A Cross-Layer Multihop Data Delivery Protocol With Fairness Guarantees for Vehicular Networks

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. 2R 2R 2R

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 Segment Length=R S6 S5 S4 S3 S2 S1 Slot1 Slot2 Slot3 Slot4 Slot5 Slot6 Slot7 time VTR (Virtual Transmission Radius) VTR length N=5 Slot Length=Tslot

10. CVIA-Overview the direction of packet dissemination Si－ Si Si＋ S6 S5 S4 S3 S2 S1 S6 S5 S4 S3 S2 S1 Si＋：The neighboring segment in the same direction of the packet dissemination. Si－：The neighboring segment in the opposite direction of the packet dissemination.

11. CVIA-Overview Segment Length=R S6 S5 S4 S3 S2 S1 Interference parameter (r)：

12. CVIA-Overview S6 S5 S4 S3 S2 S1 Active segment：Siis active in TSj if (i mod r)=(j mod r) For example, r=2 when the current time slot is T5 , the segments S1, S3, and S5 become active. Interference parameter (r)： when the current time slot is T6 , the segments S2, S4, and S6 become active.

13. CVIA-Overview S6 S5 S4 S3 S2 S1 Inbound temporary router ( )：The vehicle closet to Si－. All packets entering a segment go through. Outbound temporary router ( )：The vehicle closet to Si＋. All packets leaving the segment go through . Si－ Si Si＋

14. CVIA-Overview the direction of packet dissemination S6 S5 S4 S3 S2 S1 Si－Si Si＋ 1. deliver packet train to . 2. gather local packets. 3. move out packet train to .

15. CVIA-State diagram End of active slot New active slot tg tu Routers are selected End of packet train

16. CVIA

17. CVIA - Inactive Vehicles in inactive segments do not access the channel. Siis active in TSj 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. Δd R S5 S4 S3 S2 S1

18. CVIA

19. CVIA - Vehicle Position Update Phase Si－Si Si＋ tu：A time interval reserved for position update packets (PUPs). tPUP：The duration of a PUP. Vehicles pick a random waiting time (RWT) from [0,tu-tPUP). 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.

20. CVIA

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

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 TSj . 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 Si to . Si－Si Si＋

23. CVIA - Temporary Router Selection Phase … Slotj Slotj+1Slotj+2… Slotj+r time Temporary router must stay inside the segment for an amount of time called the router lifetime. When and are selected at the beginning of TSj . 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 Si to . becomes active immediately and stays active until the end of TSj . lifetime of is Tslot

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 TSj . is responsible for 1. Receiving packet train coming from in TSj+1. 2. Selecting and announcing and at the beginning of TSj+r . 3. Relaying the packet train to in TSj+r . Si－Si Si＋

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

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

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

28. CVIA

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

30. CVIA

31. CVIA - Local Packet Gathering Phase Si－Si Si＋ 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 100 packets S5 S4 S3 S2 S1 Si－Si Si＋ 20 packets 20 packets 20 packets 40 packets 20 packets 60 packets 80 packets 20 packets 100 packets 20 packets • accesses the channel and ends the LPG phase in 2 cases： • When the total number of packets queued up in reaches . • When the time left in the active slot is just enough to move out all packets in the queues.

33. CVIA

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

35. CVIA

36. CVIA-State diagram-Download

37. CVIA-State diagram-Download

38. CVIA-State diagram-Download

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

40. Performance Evaluation

41. Performance Evaluation

42. Performance Evaluation

43. Performance Evaluation

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

45. Performance Evaluation In LPG phase, the contention-based channel access is used. Probability of packet collision ≈ 0.3 Probability of packet dropping ≈ 0.310 ≈ 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.

47. Thanks~~~