges, remote controls, and home automation.


ZigBee End Device (RFD or FFD)

ZigBee Router (FFD)

ZigBee Coordinator (FFD)

Mesh Link

Star Link

Specify three DevicesNetwork CoordinatorFull Function Device (FFD) Powerful and can talk to any deviceReduced Function Device (RFD) Can only talk to a FFD.


CoordinatorNetwork Device

DATA RequestACK

DATAACK

Communication in a non-beacon Enabled Network


DATA

ACK (Optional)


DATA Request

ACK

DATA

ACK

Beacon


DATA

ACK (Optional)

Beacon

Communication in a beacon Enabled Network

BeaconControl information

Allocate GTS

Synchronization

CAPAllows contention via CSMA

CFPTime slot allocation specified in the beacon

Reserved bandwidth for DEV

Medium Access for Sensor Networks

Characteristics of MAC in WSNEnergy Efficiency

QoS provision

Self-Organization

Simple and distributed solution

Energy Waste of MAC

Idle listeningListening to receive possible traffic that is not sent

major source of energy inefficiency

CollisionPacket retransmission increases energy consumption.

Overhearingpicking up packets that are destined to other nodes

Communication OverheadRequired frame header to implement MAC

SMAC

Approaches: Contention-based MAC with various energy-conserving features.

Periodic listen and sleep

Collision avoidance ( IEEE 802.11)

Overhearing avoidance

Put nodes into sleep when neighbors are talking

Massage passing

Long message is fragmented and sent in burst

RTS/CTS reserve time for entire message

Problems:Fix duty cycle while traffic in sensor networks are busty.

Multiple on/sleep schedule on the edge nodes which results in unbalanced energy cost and losing communication coverage

On/sleep synchronization involves much communication overhead.

sleeplisten listen sleep

TMAC

Fixed duty cycle like S-MAC, is not optimal.The nodes must be deployed with an active time that can handle the highest expected load.Whenever the load is lower than that, the active time is not optimally used and energy will be wasted on idle listening

An active period ends when no activation event has occurred for a time TA

TA > CW + R + TCW : the length of the contention intervalR : the length of RTS packetT : the short time between the end of the RTS packet and the beginning of the CTS packet

EQ-MAC ROADMAP

MotivationEnergy Efficiency

Why energy efficiency? - Sensor networks consist of a couple of powerful sensor nodes which we call sensor sink and a bunch of battery powered sensor nodes which can not be recharged once running out of power. How? - Communication is the main contributor for energy consumption.

To avoid unnecessary communications ( transmitting, receiving and idle listening)

To balance energy cost among all nodes based on their residual energy.

A simple MAC Layer Protocols is preferred.

Differentiate ServicesWhat is service for sensor node? -- Clearly data transmissionWhy needs to differentiate Services?

Sensing reading from different types of sensors may have different importance for a specified application

Aggregated data is clearly more important the normal sensing data.

A data packet which is already transmitted for several hops maybe need higher transmitting bandwidth compared with new generated data packets or data packets which have been transmitted for less hops.

Any More Reasons?

Assumptions

Energy Constrains

Highly redundancy ( in-network processing are used to reduce the redundancy )

Periodical and busty traffic are both existed

Periodical traffic: periodical sensing the fieldBusty traffic: the traffic triggered by sensing event.

Multi-hops wireless data communications.

Queue Architecture

Weighted Queuing Algorithm

Parameters: weights for each queueWhile (at the starting of a new frame)

if the size of instant queue > 0

next packet = the first packet in the instant queue

else

Calculate rate using MAX-MIN Fairness Algorithm;

Find out which packet needed to be served first by using Packetized Generalized Processor Sharing (GPS) and set it

to next packet;

end

End

MAX-MIN Fairness Algorithm

R

W1

W2

WiR1

N flows share a link of rate C, flow i's expectation rate is wi. It is allocated rate ri.

Algorithm Outline:1. Pick the flow i with the smallest requested rate.

2. If wi < C/N, then set ri = wi ;

3. If wi > C/N, then set ri = C/N ;

4. Set N = N -1, C = C - ri ;

5. If N > 0 goto 1

Packetized GPS

R(f1) = 0.1

R(f3) = 0.3R1

C

R(f4) = 0.3

R(f2) = 0.3

Order of service for the four queues:

… f1, f2, f2, f2, f3, f3, f3, f4, f4, f4, f1,…

Generalized Processor Sharing (GPS)

Packetized GPSserve a whole packet at a timeDetermine what time a packet, p, would complete if we served flows bit-by-bit. Call this the packet’s finishing time, F.Serve packets in the order of increasing finishing time.

The time of each sensor node has been divided into frames and then further divided into time slots.

Each frame consists of Contention Period (CP) and data Transmission Period (TP).RTS/CTS/TS/DATA/ACK are used for packet transmission.Energy saving achieves from the node (except sender and receiver ) sleeping during TP.The starting time of the first frame can be decide through network-wide broadcasting by sensor sink and can be resynchronized by the same way.Only Loosely synchronization is required because of random access nature

Power Saving MACAW

The reasons for framing: Each node contend for channel at the starting time of a frame which ensure the fairness of node transmission. It also provides the fundamental requirement of our next proposed Loosely Prioritized Random Access protocols. Since each node contends the channel at the same time, by assigning them with different waiting time, we can coordinate the data transmission among multiple nodes.

The same frame schedule of all sensor nodes make communication easier since all the nodes can always hear the RTS/CTS

messages. In order to achieve the same thing,

SMAC has extra sacrifice for consuming more

energy for the edge nodes.

Simple and have a very good scalability for largeScale Wireless Sensor Networks.

Power Saving MACAW

Loosely Prioritized Random Access

Provide a mechanism to assign the channel access to high transmission priority sensor nodes.Solution:

Measuring the transmission priority of a sensor node

Divide the Contention Period into N priority levels. Distribute the random access into sub contention period

which is decided by the priority level of sensor node.

Loosely Prioritized Random Access

Solution: (cont)Division of Contention Periods.

LPRA algorithm for random access

Loosely Prioritized Random AccessLPRA Algorithm:If ( next transmitting packet != null ) && ( at the starting time of a frame)

calculate the urgency of the packet

calculate the priority level of the packet

calculate the contention time (CT)

wait for CT;

if ( no packet received during CT )

while ( current time < end of CP of this frame )

transmit RTS and wait for CTS;

listening the channel;

if ( collision happen )

recalculate CT;

retransmit RTS and wait for CTS;

end

if (receive CTS)

break;

end

end

transmit TS;

transmit Data and Wait for ACK;

else

Loosely Prioritized Random AccessLPRA Algorithm: (cont)

else

while ( current time < the end of CP of this frame)

if ( receive packet is RTS or CTS )

stop contend for channel;

wait for TS;

end

if ( receive packet is TS )


sleep to the duration specified in TS;

end

if ( detect collision) && ( collision time < CT )


wait for TS;

end

end

if ( current time > the end of CP of this frame ) && ( no TS received )

sleep to the beginning of next frame;

end

end

if ( time > the end of CP of this frame ) && ( nothing happened )

sleep until the beginning of next frame;

end

Loosely Prioritized Random AccessCollision Reduction:

High priority transmission nodes continue to access the channel when meeting collision during the Contention Period (CP). Low priority nodes constrain accessing the channel when they detection collision from high priority node or successfully receives RTS, CTS, TS.Apply a pseudo random number generator which is based on optimal Probability distribution function which minimizing the happening of

collision.

1 CWcollision

1 CW without collision

Differentiate ServicesApplication Level

Extra four bits in application header to represent the criticality of packets

Sensor Types ( different sensors )

Data Types ( Periodic Sensing data or aggregation data )

Control Messages ( for routing, security usage )

Synchronization packets, reconfiguration packets of sensor networks has the highest priority and needed to be transmit instantly.

Network LevelExtra three bits at network header to represents the number of hops that a packet has been transmitted.

MAC LevelCombine both information from Application layer and network layer to form the MAC level priority which is used for real transmitting.

Collision RecoveryRTS/CTS control packets collision recovery can been seen in LPRAFor data packet collision, it involves two scheme

High priority data packets, generate a CT from [0, CW/2]Low priority data packets, increase the priority level by one which means increasing the CT to average CW.Since most low priority packets are periodic sensor reading, therefore,

it is

Possible that other nodes have already transmitted the sensing data. So we

Can defer the data transmission by increasing priority levels. Furthermore,

If difference of sensing time and current time is beyond certain threshold,

We can delete the sensing data directly.

Next TimeTalk on related work of Sensor Mac designPresents Comprehensive simulation environment and simulation results.

Any Questions?

mac for wireless sensor networks

Documents

access of multiple sending

multiple sensor nodes

multiple access problemexample

n nodes

channel capacity

chance of collisions

alohaslotted alohapure

shared communication