1. A Cross-Layer Multihop Data Delivery Protocol With Fairness Guarantees for Vehicular Networks Gokhan Korkmaz, Eylem Ekici, and Fusun Ozguner Department of Electrical and Computer Engineering, Ohio State University IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, TVT 2006

2. Outline Introduction Controlled Vehicular Internet Access (CVIA) Simulation Conclusion

3. Introduction As mobile wireless devices became the essential parts of our lives, “anytime, anywhere” connectivity gains a growing importance. Inasmuch as an average user spends hours in the traffic everyday, Internet access from vehicles is in great demand.

4. Introduction DSRC(Dedicated Short Range Communication) is one of the ITS(Intelligent Transport System) standards that allows high-speed communications between vehicles and the roadside, or between vehicles. DSRC systems use the IEEE 802.11 protocol as their MAC layer. Multihopping with the IEEE 802.11 protocol suffers from several problems, leading to low throughput and starvation of packets originating from vehicles far away from gateways. 2R2R2R2R2R2R

5. Goal To increase the end-to-end throughput. Achieving fairness in bandwidth usage between vehicles. Mitigating the hidden node problem. Avoiding contention.

6. Network Assumption Vehicular network that accesses the Internet through fixed IGWs (Internet gateways) along the road. Gateways send periodic service announcements to indicate the availability of the service in their service area. The uplink and the downlink packets are transmitted over two frequency-separated channels.

7. Network Assumption Vehicles are equipped with GPS devices used for time synchronization and obtaining vehicle positions. Vehicle positions obtained via GPS are exchanged among one-hop neighbors.

8. Basic Idea Divides the time into slots and the service area of the gateway into segments. Controls time slots the vehicles are allowed to transmit in, how the vehicles access the channel, and to which vehicles the packets are sent.

9. CVIA-Overview S 6 S 5 S 4 S 3 S 2 S 1 Segment Length=R VTR (Virtual Transmission Radius) VTR length N=5 Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6 Slot 7 time Slot Length=T slot

10. S 6 S 5 S 4 S 3 S 2 S 1 CVIA-Overview S i ＋ ： The neighboring segment in the same direction of the packet dissemination. S i － ： The neighboring segment in the opposite direction of the packet dissemination. S 6 S 5 S 4 S 3 S 2 S 1 the direction of packet dissemination SiSi Si＋Si＋ Si－Si－

11. CVIA-Overview Interference parameter (r) ： S 6 S 5 S 4 S 3 S 2 S 1 Segment Length=R

12. CVIA-Overview Active segment ： S i is active in TS j if (i mod r)=(j mod r) For example, r=2 when the current time slot is T 5, the segments S 1, S 3, and S 5 become active. S 6 S 5 S 4 S 3 S 2 S 1 when the current time slot is T 6, the segments S 2, S 4, and S 6 become active. Interference parameter (r) ：

13. S i － S i S i ＋ CVIA-Overview S 6 S 5 S 4 S 3 S 2 S 1 Inbound temporary router ( ) ： The vehicle closet to S i －. All packets entering a segment go through. Outbound temporary router ( ) ： The vehicle closet to S i ＋. All packets leaving the segment go through.

14. S 6 S 5 S 4 S 3 S 2 S 1 CVIA-Overview S i － S i S i ＋ 1. deliver packet train to. the direction of packet dissemination 2. gather local packets. 3. move out packet train to.

15. CVIA- State diagram I. Inactive II. Vehicle Position Update Phase III. Temporary Router Selection Phase IV. Intra-segment Packet Train Movement Phase V. Local Packet Gathering Phase VI. Inter-segment Packet Train Movement Phase New active slot tutu Routers are selected End of packet train End of active slot tgtg

16. CVIA I. Inactive II. Vehicle Position Update Phase III. Temporary Router Selection Phase IV. Intra-segment Packet Train Movement Phase V. Local Packet Gathering Phase VI. Inter-segment Packet Train Movement Phase

17. CVIA - Inactive Vehicles in inactive segments do not access the channel. S i is active in TS j if (i mod r)=(j mod r) i =, ∆d ： The distance of the vehicle to the gateway. j =, ∆t ： The time passed since an absolute reference point. Vehicles obtain their positions and synchronize their clocks using GPS. Vehicles learn the positions of gateways from a digital road map database or the service announcement packets broadcast periodically by gateways. S 5 S 4 S 3 S 2 S 1 ΔdΔd R

18. CVIA I. Inactive II. Vehicle Position Update Phase III. Temporary Router Selection Phase IV. Intra-segment Packet Train Movement Phase V. Local Packet Gathering Phase VI. Inter-segment Packet Train Movement Phase

19. CVIA - Vehicle Position Update Phase t u ： A time interval reserved for position update packets (PUPs). t PUP ： The duration of a PUP. Vehicles pick a random waiting time (RWT) from [0,t u -t PUP ). After an RWT, vehicles access the channel using DCF method of the IEEE 802.11 protocol. The length of a PUP is short, RTS/CTS handshake is not used before sending PUP. S i － S i S i ＋

20. CVIA I. Inactive II. Vehicle Position Update Phase III. Temporary Router Selection Phase IV. Intra-segment Packet Train Movement Phase V. Local Packet Gathering Phase VI. Inter-segment Packet Train Movement Phase

21. CVIA - Temporary Router Selection Phase New and are selected by. The selected routers are called and until they become active. Use “router lifetime” and “safe area” concept. S i － S i S i ＋

22. CVIA - Temporary Router Selection Phase Temporary router must stay inside the segment for an amount of time called the router lifetime. When and are selected at the beginning of TS j. S i － S i S i ＋ is responsible for 1. Receiving packet train relayed by. 2. Gathering local packets. 3. Creating a new packet train and sending this train out of S i to.

23. CVIA - Temporary Router Selection Phase Temporary router must stay inside the segment for an amount of time called the router lifetime. When and are selected at the beginning of TS j. is responsible for 1. Receiving packet train relayed by. 2. Gathering local packets. 3. Creating a new packet train and sending this train out of S i to. becomes active immediately and stays active until the end of TS j. Slot j Slot j+1 Slot j+2 … Slot j+r time … lifetime of is T slot

24. CVIA - Temporary Router Selection Phase Temporary router must stay inside the segment for an amount of time called the router lifetime. When and are selected at the beginning of TS j. is responsible for 1. Receiving packet train coming from in TS j+1. 2. Selecting and announcing and at the beginning of TS j+r. 3. Relaying the packet train to in TS j+r. S i － S i S i ＋

25. CVIA - Temporary Router Selection Phase Temporary router must stay inside the segment for an amount of time called the router lifetime. When and are selected at the beginning of TS j. is responsible for 1. Receiving packet train coming from in TS j+1. 2. Selecting and announcing and at the beginning of TS j+r. 3. Relaying the packet train to in TS j+r. becomes active throughout TS j+1 and TS j+r. Slot j Slot j+1 Slot j+2 … Slot j+r time … lifetime of is (r+1)T slot

26. CVIA - Temporary Router Selection Phase Temporary router must be away from the segment border for a certain amount of distance (x margin ) to stay inside to stay inside the segment during the router lifetime. x margin ＋ ： measured from the segment borders associated with (lifetime=1∙T slot ). x margin － ： measured from the segment borders associated with (lifetime=(r+1)∙T slot ). x margin ＋,v = (∆t pu,v ＋ 1∙T slot )∙V max x margin －,v = (∆t pu,v ＋ (r+1) ∙T slot )∙V max ∆t pu,v ： The elapsed time since the last powsition update from vehicle v.

27. CVIA - Temporary Router Selection Phase S i － S i S i ＋ If vehicle v’s distance to segment borders is more than x margin ＋,v, the vehicle can safely be selected as. If vehicle v’s distance to segment borders is more than x margin －,v, the vehicle can safely be selected as. Distance to segment border x margin ＋,v x margin －,v v1v1 v2v2 Among the vehicles inside the safe area, selects the vehicle closest to S i ＋ as and the vehicle closest to S i － as v1v1 v1,v2v1,v2

28. CVIA I. Inactive II. Vehicle Position Update Phase III. Temporary Router Selection Phase IV. Intra-segment Packet Train Movement Phase V. Local Packet Gathering Phase VI. Inter-segment Packet Train Movement Phase

29. CVIA - Intra-segment Packet Train Movement Phase delivering the packet train coming from S i － to. To avoid contention, has the highest access priority and waits only SIFS duration before accessing the channel. S i － S i S i ＋

30. CVIA I. Inactive II. Vehicle Position Update Phase III. Temporary Router Selection Phase IV. Intra-segment Packet Train Movement Phase V. Local Packet Gathering Phase VI. Inter-segment Packet Train Movement Phase

31. CVIA - Local Packet Gathering Phase S i － S i S i ＋ Vehicles directly send their packets to. Each vehicle starts this phase with a random backoff counter. has the highest channel access priority in this phase.

32. CVIA - Local Packet Gathering Phase accesses the channel and ends the LPG phase in 2 cases ： 1.When the total number of packets queued up in reaches. 2.When the time left in the active slot is just enough to move out all packets in the queues. S i － S i S i ＋ S 5 S 4 S 3 S 2 S 1 100 packets 20 packets 100 packets80 packets60 packets40 packets20 packets

33. CVIA I. Inactive II. Vehicle Position Update Phase III. Temporary Router Selection Phase IV. Intra-segment Packet Train Movement Phase V. Local Packet Gathering Phase VI. Inter-segment Packet Train Movement Phase

34. CVIA - Inter-segment Packet Train Movement Phase S i － S i S i ＋ forms a packet train and moved to before the end of TS j. There is only one in S i ＋, does not need to specify the vehicle ID of the destination in the data train.

35. CVIA I. Inactive II. Vehicle Position Update Phase III. Temporary Router Selection Phase IV. Intra-segment Packet Train Movement Phase V. Local Packet Gathering Phase VI. Inter-segment Packet Train Movement Phase

36. CVIA- State diagram-Download I. Inactive II. Vehicle Position Update Phase III. Temporary Router Selection Phase IV. Intra-segment Packet Train Movement Phase V. Local Packet Gathering Phase VI. Inter-segment Packet Train Movement Phase

37. I. Inactive II. Vehicle Position Update Phase III. Temporary Router Selection Phase IV. Local Packet Distribution and Intra-segment Packet Train Movement Phase VI. Inter-segment Packet Train Movement Phase CVIA- State diagram-Download

38. I. Inactive II. Vehicle Position Update Phase III. Temporary Router Selection Phase IV. Local Packet Distriburion and Intra-segment Packet Train Movement Phase VI. Inter-segment Packet Train Movement Phase CVIA- State diagram-Download

39. CVIA - Download Local Packet Distriburion and Intra-segment Packet Train Movement Phase delivering the packet train coming from S i － to. Some packets leave the train, and a shorter train keeps moving away from the gateway. S i － S i S i ＋

40. Performance Evaluation Simulation scenarios Density34 vehicles/km Speed90 ±5 km/h (do not change during simulation) Transmission range350 m Data rate27 Mbps Payload2312 or 500 bytes Base protocol802.11a Interference range to transmission range ratio1 Maximum number of packet retrials10 Simulation time10 s Simulation repetitions20

41. Performance Evaluation ScenariopayloadVTR I2304 bytes4R (N=4) II500 bytes4R (N=4) III2304 bytes8R (N=8) CVIA parameters T slot 100 ms N4 or 8 r2

42. Performance Evaluation Scenario I ThroughputDetailed segment throughputs

43. Performance Evaluation Scenario II ThroughputDetailed segment throughputs

44. Performance Evaluation Fairness Fairness Index (FI) = Thr i ： the throughput of segment i

45. Performance Evaluation Failed packets In LPG phase, the contention-based channel access is used. Probability of packet collision ≈ 0.3 Probability of packet dropping ≈ 0.3 10 ≈ 5×10 -6

46. Conclusion Mitigates the hidden node problem. Avoids contention while moving packets among road segments. Provides fairness among segments by controlling the contents of the packet trains. Unnecessary RTS/CTS overhead while moving packets among road segments. The throughput gain obtained using the proposed CVIA protocol is higher for short payload scenario.