On Scheduling Vehicle-Roadside Data Access Yang Zhang Jing Zhao and Guohong Cao The Pennsylvania State University

2 VANET 2007, Sept 10 th, Montreal, Canada The Big Picture Vehicular Ad-hoc Networks - VANET Moving Vehicles RoadSide Units(RSU) Local broadcasting infostations access point Applications Commercial Advertisement Real-Time Traffic Digital Map Downloading Task Service Scheduling of Vehicle- Roadside Data Access

3 VANET 2007, Sept 10 th, Montreal, Canada Challenges Bandwidth Competition All requests compete for the same limited bandwidth Time Constraint Vehicles are moving and they only stay in the RSU area for a short period of time Data Upload/Download The miss of upload leads to data staleness

4 VANET 2007, Sept 10 th, Montreal, Canada Assumptions and Performance Metrics Assumptions Location-aware and Deadline-aware The RSU maintains a service cycle Service non-preemptive Performance Metrics Service Ratio Ratio of the number of requests served before the service deadline to the total number of arriving requests. Data Quality Percentage of fresh data access

5 VANET 2007, Sept 10 th, Montreal, Canada FCFS, FDF, SDF First Come First Serve (FCFS): the request with the earliest arrival time will be served first. First Deadline First (FDF): the request with the most urgency will be served first. Smallest Datasize First (SDF): the data with a small size will be served first. workload

6 VANET 2007, Sept 10 th, Montreal, Canada D*S Scheduling Intuition Given two requests with the same deadline, the one asking for a small size data should be served first Given two requests asking for the data items with same size, the one with an earlier deadline should be served first Basic Idea: Assign each arrival request a service value based on its deadline and data size, called DS_value as its service priority weight DS_value=(Deadline-CurrentClock)*DataSize

7 VANET 2007, Sept 10 th, Montreal, Canada The Implementation of D*S Dual-List Search from the top of D_list Set MinS and MinD Search D_List and S_list alternatively Stops when the checked entry goes across MinD or MinS, or when the search reaches the halfway of both lists.

8 VANET 2007, Sept 10 th, Montreal, Canada Download Optimization: Broadcasting Observation some requests may ask for downloading the same data item wireless communication has the broadcast capability Basic Idea delay some requested data and broadcast it before the deadlines, then several requests may be served through a single broadcast the data with more pending requests should be served first DSN_value=(Deadline-CurrentClock)*DataSize/Number

9 VANET 2007, Sept 10 th, Montreal, Canada D*S/N: The Selection of Representative Deadline When calculating their DSN value, we need to assign each pending request group a single deadline to estimate the urgency of the whole group.

10 VANET 2007, Sept 10 th, Montreal, Canada The Problem of D*S/N Data Quality!! DSN_value=(Deadline-CurrentClock)*DataSize/Number For upload request, it is not necessary to maintain several update requests for one data item since only the last update is useful Number value of update requests is always 1, which makes it not fair for update requests to compete for the bandwidth D*S/N can improve the system service ratio but sacrifice the service opportunity of update requests, which degrades the data quality for downloading

11 VANET 2007, Sept 10 th, Montreal, Canada Upload Optimization: 2-Step Scheduling Basic Idea: two priority queues: one for the update requests and the other for the download requests. the data server provides two queues with different bandwidth (i.e., service probability) Benefits of Using Two Separate Priority Queues we only need to compare the download queue and update queue instead of individual updates and downloads update and download queues can have their own priority scheduling schemes

12 VANET 2007, Sept 10 th, Montreal, Canada Step I: Update Queue or Download Queue Service_Profit: the sum of the profit gained from upload and download Service_Profit = Update_Profit + FreshDownload_Profit + StaleDownload_Profit Assume one update request can contribute the same profit as one download request with fresh data. Assume the profit of download with stale data will degrade with α Bandwidth Allocation ( ρ ): the download requests share ρ of the bandwidth and the update requests share the rest, 1- ρ Goal: set ρ thus achieving the balance between update and download

13 VANET 2007, Sept 10 th, Montreal, Canada Step I (cont.) r u and r d : service rate of update and download requests Update profit rate depends on: service rate, r u, and allocated bandwidth, 1- ρ Update_Profit r u (1- ρ) t Download profit rate relies on service rate, r d, bandwidth allocation ρ, and data quality for each download. FreshDownload_Profit r d ρ (1- ρ) t StaleDownload_Profit r d ρ 2 α t

14 VANET 2007, Sept 10 th, Montreal, Canada Step I (cont.) Service_Profit r u (1- ρ) t +r d ρ (1- ρ) t + r d ρ 2 α t (0 ρ 1)

15 VANET 2007, Sept 10 th, Montreal, Canada Step II: D*S/N and D*S/R D*S/N for download queue D*S/R for update queue Given two update requests with the same d*s value, the request that updates hot data should have a higher service priority R is the service rate of the requests on corresponding data item in the download queue DSR_value = (Deadline-CurrentClock)*DataSize/R

16 VANET 2007, Sept 10 th, Montreal, Canada Implementation of 2-Step Scheduling The workload is examined with a time period τ (adaptation window) At the beginning of each τ, ρ is re-calculated

17 VANET 2007, Sept 10 th, Montreal, Canada Simulation Setup NS-2 based 400m*400m square street scenario One RSU server is located at the center of two 2-way roads 40 vehicles randomly deployed on each lane Each vehicle issues request with a probability p Access pattern of each data item follows Zipf distribution

18 VANET 2007, Sept 10 th, Montreal, Canada Performance Evaluation: Effect of Workload As workload increases, D*S/N can achieve the highest service ratio while its data quality degrades dramatically

19 VANET 2007, Sept 10 th, Montreal, Canada Performance Evaluation: Effect of Access Pattern(θ) Change of θ does not have too much impact on the performance of FCFS, FDF, SDF and D*S D*S/N and 2-Step can benefit from the skewness of the data access pattern with the increase of θ

20 VANET 2007, Sept 10 th, Montreal, Canada Performance Evaluation: Effect of Access Pattern(Download/Update Ratio)

21 VANET 2007, Sept 10 th, Montreal, Canada Performance Evaluation: Adaptivity to Workload Condition Change 2-Step scheme can achieve good performances in almost all scenarios. ρ adapts quickly when workload condition changes

22 VANET 2007, Sept 10 th, Montreal, Canada Conclusion We addressed some challenges in vehicle-roadside data access We proposed a basic scheduling scheme called D*S to consider both service deadline and data size when making scheduling decisions. To make use of the wireless broadcasting, we proposed a new scheduling scheme called D*S/N to serve multiple requests with a single broadcast. We also proposed a Two-Step scheduling scheme to provide a balance between serving download and update requests. Simulation results show that the Two-Step scheduling scheme outperforms other scheduling schemes. Further, the Two-Step scheduling scheme is adaptive to different workload scenarios.

Thank You Yang Zhang Q & A ?