MAC Protocol for PAN

MAC Protocol for PAN

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Medium Access Control (MAC)Medium Access Control (MAC)

Controlling when to send a packet and when to listen for a packet are perhaps the two most important operations in a wireless network Especially, idly waiting wastes huge amounts of energy

This chapter discusses schemes for this medium access control that are Suitable to mobile and wireless networks Emphasize energy-efficient operation

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OverviewOverview

Principal options and difficultiesContention-based protocolsSchedule-based protocolsIEEE 802.15.4

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Main optionsMain options

Wireless medium access

Centralized

Distributed

Contention-based

Schedule-based

Fixedassignment

Demandassignment

Contention-based

Schedule-based

Fixedassignment

Demandassignment

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Principal options and difficultiesPrincipal options and difficulties

Medium access in wireless networks is difficult mainly because of Impossible (or very difficult) to send and receive at the

same time Interference situation at receiver is what counts for

transmission success, but can be very different from what sender can observe

High error rates (for signaling packets) compound the issues

Requirement As usual: high throughput, low overhead, low error rates,

… Additionally: energy-efficient, handle switched off devices!

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Requirements for energy-efficient MAC protocolsRequirements for energy-efficient MAC protocols

Recall Transmissions are costly Receiving about as expensive as transmitting Idling can be cheaper but is still expensive

Energy problems Collisions – wasted effort when two packets collide Overhearing – waste effort in receiving a packet

destined for another node Idle listening – sitting idly and trying to receive when

nobody is sending Protocol overhead

Always nice: Low complexity solution

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Centralized medium accessCentralized medium access

Idea: Have a central station control when a node may access the medium Example: Polling, centralized computation of TDMA

schedules Advantage: Simple, quite efficient (e.g., no collisions),

burdens the central station

Not directly feasible for non-trivial wireless network sizes

But: Can be quite useful when network is somehow divided into smaller groups Clusters, in each cluster medium access can be controlled

centrally – compare Bluetooth piconets, for example

Usually, distributed medium access is considered

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Schedule- vs. contention-based MACsSchedule- vs. contention-based MACs

Schedule-based MAC A schedule exists, regulating which participant may use

which resource at which time (TDMA component) Typical resource: frequency band in a given physical space

(with a given code, CDMA) Schedule can be fixed or computed on demand

Usually: mixed – difference fixed/on demand is one of time scales

Usually, collisions, overhearing, idle listening are not issues Needed: time synchronization!

Contention-based protocols Risk of colliding packets is deliberately taken Hope: coordination overhead can be saved, resulting in

overall improved efficiency Mechanisms to handle/reduce probability/impact of

collisions required Usually, randomization used somehow

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OverviewOverview

Principal options and difficultiesContention-based protocols MACA S-MAC, T-MAC Preamble sampling, B-MAC PAMAS

Schedule-based protocolsIEEE 802.15.4

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ALOHAALOHA

The simplest possible medium access protocol: Just talk when you feel like it

Formally: Whenever a packet should be transmitted, it is transmitted immediately

Introduced in 1985 by Abrahmson et al., University of Hawaii

Goal: Use of satellite networks

Packets are transmitted at arbitrary times

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• The earliest random-access method, was developed at the

Univ. of Hawaii in the early 1970s.

• Base station is central controller

• Base station acts as a hop

• Potential collisions, all incoming data is @ 407 MHz

ALOHA network – Multiple Access

12 Frames in a pure ALOHA network

Pure ALOHA Random Access

Each station sends a frame whenever it has a frames to send.

However, there is only one channel to share, there is the possibility of collision between frames from different stations.

ALOHA

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Procedure for pure ALOHA protocol

ALOHA

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ALOHA – Analysis ALOHA – Analysis

ALOHA advantages Trivially simple No coordination between participants necessary

ALOHA disadvantages Collisions can and will occur – sender does not check

channel state Sender has no (immediate) means of learning about the

success of its transmission – link layer mechanisms (ACKs) are needed

ACKs can collide as well

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ALOHA – Performance ALOHA – Performance

Assume a Poisson arrival process to describe packet transmissions

Infinite number of stations, all behave identically, independently

Time between two attempts is exponentially distributed Let G be the mean number of transmission attempts per

packet length All packets are of unit time length Then:

For a packet transmission to be successful, it must not collide with any other packet

How likely is such a collision? Question: How long is a packet “vulnerable” by other

transmissions?

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ALOHA – Performance ALOHA – Performance

A packet X is destroyed by another packet either Starting up to one

packet time before X Starting up to

immediately before the end of X

Hence: Packet is successful if there is no additional transmission in two packet times

– Probability: P0 = P (1 transmission in two packet times) = 2Ge-

2G

– Throughput S (G) = 1 Packet / 2 time units * Probability = Ge-2G

– Optimal for G = 0.5, S = 1/(2e) = 1/(2*2.718) = 0.185

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A slight improvement: Slotted ALOHAA slight improvement: Slotted ALOHA

ALOHA’s problem: Long vulnerability period of a packet

Reduce it by introducing time slots – transmissions may only start at the start of a slot Slot synchronization is assumed to be “somehow”

available

Result: Vulnerability period is halved, throughput is doubled S(G) = Ge-G

Optimal at G=1, S=1/e

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Slotted ALOHA

We divide the time into slots of Tfr s and force the station to send only at the beginning of the time slot.

Slotted ALOHA

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The throughput for slotted ALOHA is

S = G × e−G .The maximum throughput Smax = 0.368 when G = 1.

Note

Slotted ALOHA

S is the average number of successful transmissions,

called throughput.

G is the average number of frames generated by the

system during one frame transmission time.

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Vulnerable time for slotted ALOHA protocol

Slotted ALOHA vulnerable time = Tfr

Slotted ALOHA

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Performance dependence on offered loadPerformance dependence on offered load For (slotted) ALOHA, closed form analysis of throughput

S as function of G is simple

! Anything but a high-performance protocol In particular: throughput collapses as load increases!

1 G

S1

Ideal

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CAMA/CA and NAVCAMA/CA and NAV

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DIFS – distributed interframe space, NAV – network allocation vector

SIFS – short interframe space, RTS – request to send, CTS – clear to send

* Collision only occurs during handshake period (RTS, CTS)

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802.11802.11

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RTS : Request to Send CTS : Clear to SendACK : AcknowlegmentDIFS : Distributed Interframe SpaceSIFS : Short Interfrace Space

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Problems for the MAC-ProtocolProblems for the MAC-Protocol

Hidden Terminal Problem

Exposed Terminal Problem

A B C

A B C D

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Exposed Terminal ProblemExposed Terminal Problem

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A

Distributed, contention-based MACDistributed, contention-based MAC

Basic ideas for a distributed MAC ALOHA – no good in most cases Listen before talk (Carrier Sense Multiple Access,

CSMA) – better, but suffers from sender not knowing what is going on at receiver.

Receiver additionally needs some possibility to inform possible senders in its vicinity about impending transmission (to “shut them up” for this duration

B C D

Hidden terminal scenario:

Also: recall exposed terminal scenario

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Main options to shut up senders Main options to shut up senders

Receiver informs potential interferers while a reception is on-going By sending out a signal indicating just that Problem: Cannot use same channel on which actual

reception takes place Use separate channel for signaling Busy tone protocol

Receiver informs potential interferers before a reception is on-going Can use same channel Receiver itself needs to be informed, by sender, about

impending transmission Potential interferers need to be aware of such information,

need to store it

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Multiple Access with Collision Avoidance (MACA)Multiple Access with Collision Avoidance (MACA)

Sender B asks receiver C whether C is able to receive a transmissionRequest to Send (RTS)

Receiver C agrees, sends out a Clear to Send (CTS)

Potential interferers overhear either RTS or CTS and know about impending transmission and for how long it will last

Store this information in a Network Allocation Vector

B sends, C acks MACA protocol (used

e.g. in IEEE 802.11)

A B C D

RTS

CTS

Data

Ack

NAV indicates busy medium

NAV indicates busy medium

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RTS/CTS RTS/CTS

RTS/CTS ameliorate, but do not solve hidden/exposed terminal problemsExample problem cases:

A B C D

RTS

CTS

Data

A B C D

RTS

RTS

CTS

RTS

RTSCTS

CTSData

Data

Ack

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MACA Problem: Idle listeningMACA Problem: Idle listening

Need to sense carrier for RTS or CTS packets In some form shared by many CSMA variants; but e.g. not by

busy tones Simple sleeping will break the protocol

IEEE 802.11 solution: ATIM (Ad-hoc Traffic Indication Message) windows & sleeping Basic idea: Nodes that have data buffered for receivers send

traffic indicators at pre-arranged points in time Receivers need to wake up at these points, but can sleep

otherwise Parameters to adjust in MACA

Random delays – how long to wait between listen/transmission attempts?

Number of RTS/CTS/ACK re-trials? …

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STEM Sparse Topology and Energy Management ProtocolSTEM Sparse Topology and Energy Management Protocol

Two channels– Wakeup channel

• On the wakeup channel data is announced

– Data Channel• Otherwise the data channel is

always in sleep modeStatus of a sensor

– Monitor state • nodes are idle, no transmission

– Transfer stateSTEM-B

– Transmitter wakes up the receiver by a beacon on the wakeup channel

– no RTS/CTSSTEM-T

– Transmitter sends busy tone signal on the wakeup channel to get the receiver‘s attention

A node periodically wakes up to listen to the wakeup channel, and keeps awake upon receiving a wakeup packet intended for itself.

The wakeup packets are sent by some nodes that have packets to send.

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Sensor-MAC (S-MAC)Sensor-MAC (S-MAC)

MACA’s idle listening is particularly unsuitable if average data rate is low–Most of the time, nothing happens

Idea: Switch nodes off, ensure that neighboring nodes turn on simultaneously to allow packet exchange (rendezvous)

–Only in these active periods, packet exchanges happen

–Need to also exchange wakeup schedule between neighbors

–When awake, essentially perform RTS/CTS

Use SYNCH, RTS, CTS phases

Wakeup period

Active period

Sleep period

For SYNCH For RTS For CTS

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S-MAC FramesS-MAC Frames

S-MAC adopts a message passing concept long messages are

broken into small frames

only one RTS/CTS communication for each messages

each frame is acknowledged separately

each frame contains the information about the message length

The NAV (not available) variable of suppressed neighbors is adjusted appropriatelyProblems: Fairness

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Mediation Device ProtocolMediation Device Protocol

Goal: Avoid useless listening on the channel for messagesUses: mediation device (MD) which is available all the timeProtocol

– Sender B sends RTS to MD– MD stores this information– Receiver C sends query to MD– MD tells reciever C when to wake up– C sends CTS to B (now in sync)– B sends data– C acknowledges– C returns to old timing

Main disadvantage:– MD has to be energy independent– Solution: Distributed Mediation

Device Protocol• Nodes randomly wake up and

serve as mediation deviceProblem: no guarantees on full coverage of MD

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B-MAC (Berkeley MAC)B-MAC (Berkeley MAC)

Combines several of the above discussed ideas Takes care to provide practically relevant solutions

Clear Channel Assessment (CCA) Adapts to noise floor by sampling channel when it is assumed

to be free Samples are exponentially averaged, result used in gain

control For actual assessment when sending a packet, look at five

channel samples – channel is free if even a single one of them is significantly below noise

Optional (by multiple senders): random backoff if channel is found busy

Optional: Immediate link layer acknowledgements for received packets

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B-MACB-MAC

Low Power Listening Uses the clear channel assessment techniques to decide

whether there is a packet arriving when node wakes up Timeout puts node back to sleep if no packet arrived

B-MAC does not have Synchronization RTS/CTS Results in simpler implementation Clean and simple interface

Currently: Often considered as the default WSN MAC protocol

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B-MACB-MAC

Asynchronously maintained wake-up period

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Sleep

t

ReceiveReceiver

Sleep

t

PreambleSender Message

Sleep

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CSMA CSMA

State diagram of CSMA sender

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Power Aware Multi-Access with Signaling – PAMAS Power Aware Multi-Access with Signaling – PAMAS

Idea: combine busy tone with RTS/CTS Results in detailed overhearing avoidance, does not

address idle listening Uses separate data and control channels

Procedure Node A transmits RTS on control channel, does not sense

DATA channel Node B receives RTS, sends CTS on control channel if it

can receive and does not know about ongoing transmissions

B sends busy tone as it starts to receive data

Time

Controlchannel

Datachannel

RTS A B

CTS B A

Data A B

Busy tone sent by B

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PAMAS – Already ongoing transmission PAMAS – Already ongoing transmission

Suppose a node C in vicinity of A is already receiving a packet when A initiates RTS Procedure A sends RTS to B C is sending busy tone (as it receives data) CTS and busy tone collide, A receives no CTS, does not

send data

A

BC

?

Time

Controlchannel

Datachannel

RTS A B

CTS B A

No data!

Busy tone by C

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OverviewOverview

Principal options and difficultiesContention-based protocolsSchedule-based protocols LEACH

IEEE 802.15.4

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Low-Energy Adaptive Clustering Hierarchy (LEACH)Low-Energy Adaptive Clustering Hierarchy (LEACH)

Given: dense network of nodes, reporting to a central sink, each node can reach sink directlyIdea: Group nodes into “clusters”, controlled by clusterhead

Setup phase; details: later About 5% of nodes become clusterhead (depends on

scenario) Role of clusterhead is rotated to share the burden Clusterheads advertise themselves, ordinary nodes join

CH with strongest signal Clusterheads organize

CDMA code for all member transmissionsTDMA schedule to be used within a cluster

In steady state operation CHs collect & aggregate data from all cluster members Report aggregated data to sink using CDMA

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LEACH rounds LEACH rounds

Setup phase Steady-state phase

Fixed-length round

……….. ………..

Advertisement phase Cluster setup phase Broadcast schedule

Time slot 1

Time slot 2

Time slot n

Time slot 1

…..….. …..

Clusterheads compete with CSMA

Members compete with CSMA

Self-election of clusterheads

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IEEE 802.15.4IEEE 802.15.4

IEEE standard for low-rate WPAN applications Goals: low-to-medium bit rates, moderate delays

without too stringent guarantee requirements, low energy consumption

Physical layer (27 channels) 20 kbps over 1 channel @ 868-868.6 MHz 40 kbps over 10 channels @ 905 – 928 MHz 250 kbps over 16 channels @ 2.4 GHz

MAC protocol Single channel at any one time Combines contention-based and schedule-based schemes Asymmetric: nodes can assume different roles

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IEEE 802.15.4 MAC overviewIEEE 802.15.4 MAC overview

Star networks: devices are associated with coordinators Forming a PAN, identified by a PAN identifier

Coordinator Bookkeeping of devices, address assignment, generate

beacons Talks to devices and peer coordinators

Beacon-mode superframe structure GTS assigned to devices upon request Active period Inactive period

Contention access period

Guaranteed time slots (GTS)

Beacon

Coordinator Device

Beacon

Data request

Acknowledgement

Data

Acknowledgement

Thank you ! Q & A

Thank you ! Q & A