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MAC Performance Analysis for Vehicle to Infrastructure Communication Tom H. Luan*, Xinhua Ling§ , Xuemin (Sherman) Shen* *BroadBand Communication Research Group University of Waterloo § Research In Motion Outline 1. Introduction to Vehicular Network 2. Model of MAC in V2I communication 3. Simulation 4. Conclusion 22 Why Vehicular Networks ? Internet becomes an essential part of our daily life Watch video on Youtube; order literature on Amzone; catch the final moments of an eBay auction … Americans spend up to 540 hours on average a year in their vehicles (10% of the waking time) Internet access from vehicles is still luxury Vehicular Network To provide cheap yet high throughput data service for vehicles on the road 33 V2V and V2I Communications Vehicle to RSU (V2R or V2I) Vehicle to Vehicle (V2V) RSU (roadside unit) Infotainment: Internet access, video streaming, music download, etc. MAC throughput performance evaluation of V2I communication 44 Standard and Research Efforts IEEE drafts 802.11p standard to permit vehicular communication 802.11a radio technology + 802.11e EDCA MAC Multi-channel: 6 service channels + 1 control channel Drive-thru Internet Using off-the-shelf 802.11b hardware, a vehicle could maintain a connection to a roadside AP for 500m and transfer 9MB of data at 80km/h using either TCP or UDP Image from http://www.drive-thru-internet.org/ [1] J. Ott and D. Kutscher, "Drive-thru Internet: IEEE 802.11 b for 'automobile' users," in IEEE INFOCOM, 2004 55 Standard and Research Efforts (cont’d) CarTel in MIT [2] City-wide experiment showing the intermittent and short-lived connectivity, yet high throughput while available Small scale network without considering MAC Link layer and transport layer performance What if a great number of vehicles moving fast? [2] V. Bychkovsky, B. Hull, A. Miu, H. Balakrishnan and S. Madden, "A measurement study of vehicular internet access using in situ Wi-Fi networks," in ACM MobiCom, 2006 66 Problem Statement MAC performance evaluation for fast-moving large scale vehicular networks We consider 802.11b DCF Used by most trail networks, e.g., Drive-thru Compatible to WiFi device (e.g., iPod Touch) The basis of 802.11p MAC 77 Network Model Perfect channel without packet loss and errors Saturated case: nodes always have a packet to transmit Multi-rate transmission according to the distance to RSU Spatial zones: the radio coverage of one RSU is divide into Z = {0, 1, …, N} zones according to node transmission rate p-persistent MAC: nodes transmit with a constant probability pz for different zone n in Z RSU Mobility Model n Mirror zones along RSU nmap Received SNR (dB) Sojourn time of vehicles in each zone n is geometrically distributed with mean tn Within a period , vehicle moves from zone n to n+1 with the probability /tn, and no change with the left probability RSU Markov chain 1 2 1 2 N-1 N N-1 N Zone 88 Markov Model of Vehicle Nodes Back off Interval Countdown Each node can be represented by {z(t), b(t)} z(t): zone the vehicle is current in at time t b(t): the value of backoff counter of the node at time t Geometric distribution (p1) 1,0 1,1 1,2 1,W-1 Movement of Vehicles 2,0 2,1 2,2 2,W-1 N-1,0 Geometric distribution (pN) N,0 N,1 N,2 N,W-1 2D Markov chain embedded at the commencement of the backoff counter countdown Upon the decrement of backoff counter, vehicle may either move to the next zone or stay in the original zone When coming into a new zone, different transmission probability is applied 99 Simulation Setup Zone 2 ps ps Zone 1 1 1 Mb 5.5 Mb Zone 0 ... 2M b ps ps By default, 50 vehicles move at constant speed with v = 80 km/h RSU Mb 11 Radio coverage of RSU is 250m, which is divided into 8 zones ... Zone N Zone 0 When arriving at the end of the road session (zone N), vehicles reenter zone 0 and start a new iteration of communication Two schemes Equal contention window (transmission probability p) in all zones Differential contention window in zones 10 10 Nodal Throughput in Each Zone n Nodal Throughput in Each Zone sn = Average pkt length in each trans. Mean interval between consecutive trans. Integrated Throughput S = ∑ Xn sn n Where Xn is the node population in zone n Using equal CW in all zones would suffer from performance anomaly 11 11 Increasing Velocity With enhanced node velocity, nodes in front zones have higher throughput than the back zones The small CW in zone 4 benefits the following zones System throughput reduces when velocity increases 12 12 Conclusion Throughput performance evaluation of DCF in the vehicle to infrastructure communication Increase the velocity would reduce the system throughput Future work Optimal design of DCF (contention window) QoS provision with call admission control etc. 13 13 Question and Answers ? Thank you ! bbcr.uwaterloo.ca/~hluan 14