Presented by: Zakhia Abichar Computer Systems Lab Group Meeting Nov. 11, 2009

1. Reliable Broadcast of Safety Messages in Vehicular Ad Hoc Networks Farzad Farnoud (Hassanzadeh) and Shakrokh ValaeeUniversity of Toronto, Canada In proceedings of IEEE Infocom 2009 Presented by: Zakhia Abichar Computer Systems Lab Group Meeting Nov. 11, 2009

2. Vehicular Wireless Networks • Designed by Intelligent Transportation Systems (ITS) • ITS is a comprehensive system for vehicles (including on-road cameras, sensors, weather equipment, etc.) • Communications is a big part of it • Wireless is supported by Dedicated Short Range Communications (DSRC) • DSRC is supported by IEEE and ASTM (American Society for Testing and Materials)

3. Protocol used for DSRC • There are 75 MHz of bandwidth allocated for public and private use • They are at 5.9 GHz • There are 7 channels of 10 MHz each • One control channel use mainly for broadcast • The IEEE standard used for DSRC is 802.11p • It’s also called Wireless Access in Vehicular Environments (WAVE)

4. Topic of Paper • There are safety-related messages that need to be transmitted with low delay • Proposing a Medium Access Control (MAC) scheme for broadcast communications • We need to: broadcast traffic, have low delay, support mobility, and high reliability

5. Outline • Characteristics of safety traffic in vehicular networks • Related work • Proposed broadcast protocol • Evaluation of the MAC protocol

6. Safety-Related Data • Each vehicle generates about 5 messages/second • A message is short. It has 100 bytes • It contains, vehicle ID, message ID, position • It also has: position of vehicles in front, behind, at left and right • Other fields are: detection of obstacle, its position, emergency car and its position, emergency braking

7. Delay Requirement • For an event, the driver reaction time is from 500ms to 1.2 s • There is also other processing delay • The communication delay should be in range of 100-200ms

8. Synchronous p-Persistent Retransmission (SPR) Time is divided into frames with L slots each With probability p, a node transmits in each timeslot It remains idle with probability 1-p Synchronous Fixed Retransmission (SFR) Similarly, time is divided into frame with L slots each A packet is transmitted w times in a frame (w <= L) The w slots are chosen randomly Frame1 Frame2 Frame3 Frame1 Frame2 Frame3 Related Work Transmit everywhere with proba. p w=2

9. 10100 01001 00110 Frame1 Frame2 Frame3 Transmission Using Codes • Consider two code words 1011101 1001001 • The distance is the number of bits that are different. distance = 2 • Using the code in a frame, transmit at 1, listen at 0

10. Positive Orthogonal Codes (POC) • Definition of POC: With two code words x and y, we have: • In this paper, the codes are constant-weight and • Strict orthogonality is when

11. Code Assignment to Vehicles • The number of nodes in a vehicular networks maybe very large • However, the codes are reused due to the limit of coverage • Two nodes in the same coverage area should have different codes

12. Code Assignment Protocol • A subset Ca of code C is used for network association • A new vehicle uses a tentative code from Ca • It also sends a Code Information Request (CIQ) packet • The response is a Code Information Response (CIR) packet • CIR from node i contains: ID and code of node i and all its neighbors, subset called Ci • After receiving several CIRs, the new node chooses a code from: Cp = C\Ca\Ui Ci Also, nodes with a permanent code send CIRs periodically, every few seconds

13. Code Assignment and Congestion • When a joining node sends a CIQ to all its neighbors, all of the CIRs response might cause congestion • Use Code Information Response Window (CIRW), like the CW in 802.11 • A node that received a CIQ sets a counter randomly: 1<= counter <= CIRW (uniform distribution) • The counter is decreased by 1 at the elapse of a frame • The joining nodes collects CIRs and chooses a code after elapse of CIRW frames CIQ: Code Information Request; CIR: Code Information Response

14. Packet from user 1 Adaptive Elimination • Eliminating codes with colliding slots • User 1 and User 2 have a colliding slot • User 1 transmits first. It will include its code in the packet • User 2 can hear the packet and know the upcoming collision; and refrain from transmitting User 1 with code 01010 User 2 with code 00011

15. Numerical Results • Analysis of probability of collision and delay • L=128 (code length); N=31 (number of neighbors) Delay, w ranging from 2 to 12, Up=0.1, 0.4, 0.7 and 1 (packets/user/frame) Optimum probability of failure vs. load Adaptive elimination not considered

16. Simulation Setup • One road stretch with 3 lanes • Lane width is 4 m • Two following cars at distance of 30 m • Communication range is 300 m • Frame length L=64 slots • Data rate is 5 Mbps • Safety message is 200 bytes • Time slot is 320 microseconds and the frame is 20.48 ms

17. Probability of Failure • It is good when more than 90% of nodes receive a safety message designated by Ps(0.9) • The inverse is: Pf(0.1) = 1 – Ps(0.9) Good selection of w gives good result. POC performs better than the other. Load: up=0.2 messages/user/frame

18. Average Delay • The average delay is less than 24 timeslots, or 8 ms • If we want to consider the time in buffer, add 10.24 ms to values in the figure “The average delay of all protocols is more or less the same…” Load: up=0.2 messages/user/frame

19. Simulation vs. Analytical Results • In simulation results, channel had Ricean model and adaptive elimination was used • Analytical results didn’t use these mechanisms (ideal channel) • Now, they are removed from simulation to have a comparison with analytical

20. Conclusion • Comparison of SPR, SFR and POC • POC has better probability of successful transmission • We can transmit the safety messages more reliably • The delay of POC isn’t improved from others • But POC transmit more messages

21. Notes • What’s good in this paper? • Analysis, comparison • What’s not so clear? • No explanation, no insight into results