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Enabling Technologies

Part 4Mobile Ad Hoc NetworkingWuhan University

Why Enable?

Reliable point-to-point communication via media access control (MAC)Challenges in medium shareCategories of MAC protocolsRandom access principlesCase studiesn IEEE 802.11, Bluetooth

More PHY/MAC standardsn WiMax, ZigBee, UWB

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Digital Radio Properties

Higher bit error rate (BER)n Several orders compared to wired medium

Path loss – signal attenuationn At least quadratic of distance; could be 4th

power

Fading effectsn Shadow fading (large-scale) n Doppler and multipath fading (small-scale)

MAC in Wireless Networks

Contention resolution and collision handlingALOHA & CSMA/CD – recallCSMA/CD in Ethernet not working heren Half duplex transceiverMore challengesn Hidden terminal problemn Exposed terminal problems

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Hidden Terminal Problem

A, B, C – three nodesB can talk to A and C directly, but A and C

can’t talk to each otherWhen A transmits to B, node C, not aware of

the ongoing transmission, may transmit and collide with A’s transmission

A B C

Exposed Terminal Problem

A, B, C, D – four nodesOnly AB, BC, and CD can talk to each other

directlyWhen B transmits to A, node C, aware of the

ongoing transmission, can’t transmit to D, even though there will not be interference at D

A B C D

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A Simple Categorization

WirelessMAC

Distributed

Guaranteed orControlled access

Randomaccess

Centralized

How to Solve Hidden Terminals?

Your inputs …

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Busy Tone [Tobagi75,Haas98]

A receiver transmits busy tone when receiving dataAll nodes hearing busy tone keep silentAvoids interference from hidden

terminalsRequires a separate channel for busy

tone

MACA [Karn90]

Three-way handshaken When A wants to transmit to B, it first

sends an RTS (Request-To-Send) packetn After receiving A’s RTC, B sends a CTS

(Clear-To-Send) packetn After receiving B’s CTS, B sends the DATA

packet

A B C

DataRTS CTS

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MACA (Cont’d)

When should C reactivate?n Both RTS and CTS contain duration

information for overhearing nodes, e.g. C, to reactivate after specified silent period

A B C

RTSCTSData

MACA (Cont’d)

When should C reactivate?n Both RTS and CTS contain duration

information for overhearing nodes, e.g. C, to reactivate after specified silent period

A B C

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MACAW [Bharghavan94]

Simple modification to improve reliabilityFour-way handshakeRTS/CTS/DATA/ACKEssential ingredient of IEEE802.11’s

DCF

A B C D

RTSCTSDataACK

MACAW [Bharghavan94]

Simple modification to improve reliabilityFour-way handshakeRTS/CTS/DATA/ACKEssential ingredient of IEEE802.11’s

DCF

A B C D

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802.11 WLAN802.11 WLAN componentsn Access Point (AP) + Mobile Stations (MS) + Distribution

System (DS)

802.11 WLAN Modesn BSS – basic service set (infrastructure mode)

w ESS – extended service set (multiple AP’s)n IBSS – independent BSS (ad hoc mode)

IEEE 802.11– Scope of Standard

Logic Layer Control (LLC) – 802.2

Medium Access Control (MAC)

PHYFH, DS, IR

Data link layer

Physical layer

IEEE

802

.11

Upper layer protocols of OSI

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IEEE 802.11 Family802.11 (legacy)1997

2Mbps/2.4GHz

802.11a - 199954Mbps/5.2GHz

802.11b – 199911Mbps/2.4GHz

802.11g - 20254Mbps/2.4GHz

2.4GHz

Yr 1999

54MbpsOFDM

802.11eQoS

802.11hEuropean

802.11iSecurity

IEEE 802.11 Legacy

Standardized1-2Mbps wireless LANMultiple PHY layer choicesCentralized and ad hoc controlGreat start point

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PHY – Physical LayerDiffused Infrared (IR)

2.4~2.483GHz ISM band:Frequency-Hopping Spread Spectrum (FHSS)n Dwell time = 20msn 79 1MHz channels with 22 hopping patternsDirect-Sequence Spread Spectrum (DSSS)n Barker sequence – 11-bit chipping sequencen 14 partially overlapping 22MHz channelsw Center frequencies 5MHz apart

n Channels 1~11 available in N. Americaw 1, 6, 11 can operate at the same time with 25 MHz apart

MAC Main RequirementsSingle MAC to support multiple PHYsn Support single and multiple channel PHYsn and PHYs with different Medium Sense Characteristics

Should allow overlap of multiple networks in the same area and channel spacen Need to be able to share the mediumn Allow re-use of the same medium

Need to be robust for interferencen Microwave interferersn Other un-licensed spectrum usersn Co-channel interference

Need mechanisms to deal with Hidden TerminalsNeed provisions for Time Bounded ServicesNeed provisions for Privacy and Access Control

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Basic MAC FeaturesUse Distributed Coordination Function (DCF) for efficient medium sharingn Use CSMA/CAn Based on carrier sense function of PHY – Clear Channel Assessment

(CCA)

Robust for interferencen CSMA/CA + ACK for unicast frames – MAC level recoveryn CSMA/CA for broadcast frames

Parameterized use of RTS/CTS to provide virtual carrier sense function to protect against hidden nodesn Duration information is distributed by both sender and receiver

through separate RTS/CTS frames

Support fragmentation to cope different MSDU lengths

Coordination FunctionsPCF – Point Coordination Functionn Infrastructure moden OptionalDCF – Distributed Coordination Functionn Infrastructure and ad hoc modesn Mandatoryn MAC protocol intended to implement ad hoc

networks

Implement different fixed priority levelsn DIFS – distributed inter-frame spacen PIFS – point inter-frame spacen SIFS – short inter-frame space

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CSMA/CA Explained

Reduce collision probability where mostly neededn STAs are waiting for medium to become free for longer than DIFS

w CCA from PHY and virtual carrier sense

n Select random backoff in current collision window after a defer, resolving contention to avoid collisions

Efficient backoff algorithm stable at high loadsn Exponential contention window increases for retransmissions (up to max

limit)n Resets to min value when transmission succeeds

CSMA/CA + ACK

Receiver sends ACK after receiving frame with correct CRC

Direct access when medium is sensed free for longer than SIFSn No random backoff needed

SIFS < DISF to give ACK higher priority than a new transmission of data framen Such technique of using different inter-frame spaces to

differentiate priorities is extensively used in 802.11 MAC

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Hidden Terminal

Separate control frames (RTS/CTS) between sender and receiver will reserve the mediumfor subsequent data exchangen Duration information included in all frames

Hidden Terminal Resolution

Duration fields in RTS and CTS frames distributes medium reservation information, stored in an overhearing node’s counter– Network Allocation Vector (NAV)Medium “busy” decided by CCA and NAVUse of RTS/CTS is optional but must be implementedn Use is controlled by an RTS_Threshold parameter in each STA to limit

overhead

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Optional Point Coordination Function (PCF)

Contention-free service uses PCF on a DCF foundationn PCF can provided lower transfer delay variations to support time

bounded servicesn Async data, voice or mixed implementations are possiblen Point Coordinator resides in APCoexistence of contention and optional contention-free does not burden the implementation

Contention Free operation

Alternating contention-free and contention operation under PCF controlNAV prevents contention traffic until reset by the last PCF transfern Thus, variable length of contention-free period per interval

Both PCF and DCF defer to each other

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PCF Burst

CF-burst by “polling” bit set in CF_Down framesImmediate (w/SIFS) response by STA on CF_poll STAs to maintain NAV to protect CF_trafficResponses can be of variable length“reset NAV” by last (CF_End) frame from AP“ACK Previous Frame” bit in headers

Fragment Burst

Long MSDU (MAC Service Data Unit) fragmentedBurst of fragments are ACKed individuallyn Separated by SIFSsn Unicast frames only

Random backoff and retransmission of failing fragment when no ACK is returnedn Can be interruptedDuration information in data fragments and ACKs causes NAV to be set, for medium reservation

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Frame Formats

MAC header format differs per frame typen Control frames (several fields are omitted)n Management framesn Data frames

Includes sequence control field for filtering of duplicate caused by ACK and retransmissions

Address Fields Description

Addr1 – all STAs filters on this addressAddr2 – Transmitter Address (TA)n Identifies transmitter to address ACKs to

Addr3 – Depending on To and From DS bitsAddr4 – Only needed to identify the original source of WDS (Wireless Distribution System)

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Privacy and Access ControlGoal of 802.11 is to provide “Wired Equivalent Privacy (WEP)”Provides for an authentication mechanismn To aid in access controln Has provisions for “open”, “shared key” or proprietary

authentication extensions

Optional (WEP) privacy mechanism defined by 802.11n Limited to STA to STA, not “end to end”n Only implements “confidentiality” functionn Uses RC4 PRNG algorithm based on

w A 40 bit secret key (no key distribution standardized)w And a 24 bit IV that is sent with the dataw Includes an ICV to allow integrity check

n Only payload of data frames are encryptedw On per MPDU basis

Privacy Mechanism

WEP bit in frame control field indicates WEP in usen Each frame can have a new IV or IV can be reused for a

limited timen If integrity check fails then frame is ACKed but discarded

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Support for Mobility

MAC Management LayerSynchronizationn Finding and staying with a WLANn Synchronization functionsw TSF timer, beacon generation

Power managementn Sleeping without missing any messagen Power management functionsw Periodic sleep, frame buffering, traffic indication map

Association and de-associationn Joining a networkn Roaming – moving from AP to APn Scanning

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Synchronization in 802.11

Timing Synchronization Function (TSF)Used for power managementn Beacons sent at well known intervalsn All STA timers in BSS are synchronized

Used for PCF timingn To predict CP/CFP alternation period

Used for hop timing for FH PHYn Dwell intervaln STA synchronization

Synchronization ApproachEach STA maintains a local timerTiming Synchronization Function (TSF)n Keeps timers from all STAs in synchn AP controls timing in infrastructure moden Distributed function for ad hoc mode

Timing conveyed by periodic beacon transmissionsn Beacons contain timestamps for the BSSn Timestamps in beacons used to calibrate local clocks

n Not required to hear each beaconn Beacons contain other management information

w Also used in power management and roaming

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Infrastructure Beacon Generation

AP’s send beacons in infrastructure mode

Beacons scheduled at beacon intervalsTransmission may be delayed by CSMA deferraln Subsequent transmissions at expected beacon intervaln Not relative to last transmissionn Next beacon sent at target beacon transmission time

Timestamps contains timer values at transmit time

Power ManagementMobile devices are battery poweredn Power management is important for mobility

Current LAN protocols assume stations are always ready to receiven Idle receive state dominates LAN adapter power

consumption over time

How can we power off during idle periods, yet maintain an active session?802.11 power management protocoln Allows transceivers to be off as much as possible

n Transparent to existing protocolsn Flexible to support different applications

w Possible to trade off throughput for battery life

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Power Management ApproachAllows idle STAs to go to sleepn STA’s power save mode stored in AP

AP buffers packets for sleeping STAsn AP announces which STA have frames bufferedn Traffic Indication Map (TIM) sent with every beacon

Power saving STAs wake up periodicallyn Listen for beacons

TSF assures AP and power save STAs in synchn STAs will wake up to hear a beacon

n TSF timer keeps running when STAs sleepingn Synchronization allows extreme low power consumption

IBSS also have power managementn Similar in concept, distributed approach

Infrastructure Power Management

Broadcast frames also buffered in APn All broadcasts/multicasts are bufferedn Broadcasts/multicasts only sent after DTIMn Delivery TIM – sent every several TIM intervals

STAs wakeup prior to an expected (D)TIMIf (D)TIM indicates frame bufferedn STA sends PS_Poll and stays awake for the TIM interval

to receive datan Else STA sleeps again

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WLAN Infrastructure Network

Each STA is associated with a particular APn STAs 1, 2 and 3 -> An STAs 4 and 5 -> Bn STAs 6 and 7 -> C

Roaming

Mobile devices may move …

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Roaming

Mobile devices may move …n Beyond the coverage area of the AP

Roaming

Mobile devices may move …n Beyond the coverage area of the APn But within range of another AP

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Roaming

Mobile devices may move …n Beyond the coverage area of the APn But within range of another AP

Re-association allows STA to continue operation

Roaming Approach

STA decides that link to current AP is too weakSTA uses scanning to find another APn Or uses information from previous scans

STA sends re-association request to new APIf re-association response successfuln STA has roamed to new APn Else scans for another AP

If AP accepts re-association requestn AP indicates re-association to the DSn DS information is updated

n Normally old AP is notified through DS

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ScanningScanning required for many functionsn Finding and joining a networkn Finding a new AP while roamingn Initializing an IBSS (ad hoc) network

802.11 MAC uses a common mechanism for all PHYn Single/multiple channeln Passive/active scanning

Passive scanningn Find networks simply by listening for beacons

Active scanningn On each channel, send probe and wait for response

Beacon or probe response contains information needed to join the network

Roaming

Initial connection to an AP

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Roaming

Initial connection to an AP

Roaming

Initial connection to an AP

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Roaming

Initial connection to an AP

Roaming

Initial connection to an AP

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Roaming

Initial connection to an APn Re-association follows a similar process

MAC Management FramesBeaconn Timestamp, beacon interval, capabilities, ESSID, supported

rates, parametersn TIM

Proben ESSID, capabilities, supported rates

Probe responsen Timestamp, beacon interval, capabilities, ESSID, supported

rates, parametersn Same as beacon except for TIM

Association requestn Capabilities, listen interval, ESSID, supported rates

Association responsen Capabilities, status code, STA ID, supported rates

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More MAC Management FramesRe-association requestn capabilities, listen interval, ESSID, supported

rates, current AP address

Re-association responsen capabilities, status code, STA ID, supported ratesDisassociationn Reason code

Authenticationn Algorithm, sequence, status, challenge text

De-authenticationn Reason

HIPERLAN & HIPERLAN/2

ETSI’s Project BRANn High performance radio LANn Frequency band: 5.2GHz U-NIIn Ver. 1 (1997): 23.5Mbpsn Ver. 2 (2000): 54MbpsComponents:n Distribution System (DS)n Access Point (AP)n Mobile Terminal (MT)

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HIPERLAN & HIPERLAN/2PHYn OFDM (like 802.11a and g)

MACn TDMA/TDDn Under control of an AP or CC (central

controller, regular MT acting as AP)n Transmission through AP/CC or directly

between MT’sn Better QoS provisioning

IEEE 802.15 Working Group

Personal Area Network (PAN)n Communication within a persons operating

spaceIEEE 802.15.1 – BluetoothIEEE 802.15.3a – High data rate (UWB)IEEE 802.15.4 – Low energy (ZigBee)(802.15.2 – interoperability between

802.15 and 802.11)

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Bluetooth

Originally by Ericsson 1994Cable replacementn Short-range (~m)n Low data rate (<1Mbps)n Low power (~40mA/0.2mA)Bluetooth Special Interest Group (SIG)n 1998 – started with 5 companiesw Ericsson, Nokia, IBM, Toshiba, Intel

n Now – thousands

Bluetooth

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Bluetooth PHY

2.4 GHz ISM bandModulation - GFSKFrequency hoppingn Over 79 1MHz carriers world wide (except

for France, Spain and Japan – 23 carriers)n 1600/sec (dwell time = 625µs)Long repetition interval of frequency-

hop sequencen Over 23 hours

Piconet – Basic TopologyStar shape1 master node controls

up to 7 active slave nodesn And controls up to 255

parked nodes (in energy save mode)

All nodes follow the same hopping sequence as the master

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Links in Piconet – TDD/TDMASCO – synchronous connection orientedn Period negotiated with mastern Supports symmetric, circuit -switched connectionsn Usually for voice

ACL – asynchronous connectionlessn Master polls each slaven Slave responds immediatelyn Response can take 1, 3, or 5 slots

ScatternetPiconets interconnectedn Slave in multiple piconetsn Master also a slaveNodes in the same

piconet hop with the masterInter-piconet

communication not specified in specification

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IEEE 802.15.3 – UWB

High data rate WPANUWBMotivation: The need for higher

bandwidths than currently supported by 802.15.1n 100 Mpbs within 10 metern 400 Mpbs within 5 meterData, High quality TV, Home cinema

IEEE 802.15.4 & ZigBeeShort rangen <30mLow rate n < 250kbpsLow power consumptionn Attractive for sensor networking

Frequency Bands of Operationn 16 channels in 2.4GHzn 10 channels in 915MHzn 1 channel in the European 868MHz band

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Selected Comparisons

Low power

Interoperability

Data rate

Stronghold

Sensor networks, remote control

34KB30mA TX.356mA ST

<=250kbpsZigBee

Cable replacement: wireless USB & headset

~100KB40mA TX0.2mA ST

<1MbpsBluetooth

Internet, PC networking

100+KB400mA TX20mA ST

<=54MbpsWi-Fi

ApplicationsProtocol stack size

PowerBandwidthStandard

IEEE 802.16 WiMAX

Broadband MANIEEE 802.16an Wireless last-mile alternativen stationary subscribers, e.g. transceivers mounted

on top of business or residential buildingsn 2-10GHzn Supports thousands of users simultaneouslyn TDMA+OFDM / OFDMA

IEEE 802.16en Support low speed mobile users

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IEEE 802.20 Mobile Broadband

Supports mobile users at very high speedn <= 250 km/hr

Operates in licensed 500MHz~3.5GHz bandData rate <= 1Mbps

Other Issues and Techniques

QoS and fairnessDirectional antenna and DMACMulti-channels