1. A Multihop Peer-Communication Protocol With Fairness Guarantee for IEEE 802.16-Based Vehicular Networks Kun Yang, Shumao Ou, Hsiao-Hwa Chen and Jianghua He IEEE Transactions on Vehicular Technology,2007 Mei-jhen Chen

2. Outline  Introduction  System Model  Proposed Protocol － CEPEC (  Proposed Protocol － CEPEC (coordinated external peer-communication )  Transmission Scheduling Within and Across Segments  Simulation Design and Results  Conclusion

3. Introduction (1/4)   Vehicles are an essential part of people’s everyday life.   Much work has been conducted to provide a common platform facilitating intervehicle communications (IVCs) or intelligent transportation systems.   IVC is necessary to realize traffic-condition monitoring, dynamic route scheduling, emergency- message dissemination, and, most importantly, safe driving.   Safety communication is essential for IVC, and dedicated short-range communications (DSRC) is a key enabling technology for it.

4. Introduction (2/4)   This is largely due to the fact that IEEE 802.11 is recommended by DSRC systems as the default protocol.   However, the contention nature of IEEE 802.11 generally renders itself hard to provide guaranteed QoS.   In this paper, authors consider a vehicular network that accesses the Internet through fixed roadside IEEE 802.16 base stations (BSs).

5. Introduction (3/4)

6. Introduction (4/4)   In this paper, we present a cross-layer protocol designed mainly for vehicle–roadside communication (VRC).   Authors name this protocol as coordinated external peer-communication (CEPEC) protocol for vehicular networks.   The major objective of CEPEC is to increase the end- to-end throughput in IEEE 802.16-enabled VRC while ensuring fairness guarantee in bandwidth usage among road sections.

7. System Model -Problem Definition (1/3)   As far as IEEE 802.16 is concerned, works have been conducted on providing Internet services using different QoS supporting scheduling algorithms.   This paper focuses on another equally important issue arising in vehicular networks, i.e., data transmission from moving vehicles on highways to peers connected to the Internet.   These peers are typically not commercial service/content providers but are individuals such as the driver/passenger’s family members or colleagues.   We call this type of communication as external peer communication (EPC).

8. System Model -Problem Definition (2/3)   This paper is largely inspired by the Controlled Vehicular Internet Access (CVIA) protocol. Data relaying is also needed within segments and this leads to longer delay. Select two routers in a segment: The routers should be as close to the segment borders as possible. They need to stay inside the segment during the active period of the segment given their high moving speeds. The way fairness is measured is at segment level and ignores the unevenness of traffic requests from different vehicles.

9. System Model -Problem Definition (3/3)   In CEPEC, R : the physical transmission range of the vehicles and BSs.

10. System Model -Network Model   The network model employed in this paper is briefed as follows. 1. 1. Each vehicle is equipped with a global-positioning system (GPS) device for time synchronization and providing vehicle positions. 2. 2. The physical layer of mesh mode uses time-division multiple access (TDMA) for channel allocation. 3. 3. Time-division duplex is utilized to share the channel between uplink and downlink. 4. 4. A mesh frame consists of a control and a data subframe. The multiple- access scheme at the radio interface is orthogonal frequency-division multiple access.

11. Proposed Protocol － CEPEC -Overview   CEPEC is a multihop cluster-based protocol.   Each segment represents a cluster, and data delivery from vehicles to a BS is carried out on a segment-to- segment basis. LP i :All packets collected locally from segment S i. AP i :aggregated with the packets received from the neighboring segment S i+1. AP i = LP i + AP i+1 AP i AP i+1 LP i

12. Proposed Protocol － CEPEC -Protocol Operation   In CEPEC, each segment Si normally goes through six phases as a lifecycle. 1. Inactive 2. Inter- segment Packet (AP i+1 ) Receiving 3. Local Packet (LP i ) Collecting 6. Inter- segment Packet (AP i ) Sending 5. Next Head (SH i-1 ) Selection 4. Aggregated Packet (AP i ) Generating wiwi aiai bibi hihi cici

13. Proposed Protocol － CEPEC -Protocol Operation 1. Inactive phase: A segment becomes inactive for duration w i if no time slot is allocated to it. In CEPEC, inactive phase is employed to ensure no interference from Si to the ongoing data transmission in the neighboring segments. The duration of w i depends on the scheduling algorithms. If a mesh frame can be shared by multiple segments, then wi can be very small. Otherwise, w i can be as long as the duration of a segment’s lifecycle.

14. Proposed Protocol － CEPEC -Protocol Operation 2. Intersegment packet-receiving phase: In CEPEC, each S i (i≠N) needs to relay packets from S i+1 in uplink. S N, as the farthermost segment away from the BS, does not have this phase. After S i becomes active, SH i starts receiving packets from SH i+1. In TDMA-based IEEE 802.16, the AP-sending phase of SH i+1 can be synchronized to the AP-receiving phase of SH i. A time interval of a i is reserved for this purpose. the amount of traffic to be relayed from SH i+1 to SH i the fairness mechanism the number of hops to the BS and the bandwidth capacity of the BS

15. Proposed Protocol － CEPEC -Protocol Operation 3. Local-packet-collecting (LPC) phase of duration b i : After receiving AP i +1 from SH i+1, the vehicles start competing for transmission opportunities in the control subframe. The vehicles that are successfully granted a transmission opportunity can send their data transmission requests to SH i. Authors assume that all traffics are of same type (BE) and have the same priority. Upon receipt of the requests, a scheduling algorithm is activated to allocate time slots to vehicles. Vehicles in Si send their data packets directly to SHi. All these packets received by SHi are aggregated into LPi.

16. Proposed Protocol － CEPEC -Protocol Operation 4. AP generating phase: AP i+1 and LP i are merged together to create a new AP: AP i. Only some local processing is performed in this phase, so the duration for this phase is neglected.

17. Proposed Protocol － CEPEC -Protocol Operation 5. Next head-selection phase: Before AP i is transmitted to S i−1 (or BS if the current segment is S 1 ), SH i−1 needs to be selected. Vehicles in S i−1 start to compete to become the SH i−1. Before competition, each vehicle calculates the segment ID it belongs to using x v : the X-coordination of vehicle v obtained from its onboard GPS x BS :the BS’s X-coordination obtained via the BS’s EAP (existence announcement packets)

18. Proposed Protocol － CEPEC -Protocol Operation A BS periodically broadcasts the following information to all vehicles within its SvC: its capacity C the number of segments N the segment length L Otherwise, they can be informed of to vehicles by the BS when vehicles register themselves A competing vehicle v has to satisfy the following two conditions. C1: x v + (a i + b i + c i ) × V max < x b- i x b- i : the X-coordinate of the segment border that is closer to the BS. i :the segment ID, where vehicle v is located in. V max : the maximum velocity of vehicles. C2: |x C i − s v | < d d = |x C i − s a | ∧ a ∈ Si ∧ a ≠ v.

19. Proposed Protocol － CEPEC -Protocol Operation 6. Intersegment packet-sending phase: AP i is forwarded to SH i−1. In CEPEC, duration of c i is reserved for this phase. c i should be long enough to forward packets from both local segment and packets collected from other preceding segments.

20. Proposed Protocol － CEPEC -Fairness   Packets originated from vehicles locating at S i (i =2,..., N) have to go through i segments to reach the BS. C : a fixed overall bandwidth capacity at the BS. C/N : each segment is granted an equal portion of C.   SH allocates bandwidth to requesting vehicles in proportion to their packet-transmission requirements.   Req i : the overall requested packet transmission in S i.   p = (C/N)/Req i :an equal proportion of each vehicle’s requested packet is transmitted to SH i. Req i > C/N : saturation status. Req i ≤ C/N : all Req i is transmitted.

21. Transmission Scheduling Within and Across Segments   Authors use a centralized-scheduling scheme to allocate aggregate transmission time to the SHs. the bandwidth request-and-grant mechanism specified in the 802.16 standard   Goal: to maximize throughput delivered to the BSs while ensuring bandwidth-usage fairness.

22. Transmission Scheduling Within and Across Segments -Allocation of Time to Segments   The scheduling is made with a period of n frm frames.   t frm : the duration of a frame.   In a schedule, each SH will be granted by the BS the transmission time of n i frames for segment i. n i = n i,i + n i,i−1

23. Transmission Scheduling Within and Across Segments -Allocation of Time to Segments   The centralized-scheduling problem is formulated by an optimization model.

24. Transmission Scheduling Within and Across Segments -Allocation of Time to Segments

25. Simulation Design and Results   A vehicular-network simulator has been developed by us to simulate the performance of both the CEPEC protocol and the IEEE 802.16 protocol.   There is a two-lane straight highway of limited length with each lane for each direction of traffic flow.   There is more than one sublane all carrying traffic to same direction.   The highway is divided into a few segments of fixed length L starting from a BS.   These segments constitute the SvC of this BS.

27. Simulation Design and Results   Simulated vehicles randomly enter the SvC with exponentially distributed interarrival time at a rate of 100 vehicles per minute.   Vehicle speed follows a Gaussian distribution with a mean of 85 km/h and a standard deviation of 10 km/h.   Uplink data request of each vehicle follows an exponential distribution with the rate of 512 kb/s.   The capacity of the BS is 40 Mb/s.   Channel data rate varies between 5–40 Mb/s.   Transmission ranges of all vehicles and the BS are same: R = 2.4 km.   Payload of packets varies between 500 and 2200 B.

28. Simulation Design and Results Fig 6. Overall uplink throughput Scenario 1: payload = 500 B, N = 4. Scenario 2: payload = 2200 B, N = 4. Scenario 3: payload = 2200 B, N = 8.

29. Simulation Design and Results Fig 7. Overall delay Scenario 1: payload = 500 B, N = 4. Scenario 2: payload = 2200 B, N = 4. Scenario 3: payload = 2200 B, N = 8.

30. Simulation Design and Results  SvC size : N=4 and 8   L1 = 2R/3, L2 = 2R√10, and L3 =1R/2 (L1 < L2 < L3) Fig 8. Lengths of segments on delayFig 9. Lengths of segments on throughput

31. Conclusion   This paper proposes a novel communication protocol for vehicular networks that supports vehicles, on highways, communicating with peers at home or office.   The CEPEC protocol coordinates the functions of physical, MAC, and network layers to provide a fair and handoff-free solution for uplink packet delivery from vehicles to BSs.   The simulation results have showed that the proposed protocol has better performance than the standard IEEE 802.16 protocol in terms of both end-to-end throughput and packet delay.