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Reliable Broadcast of Safety Messages in Vehicular Ad Hoc Networks Farzad Farnoud (Hassanzadeh) and Shakrokh Valaee University of Toronto, Canada In proceedings of IEEE Infocom 2009. Presented by: Zakhia Abichar Computer Systems Lab Group Meeting Nov. 11, 2009. Vehicular Wireless Networks.
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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
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)
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)
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
Outline • Characteristics of safety traffic in vehicular networks • Related work • Proposed broadcast protocol • Evaluation of the MAC protocol
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
Notes • What’s good in this paper? • Analysis, comparison • What’s not so clear? • No explanation, no insight into results