Wireless Access in Vehicular Environments - CITI …perso.citi.insa-lyon.fr/rstanica/cours/RTS-RF-V2X.pdf · · 2014-11-24Wireless Access in Vehicular Environments IST Semester

TCTC

Département TélécommunicationsDépartement TélécommunicationsServices & UsagesServices & Usages

Wireless Access inVehicular Environments

IST Semester

Advanced Wireless Networks

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● WAVE and DSRC

OSI Layering

WAVE

WirelessAccess forVehicularEnvironments

DSRC

DedicatedShortRangeCommunication

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● IEEE 802.11p

OSI Layering

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● Physical (PHY) layer● Basically identical to that of 802.11a● Slightly different frequencies: dedicated @ 5.9 GHz● Smaller 10-MHz channels to combat reduced

coherence bandwidth

● Medium Access Control (MAC) layer● Standard 802.11 family CSMA/CA● Inclusion of traffic priority (as in 802.11e)● Introduction of Wildcard Basic Service Sets

(WBSS)

IEEE 802.11p

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● Traffic priority in IEEE 802.11p● Enhanced Distributed Channel Access (EDCA)● Employed to prioritize traffic

● Important in vehicular networks, where safety (real-time) and accessory (best-effort) traffic coexist

● 4 Traffic Categories (TCs), implemented as different MAC-layer queues

● TCs are distinguished by1. minimum/maximum contention window size2. different AIFS (Arbitration InterFrame Space) that replace the original DIFS, that is identical for all traffic

IEEE 802.11p / MAC

smaller CWs and shorter AIFS imply higher priority

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● Traffic priority in IEEE 802.11p● Enhanced Distributed Channel Access (EDCA)

IEEE 802.11p / MAC

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● Basic Service Sets in IEEE 802.11p● Standard 802.11a/b/g, WiFi

IEEE 802.11p / MAC

BSS

BSS

DS

IBSS

Basic Service SetInfrastructured modein presence of WiFi AP

Distribution SetWiFi AP backbone ininfrastructured mode

Independent BSSWiFi ad hoc mode

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● Basic Service Sets in IEEE 802.11p● Standard 802.11, WiFi● BSS join process

1. scanning to obtain the (I)BSS identifiers (BSSIDs)● passive – wait for beacons broadcasted by APs every 100ms● active – poll APs for immediate beacon transmission

● 2. authentication● open – send MAC, get ACK/NACK from AP● shared key – challenge & response with AP using known key

● 3. association● association request/response with AP● new AP possibly informs old AP via DS

● IBSS join process: scanning only

IEEE 802.11p / MAC

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● Basic Service Sets in IEEE 802.11p● Adaptation to vehicular environments

● Same architecture, but much faster dynamics● fast alternation of short-lived communication links

● Scanning to acquire BSSIDs takes too long● BSS association is even worse

IEEE 802.11p / MAC

DS

IBSS

BSS

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● Basic Service Sets in IEEE 802.11p● Adaptation to vehicular environments

● Introduction of a wildcard BSSID● allows broadcast-like access to vehicles or APs within

any BSS, IBSS, DS● a WAVE BSS (WBSS) can be instantly formed / deleted

no need for association, authentication or even BSSID acquisition!

IEEE 802.11p / MAC

IBSS

BSS

DS

WBSS

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● IEEE 1609.x

OSI Layering

protocolsdesigned for vehicularcommunication

traditionalnetworkprotocolstack

multichannelmanagement

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● IEEE 1609.4● Management of multiple channels

● 1 control channel (CCH)● transmission of safety messages (IEEE 1609.3 WSM data)● advertisement of service availability on SCHs (WSA data)

● 6 service channels (SCHs)● transmission of service messages (WSM or IP data)

IEEE 1609.x

US frequencyspecification:7 channels of10 MHz eachat 5.9 GHz

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● IEEE 1609.4● Management of multiple channels

● Time divided into CCH intervals and SCH intervals● nodes are synchronized (e.g., via GPS)● guard intervals allow switching at single-interface nodes

● CCH interval: all nodes must listen on the CCH ● SCH interval: nodes can exchange IP/WSM data on SCHs

● inefficient resource utilization (50%) early departure from CCH during CCH intervals is possible

IEEE 1609.x

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● IEEE 1609.3● Network and transport layers● V2I and V2V communications will often be 1-hop

transmissions limited to the vehicular network● no need to employ TCP or UDP over IP

● IEEE 1609.3 defines two types of messages● WAVE Short Message (WSM)

- short messages used for safety purposes● WAVE Service Advertisement (WSA)

- more complex messages used to announce the availability of some service by one or more nodes: traffic alerts, tolling, navigation, restaurant and shopping information, entertainment, and Internet access

IEEE 1609.x

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● IEEE 1609.3● Network and transport layers● WAVE Short Message (WSM)

IEEE 1609.x

protocolversion

service id,similar toL4 ports

optional information,mainly cross-layer data

end of optional fields

payload length

payload

minimum overhead: 8 bytes

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● IEEE 1609.3● Network and transport layers● WAVE Service Advertisement (WSA)

IEEE 1609.x

security information

version

list of servicesand SCHsassociated

IP routing infofor servicesrequiring accessto the Internet

repetitions in a sync period

optional sender info identity, position,transmission power

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● IEEE 1609.2● Data exchange security● goals

● enable privacy (i.e., no eavesdropping)● allow authentication/integrity (i.e., certify sender and content)● protection against replays (i.e., avoid malicious forwarding)

● hybrid cryptographic approach● asymmetric cryptography for (1) authentication

and (2) distribution of symmetric keys● symmetric cryptography (faster) for data exchanges

● algorithms and solutions● ECDSA with SHA-256, ECIES-256● adding time stamps and location information

IEEE 1609.x

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Experimental tests

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● Field tests with IEEE 802.11● typical controlled vehicular environment

Experimental assessments

Laptop + WiFi card connectedto the external antenna

High-gain externalantenna withmagnetic base

GPS receiverconnected tolaptop

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● Field tests with IEEE 802.11● vehicle-to-infrastructure (V2I) communication

Experimental assessments – part I

movement

supremum goodput:achieved in one singlebest run

average goodput:mean over multipleruns, with standarddeviation

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movement

entry phase- scan & AP selection- BSSID association- ARP run- TCP handshake

production phase- maximum throughput

exit phase- connection loss

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movement

very diverse result overmultiple identical runs

not due to signal fluctuations!

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movement

suboptimal performancedue to MAC / transportprotocol operation!

1. lengthy scan2. association timeouts +3. slow data rate adaptation +4. data rate overestimation5. TCP timeouts

1.2.

3.

4.5.

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● Field tests with IEEE 802.11● vehicle-to-infrastructure (V2I) handoff

Experimental assessments – part II

typical 802.11 AP with antenna

APs deployment

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overall networkcoverage with signal strength

single AP coverage

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● Field tests with IEEE 802.11● vehicle-to-infrastructure (V2I) handoff● handoff : association process to a new AP

● when to perform an handoff? To which new AP?● experimental techniques

1. maintain until broken● keep current AP as long as possible

2. always strongest signal● follow the strongest signal

3. average with hysteresis● take the average signal over a time

window + introduce a minimum signalimprovement threshold to handoff


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always strongest signalexcessive (de)associationrate results in suboptimalthroughput

average w/ hysteresisuses an EWMA filter anda gain threshold on SINRfrom each AP in range

maintain until brokendefault handoff policyin many 802.11 cards

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considering the meshnetwork peculiarities,the handoff can befurther refined!

- maintain until broken- always strongest signal- average w/ hysteresis

mesh-specific handofftechniques, consideringAPs diversity induced bythe backbone topology

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● Field tests with IEEE 802.11● vehicle-to-infrastructure (V2I) data rate adaptation● experimental performance comparison

[1] ARF[2] window-based ARF[3] SINR-based

[4] SINR-based w/ packet bursts

Experimental assessments – part III

6 Mbps

insufficient SINR

24 Mbps

54 Mbps

ARF on a (large enough)window of sent packet

associate amodulation(data rate) toa SINR interval:requires feedback!

send bursts of packetsupon handshaking thedata rate with receiver

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● Field tests with IEEE 802.11● vehicle-to-infrastructure (V2I) data rate adaptation


depends on speed of transmitter,receiver and mobile obstacles

static / indoor

mobile / outdoor

schemes performsimilarly in static orindoor environments...

...but mobile outdoorenvironments inducesignificant differences!

results of tests in a controlled (lab) environment

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1. ARF is too pessimistic:hard to have 10 consecutivesuccessful transmissions, easyto have 2 consecutive losses

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2. window-based ARF is thebest: evaluating over a longertime period avoids underselectionof rates as in standard ARF

3. SINR-based largelyoverselect: why?

2.1.

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2. SINR-based schemes designed for static/indoor are too optimistic and overselect1. a same modulation performs differently in static and mobile cases with same SINR

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real

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results of tests in a real-world environment

1. ARF is pessimistic:same reasons as before

2. window-based ARFnow overselects: cannottrack the faster dynamicsof a real-world environment

3. SINR-based is now the best:after training for the vehicularenvironment, it can adapt to thefast channel variations and providethe best performance

3.2.

1.

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● Summary● Field tests reveal that improvements are needed

● protocol operations cause 50% under-utilization

● poor handoff support causes connectivity holes

● DS backbone nature (wired / mesh) has an impact

● rate adaptation algorithms cannot be inherited from static indoor wireless networks

Experimental assessments

802.11p MAC will answer to some ofthese problems, but not to all of them

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● Bibliography● H. Hartenstein, K. Laberteaux, “VANET: Vehicular Applications and

Inter-Networking Technologies”, Wiley, 2010● D. Jiang, L. Delgrossi, “IEEE 802.11p: Towards an International

Standard for Wireless Access in Vehicular Environments”, VTC 2008● D. Hadaller, S. Keshav, T. Brecht, S. Agarwal, “Vehicular

Opportunistic Communication Under the Microscope”, ACM MobiSys 2007

● A. Giannoulis, M. Fiore, E. Knightly, “Supporting Vehicular Mobility in Urban Multi-hop Wireless Networks”, ACM MobiSys 2008

● J. Camp, E. Knightly, “Modulation Rate Adaptation in Urban and Vehicular Environments: Cross-layer Implementation and Experimental Evaluation”, MobiCom 2008

Wireless Access in Vehicular Environments

Documents

Wireless Access in Vehicular Environments - CITI …perso.citi.insa-lyon.fr/rstanica/cours/RTS-RF-V2X.pdf · · 2014-11-24Wireless Access in Vehicular Environments IST Semester