Chapter 8 Vehicular Networks

Qing Yang

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Contentsintheslidesarebasedonthefollowingpaper:Kenney,JohnB."Dedicatedshort-rangecommunications(DSRC)standardsintheUnitedStates."ProceedingsoftheIEEE99.7(2011):1162-1182.

Motivations

• The U.S. Department of Transportation (DOT) has estimated that vehicle-to-vehicle (V2V) communication can address up to 82% of all crashes in the U.S.

• FCC allocate 75 MHz spectrum from 5.850 GHz to 5.925 GHz to Dedicated Short Range Communication (DSRC)

• Zero usage over the past 19 years

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Motivation (Cont’)

• Driven by the Google’s driverless car, or called autonomous driving

• Obama’s 10-year proposal– Nearly $4 billion investment to accelerate the

development and adoption of safe vehicle automation through real-world pilot projects.

• NSF Cyber Physical System Program– Last for 10 years, e.g., transportation CPS

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Vehicle-to-Vehicle (V2V)

• Primary motivation of having V2V communication is to enable collision prevention applications

• Vehicle periodically broadcast state information in beacon messages– Including location, speed, acceleration, etc…– E.g. 10 Hz for safety applications

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Vehicle to Infrastructure (V2I)

• V2I allows a vehicle to communicate with a roadside unit (RSU)

• Example applications:– Download the geometry of an approaching

intersection– Obtain the state of the signals at an intersection– Be notified about the existence of hazard including

disabled vehicle, emergency vehicle, ice on road, fog etc…

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Applications of Vehicular Networks

• Forward collision warning• Emergency electronic brake lights• Blind spot warning• Intersection movement assist• Do not pass warning• Control loss warning• Navigation• Electronic payments (e.g., tolls, parking, fuel)• Improve fuel efficiency• Gather traffic probes, and disseminate traffic updates• General entertainment and commercial purposes

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DSRC

• Dedicated Short Range Communication• Dedicated: U.S. FCC allocated 75 MHz of

licensed spectrum in 5.9 GHz band • Short Range: hundreds of meters• Spectrum is divided into 7 channels• V2V safety messages are expected on channel

172• Control messages on channel 178

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Overview of DSRC

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WAVE/DSRC

• PHY and MAC utilizes IEEE 802.11p, also called wireless access for vehicular environment (WAVE)

• A modified version of IEEE 802.11 standard• IEEE 1609 Working Group– 1609.4 for channel switching– 1609.3 for network services– 1609.2 for security services

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WSMP

• WAVE Short Message Protocol (WSMP)• Why not IP+TCP/UDP?• Depends on the requirements of an application– Single-hop messages, e.g., those upon which

collision prevention applications are based, typically use the bandwidth-efficient WSMP

– multi-hop packets use IPv6 for its routing capability

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SAE J2735 Messages Set Dictionary

• Specifies 15 message formats – Basic safety message, common safety request,

emergency vehicle alert msg., intersection collision avoidance, map data, NMEA correction, probe data management, probe vehicle data, roadside alert, RTCM correction, signal phase and timing msg., signal request msg., signal status msg., and traveler information

• Basic Safety Message (BSM)– conveys critical vehicle state information in support of

V2V safety applications

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DSRC: Physical Layer

• Orthogonal frequency division multiplexing (OFDM)

• DSRC uses the 10 MHz channels• Four modulation techniques– BPSK, QPSK, 16-QAM and 64-QAM

• Forward error correction (FEC) to improve reliability

• Data rate varies from 3 Mbps to 27 Mbps

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Classes of DevicesDeviceClass Max.Output Power

(dBm)Comm. Range(m)

A 0 15B 10 100C 20 400D 28.8 1000

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• Devices in V2V safety are normally in Class C• RSU could be Class D

DSRC Spectrum

• From 5.850 GHz to 5.925 GHz• Divided into seven 10 MHz channels – with a 5-MHz guard band at the low end

• Each channel is either a Service Channel (SCH) or as the Control Channel (CCH)

• Pair of channels can be combined to offer a 20 MHz channel

• 10 MHz vs 20 MHz ?

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DSRC: MAC

• WAVE is based on 802.11• Delays in communicating through AP is an

issue in vehicular network– Connection from a vehicle to an RSU last only a

few seconds– Need an alternate, light-weighted rules for

accessing the medium–Motivation of having 802.11p WAVE amendment

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Outside the Context of BSS

• Abbreviated as OCB• No MAC sub-layer setup before STAs

exchange data frames• The BSSID field of a frame sent OCB is set to

all 1 s, i.e., 0xFFFFFF• MAC is configured to pass any frame with

0xFFFFFF in the BSSID field• OCB or BSS but not both

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WAVE: Management Frames

• Timing advertisement (TA) frame– Announce the senders’ current time

• The original 802.11 Vendor-Specific Action (VSA) frame is modified– 24-bit or 36-bit organization identifier assigned by

IEEE– Payload is used to convey WAVE service

advertisement, .e.g., tolling, traffic, fuel etc…

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OCB vs BSS

• Compared to BSS, OCB communication does NOT– Use a beacon frame– Require one STA to synchronize with another

before they can communicate– Use authentication at the MAC sub-layer– Include any notion of the STAs associating before

they communicate

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Lack of Beacon

• The 802.11 beacon periodically announces the existence of a BSS

• OCB type of communication does not utilize a BSS

• Some beacon contents, e.g., supported data rates or Quality of Service (QoS) parameterscan be specified in upper layers

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Synchronization

• In 802.11, synchronization between STA and AP is used to facilitate power management

• Power is not an issue in vehicular network• A vehicle may wish to monitor a channel

continually• Vehicles used GPS signals to achieve time

synchronization• If no GPS, TA frame is used

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Authentication

• The need for authentication in OCB communication is determined at higher layers

• Provided by the IEEE 1609.2• OCB communication that uses IPv6 rather than

the 1609 upper layers can utilize a variety of well-established techniques for authentication

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Association

• In 802.1, association is used to help the AP bridge frames between a non-AP STA within the BSS and a node on the other side of the DS

• V2V safety messages have no need of bridging• Many DSRC messages (V2V V2I) reach their

destinations in a single hop• If multi-hop forwarding is desired, it can be

achieved by layer 3 routing

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Medium Access Rules• CSMA/CA• A STA that has a frame to send first senses the wireless

medium. If the medium is idle for DIFS, the STA begins transmission of its frame

• If the medium is busy, the STA performs a random backoffby choosing a number of idle time slots to wait before transmission. The countdown begins when the medium becomes idle, is interrupted during any non-idle interval, and resumes when the medium returns to idle.

• The sender of a unicast frame waits for an acknowledgment (ACK) from the recipient; if it does not receive the ACK within a timeout interval it retransmits the frame after another random backoff.

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QoS in WAVE

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• Use of EDCA equivalent to IEEE 802.11e– Enhanced Distributed Channel Access

• Definition of four Access Categories (AC)– AC0 (lowest) to AC3 (highest priority)– CWmin, Cwmax– AIFS (arbitration inter-frame spacing)–…

Example of QoS in WAVE

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Channel Management (1)

• Support one or more radios• A separate logical instance of the IEEE

802.11p MAC– Including queues and state variables

• Channel switching is optional– A DSRC device is allowed to remain tuned to a

single channel all the time• How can two device switch to the same

channel at the same time?

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Channel Management (2)

• Control Channel (CCH)– Ch. 178 is the rendezvous channel that a device

will tune to on a regular basis– Other channels are service channels

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Channel Management (2)

• Time division– Synchronization via GPS– Alternating CCH and SCH intervals– 100ms sync interval including 50ms on the CCH– Slot start with a 4 ms guard interval– Hear WSAs (Service Advertisement) on CCH

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Channel Management (3)

• Immediate departure option– Switch to a service channel immediately

• Extended access departure option– Remains on the SCH through one or more sync

period until service is completed• Remain on CCH option– If not WSA is heard – It may hear nothing in the SCH interval

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Channel Management (4)

• If a frame is queued during the 54 ms of SCH interval– Channel is treated as busy and back-off– Collision may happened if two devices choose the

same back-off timer• V2V safety message is exchanged during the

46 ms rendezvous time– Capacity of the sys. for safety message is < ½ that

of a system utilizes a full-time channel

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Network Service for DSRC

• WAVE Short Message Protocol (WSMP)• It is a 1-hop transmission protocol• Packet transmitted by WSMP is call WAVE

Short Message (WSM)• Packet overhead is less than 20 bytes and the

minimum overhead is 5 bytes

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WSMP Packet Format

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WSM Header

• WSMP Version• PSID (Provider Service Identifier)– Similar to the port numbers in TCP

• Extension Field– Channel number– Data rate using a resolution of 500 Kbps– A signed integer with resolution of 1 dBm

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WSM Header (Cont’)

• WSMP WAVE Element ID–Marks the end of extension field and indicates the

starting of WSM Data field• Length– Number of bytes in the WSM Data field

• WSM Data– The payload of the WSM

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WAVE Security

• Background about public-key cryptography• Type of asymmetric encryption• The de facto standard for computer security• https://en.wikipedia.org/wiki/Public-

key_cryptography• https://en.wikipedia.org/wiki/RSA_(cryptosyst

em)

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IEEE 1609.2 Authentication (1)

• A message carries a digital signature that is used to verify the sender’s identity

• Elliptic Curve Digital Signature Algorithm is used in IEEE 1609.2

• Sender uses its private key to encrypt/sign the message

• Send the message along with its certificate that contains its public key

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IEEE 1609.2 Authentication (2)

• Receivers use the public key to verify the signature

• Certificate include information about how a receiver can check if the certificate has be revoked

• Vehicles are given a set of certificates (100K)• For privacy protection, a vehicle uses a given

certificate for a limited time, e.g., 5 mins

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IEEE 1609.2 Authentication (3)

• When a vehicle changes certificates, it also changes other identifiers in the safety message– E.g., source MAC address

• Only one certificate will be valid at any given time

• Stolen of a portion of the certificates?• They will be revoked by the certificate

authority (CA)

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SAE J2735 DSRC Message Set

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Standard Message Type

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Basic Safety Message

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Basic Safety Message (Cont’)

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