Medium Access Control inMedium Access Control in Vehicular Ad Hoc Networks

Katrin Sjöberg, Elisabeth UhlemannKatrin Sjöberg, Elisabeth Uhlemannand Erik G. Ström

IEEE VTS Workshop on Wireless Vehicular CommunicationsHalmstad University, Sweden

October 12, 2010

IntroductionIntroduction

OSI d lTCP/IP d lMAC Predictability

Application

OSI model

HTTP, FTP

TCP/IP modelIEEE 802.11p

VANETMACReliability

Scalability

Predictability

Session

Presentation,

DHCP, DNSTelnet etc

VANET

STDMASTDMA

IEEE 802 11p

MACScalabilityScalability

VANET

Transport

Session

TCP/UDPTelnet etc.STDMA

VANETVANET

IEEE 802.11p

IEEE 802.11p STDMA Delay

Data link

Network

WiFi/Ethernet

IP LLCMAC

VANETIEEE 802.11pScalability

PhysicalWiFi/Ethernet

/ MACPredictability

IEEE 802.11p ScalabilityVANET

ScalabilityMACp

IEEE 802.11p IEEE 802.11p STDMAScalability

IEEE 802.11pScalability

STDMADelay

What is a VANET?What is a VANET?

• Vehicular ad hoc networks

• Dezentralized network topologyDezentralized network topology– No access point or base station

P i i

Direct vehicle‐to‐vehicle (V2V)

– Peer‐to‐peer communication

• Self‐organizationg

• Can contain roadside units (RSU)

Ad hoc no central entity. 

Characteristics of VANETsCharacteristics of VANETs

h i i h l• Share a common communication channel• Broadcast

– Traditional Automatic Repeat reQuest (ARQ) are not available

• The multipath environment where the radiowaves propagateradiowaves propagate– 5.9 GHz has been chosen for VANETs

Th b f ti i ti d i VANET• The number of participating nodes in a VANET cannot be restricted

Why Traffic Safety Applications?Why Traffic Safety Applications?decrease the number of traffic accidents by introducing traffic…. decrease the number of traffic accidents by introducing traffic 

safety applications, but also to reduce congestion, travel‐time, and pollution through traffic efficiency applications…

• Lane change warning

• Overtaking vehicle

M i t• Merge assistance

• Use different communication technologiesgdepending on application– IEEE 802 11p 3G Mobile WiMAX– IEEE 802.11p, 3G, Mobile WiMAX

Real Time CommunicationReal‐Time Communication

ffi f li i h• Traffic safety applications have concurrentrequirements on delay and reliability

• Packets have a deadline to meet• Time‐triggered position messages Broadcastgg p g

– 2‐10 Hz, 300‐800 byte – Cooperative awareness messages (CAM)p g ( )– Facility: Local dynamic map

• Event‐driven hazard warnings Broadcast

Here I am!

Event driven hazard warnings– Packet size and periodicity depend on traffic safetyapplication

Broadcast

pp

Medium Access Control in VANETMedium Access Control in VANET

• Responsible for scheduling channel access to minimize interference to increase reliabilityy

• How to guarantee these low delay applications that the packet arrives in time?applications that the packet arrives in time?

• The MAC method must be decentralized, scalable and predictablescalable and predictable

• Only standard supporting direct vehicle‐to‐vehicle communication is 802.11p– 5.9 GHz

WAVEWAVEWAVE = Wireless Access in Vehicular EnvironmentWAVE   Wireless Access in Vehicular Environment

HTTPA li ti 1609.1

WSMPTCP/ UDPTransport

HTTP etc

Application 1609.1 1609.0 WAVEArchitecture

LLC 802.2Logical link

IPv6 NetworkWSMPUDP p

1609.31609.2Security

1609.5 WAVEC i i

802.11pPhysical

802.11p 1609.4Medium access

LLC 802.2Logical link Communications Managerstarted during spring 2008802.11pPhysical

WSMP – WAVE Short Message protocol

spring 2008. 

WAVE = IEEE 802.11p, 1609.0, 1609.1, 1609.2, 1609.3, 1609.4 and 1609.5

IEEE 802 11pIEEE 802.11p

R tifi d J l 2010• Ratified July 2010• PHY and MAC amendment

– No support for access pointsNo support for access points– Peer‐to‐peer mode (ad hoc)

• IEEE 802.11a OFDM physical layer– 3, 4.5, 6, 9, 12, 18, 24 and 27 Mbps– 5.850‐5.925 GHz Intelligent Transportation Systems Radio 

Service (ITS‐RS)Service (ITS RS)– 10 MHz channels– 1 control channel and 6 service channels (WAVE 1609.4)

d d ( )• European standard (ETSI) – ITS G5• Worldwide standard (ISO) – CALM M5

IEEE 802 11p MACIEEE 802.11p – MAC 

C i l i l i h lli i id• Carrier sense multiple access with collision avoidance (CSMA/CA)

• IEEE 802 11e QoS• IEEE 802.11e QoS– Provides 4 different priority levels

• Starts listening to the channel during one AIFS• Starts listening to the channel during one AIFS– Arbitration InterFrame Space (58 µs, highest priority in 802.11e)

• Channel becomes busy during listening periodChannel becomes busy during listening period – Perform backoff by selecting a random number– Decrement backoff only when channel is free

• A node sends directly if the channel was free during one AIFS

CSMA/CA drawbacksCSMA/CA drawbacks

U di bl h l d l• Unpredictable channel access delay– Periodic messages need to be sent within its time periodTh d b k ff d l l h h– The random backoff may cause a delay longer than the time period

– Causes packet drops at sending nodeCauses packet drops at sending node

• Collisions – The random backoff time chosen are discrete and thus– The random backoff time chosen are discrete and thus nodes may choose the same

• For example in 802.11e highest priority – {0 µs, 13 µs, 26 µs, 39 µs}

– Two concurrently transmitting nodes may be located very close together 

CSMA is not predictable nor scalable.

STDMA a potential remedy?STDMA – a potential remedy?

S lf i i ti di i i lti l (STDMA)• Self‐organizing time division multiple access (STDMA)• Already in commercial use

Automatic Identification System (AIS)– Automatic Identification System (AIS)– VDL mode 4

• Specially designed for position messages e g CAMSpecially designed for position messages, e.g., CAM• Predictable channel access delay regardless of the number of competing nodesp g

• In overloaded situation “collisions” are scheduled to minimize interference

• Needs synchronization between nodes• Fixed packet length

STDMA is predictable and scalable.

STDMASTDMASI SI

NI …..NI…..Every node is allowed to

….. …..

NSNSS

Node 1

.NTSNTS

Every node is allowed to choose a transmission slot from 20%  of all available slots.

NI NI NI

NSNSS...

NTSNTS

…..NI…..SI

….. …..

SI SI

Node n

.A certain slot is used for 3‐8 

NS NSS NSNTSNTS

NTS

consecutive frames. 

Fixed number of slots in the frame. All nodes have a unique frame start!

STDMA needs position messages for scheduling transmissions in space when all availableSTDMA needs position messages for scheduling transmissions in space when all availableslots are occupied, i.e., the 20% of all available slots that is accessible to one particular node. 

Simulator in MatlabSimulator in Matlab

Data traffic model – time‐driven position messages, i.e., CAMsVehicle traffic model – Poisson distributed, approx. 1 vehicle/100 meter

Highway scenario g ywith 5 lanes in each direction. 

We are evaluating the sending side of the system. 

Channel access delayChannel access delay

300 byte packets300 byte packets, 6 Mbps, 10 Hz, 

Channel access delay is not a problem in pSTDMA!

Interference distanceInterference distance

Probability that two nodes initiatea transmission at the same time. 

2 Hz, 800 byte, 6 Mbps

CountermeasuresCountermeasures

• To overcome the scalability problems with CSMA/CA

• Transmit power control

C i l• Congestion control– Restrict the data traffic at the sending nodes

• Adjust the backoff window

SummarySummary

VANET i l h l f• VANET uses a common wireless channel for broadcast communication directly betweenvehicles at 5 9 GHzvehicles at 5.9 GHz

• CSMA has been selected as the MAC method for the first generation of VANETs through IEEEthe first generation of VANETs through IEEE 802.11p

• When penetration of ITS equipped vehiclesWhen penetration of ITS equipped vehiclesincreases 802.11p may experience problems with unbounded channel access delay and closeconcurrent transmissions

• Potential remedy STDMA

SummarySummary

• In STDMA the position information is required to schedule transmissions resulting in non‐overlappingtransmissions– Beneficial in order to protect the receivers located closestto the transmitter

• CSMA supports variable packet sizes and no synchronization is needed

• STDMA requires slot synchronization and position q y pmessages

Thank you!Thank you!

Questions?

katrin sjoberg@hh sekatrin.sjoberg@hh.se

PublicationsPublications• K Sjöberg Bilstrup E Uhlemann and E Ström "Scalability issues of the MAC methods STDMA and• K. Sjöberg Bilstrup, E. Uhlemann, and E. Ström,  Scalability issues of the MAC methods STDMA and 

CSMA of IEEE 802.11p when used in VANETs," in Proc. of the ICC'10 Workshop on Vehicular Connectivity, Cape Town, South Africa, 23‐27 May 2010

• K. Bilstrup, E. Uhlemann, E. Ström and U. Bilstrup, "On the ability of the IEEE 802.11p STDMA to provide predicatble channel access," in Proc. of the 16th World Congress on Intelligent Transport p p , f g g pSystems, Stockholm, Sweden, 21‐24 September 2009

• K. Sjöberg‐Bilstrup, ” Predictable and Scalable Medium Access Control in Vehicular Ad Hoc Networks,” Licenciate Thesis, Chalmers University of Technology, December 2010

• K. Sjöberg Bilstrup, E. Uhlemann, E. Ström and U. Bilstrup, "On the ability of the 802.11p MAC method and STDMA to support real‐time vehicle‐to‐vehicle communication" in EURASIP Journal on Wireless Communications and Networking, vol. 2009, Article ID 902414, 13 pages, 2009. doi:10.1155/2009/902414

• K. Bilstrup, E. Uhlemann and E. Ström, "Medium access control in vehicular networks based on the upcoming IEEE 802 11p standard " in Proc of the 15th World Congress on Intelligent Transportupcoming IEEE 802.11p standard,  in Proc. of the 15th World Congress on Intelligent Transport Systems, New York, US, 16‐20 November 2008

• K. Bilstrup, E. Uhlemann, E. Ström, and U. Bilstrup, "Evaluation of the IEEE 802.11p MAC method for vehicle‐to‐vehicle communication," in Proc. of the 2nd IEEE Int. Symp. on Wireless Vehicular Communications, Calgary, Canada, 21‐22 September 2008, g y, , p

• K. Bilstrup, "A survey regarding wireless communication standards intended for a high‐speed vehicle environment,", Technical Report IDE0712, Halmstad University, Sweden, February 2007