TDMA Slot Reservation in Cluster-Based VANETs

2,703 views

Published on

Mohammad Almalag's PhD Defense Slides
Department of Computer Science
Old Dominion University
April 3, 2013

Note: You may need to download the file to see all of the animations.

Published in: Technology, Health & Medicine
1 Comment
0 Likes
Statistics
Notes
  • pls send tis ppt to my mail lokeshwaric@gmail.com,,,,,,,,,,,,it wil useful most for me
       Reply 
    Are you sure you want to  Yes  No
    Your message goes here
  • Be the first to like this

No Downloads
Views
Total views
2,703
On SlideShare
0
From Embeds
0
Number of Embeds
938
Actions
Shares
0
Downloads
109
Comments
1
Likes
0
Embeds 0
No embeds

No notes for slide
  • This question was the starting point of research. The answer was NO Then I started to think, how can I make the answer to be YES?
  • Vehicular Ad-hoc Networks (VANETs) are important component of Intelligent Transportation Systems (ITS). VANETs enable the exchange of messages between vehicles and between vehicles and infrastructure, as shown in the figure. Such communications aim to: Increase safety on the road Improve transportation efficiency And provide comfort to drivers and passengers.
  • VANET is useful for a wide variety of applications, including both safety applications and non-safety applications [24]. Safety apps: Talk about EEBL as an example. If there is a truck in front of you that is blocking the view of the traffic ahead. And if someone used the break, the truck will block the tail lights, which will make the 1 sec not enough to react!!!! Non-safety apps: Talk about if you are traveling to Cali and you have a child that wants to watch a movie, how you can download it in the car.
  • -In the US, VANETs use 75 MHz of spectrum specially allocated by the U.S. Federal Communications Commission for Vehicle-to-Vehicle (V2V) and Vehicle-to-Infrastructure communication (V2I) using Dedicated Short Range Communication (DSRC) technology [19]. The spectrum band is divided into seven channels: Channel 178 is the control channel (used for safety/update messages) The other SIX channels are service channels. (to support non-safety messages) WAVE The IEEE has developed the 1609 family of standards for Wireless Access in Vehicular Environments (WAVE) In WAVE, the IEEE 1609.4 standard operates on top of IEEE 802.11p in the MAC layer. Which is Based on CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) mechanism IEEE 1609.4 focuses on multichannel operations of a DSRC radio. It introduces a Sync Interval
  • - scalability is the ability to handle the addition of nodes or objects without suffering a noticeable loss in performance or increase in administrative complexity. 1- Due to the shared wireless channel with a CSMA/CA medium access scheme, the limited bandwidth is further decreased by poor channel utilization. Explain the hidden terminal problem
  • Safety messages need to be sent every 100 msec. - Why? -> This desire came from the allowable latency requirements of Life-Critical safety applications. -> note that what Wang et al. did was  -- to enforce 90% and 95% reliability of safety messages, they changed the amount of time that non-safety apps can send during the 100ms interval. In high density, CCH interval will be higher than the SCH interval. So, non-safety mesg will be almost stopped. Traffic density: Low = 40 Medium = 80 High = 160
  • (JUST SAY ACRONYMS) The issues of MAC protocols for WAVE, as described above, led researchers in developing new MAC protocols for VANETs. In general, MAC protocols can be classified into three different categories: channel partitioning: channel partitioning is done using the following methods: Time Division Multiple Access (TDMA), Frequency-Division Multiple Access (FDMA) and Code-Division Multiple Access (CDMA). These protocols are based on sharing the channel efficiently at high uniform load. The main advantages of protocols developed under this category are reducing interference between nodes and providing fairness. However, they add allocation complexity and suffer from inefficient channel utilization at low loads. random access: also known as contention based protocols, are based on the notion of CSMA. The goal of MAC protocols is to increase throughput, so protocols under this category aim to keep packet collisions to a minimum. The advantage of random access protocols is that they are not sensitive to underlying mobility and topology changes. Also, CSMA protocols are efficient in low load scenarios. However, in networks such as VANET, the hidden terminal problem and exposed terminals affect the system performance. taking turns: use either polling (master-slave) or token ring techniques. Such techniques provide fairness by giving each node a turn to transmit. They also provide a real time bandwidth allocation.
  • VeSOMAC: a MAC protocol for inter-vehicular wireless networking using DSRC They designed a self-configuring TDMA slot reservation protocol capable of inter-vehicle message delivery with short and deterministic delay bounds. To achieve the shortest delay, vehicles determine their TDMA time slot based on their location and movement on the road. the TDMA slot assignment is designed to be in the same sequential order with respect to the vehicles physical location. Dis. Adv: VeSOMAC does not explain the communication between vehicles and RSUs. If the message is sent from tail to head, it will need a time frame for each hop VeMAC: TDMA two transceivers: One for CCH: safety messages and control messages for slot assignment. One for SCHs. VeMAC is designed based on having one control channel and multiple service channels in the network. Vehicles in both directions and RSUs are assigned to time slots in the same TDMA time frame. vehicles are said to be either travelling in the right (R) or left (L) direction. The time frame in VeMAC is divided into three different slots sets, L, R, and F. - To avoid the hidden terminal problem: each vehicle in VeMAC includes in the header on the CCH: the time slots used by the vehicle on the SCH the time slot used by each neighboring vehicle on the CCH the time slots used by each neighboring vehicle on the SCH the position and the current direction of the vehicle Overhead: Before accessing the CCH, vehicles has to check for the ones that are not used by other vehicles 2-hops away. This information is provided by other vehicles nearby. PP-CSMA: based on a priority scheme in CSMA using different backoff time spacing prioritization scheme as a combination of: Closeness of the transmitting vehicle to the receiving vehicle the message type Besides the priority scheme, a polling scheme: Each vehicle maintains a polling table that holds information about other vehicles positions. Dis. Adv: Did mention broadcasting MCTRP : More than one ring These rings are formed according to the velocity of vehicles and the road conditions. If a vehicle has an emergency message to be sent, it generates a tone, which is out of the frequency band used for data transmission. If the vehicle is in the receivers polling table, the receiver will clear for the sender to transmit the cd csdc
  • In this case, we guarantee that every vehicle in the cluster sends one update/safety message every 100 msec to meet the safety message requirements. What is accuracy of GPS time sync? Typical accuracies range from better than 100 nanoseconds to a few milliseconds depending on the synchronization protocol
  • Once the cluster is formed: The CH will assign local IDs to all cluster members. Then, the CH will send a table of the assigned local IDs to all cluster members
  • Once the cluster members receive their local IDs, they will be using them to access to the channel to send/receive safety/update and non-safety messages. Notice that the Integer Division Theorem guarantees that if i ≠ j then either:
  • Our goal is to design lightweight communication protocols that avoid, to the largest extent possible, the involvement of the CH in setting up connections between vehicles. The CH will also use the un-used mini-slots to rebroadcast safety messages. Remind that all vehicles in cluster within range of each other.
  • -
  • you need to add an example after this slide (before moving on to cluster).  Say you have two vehicles that need to communicate while sending their update messages.  How does this work?  Here you could explain the issue with missing update messages if their SCH is the same slot as others' CCH.  You also should explain that the CH will rebroadcast update messages during unused slots.  It's never mentioned except in the evaluation, but it's an important part of the protocol.
  • The CH will be selected based on the flow of the majority of traffic. If 2 CHs candidates have the same CHL, the one with the lowest ID will be the CH
  • A Clusterhead leaving the cluster A new vehicle joining the cluster A cluster member leaving the cluster Two clusters get in range of each other A multi-hop cluster shrinks to a one-hop cluster
  • We modified TC-MAC to overcome the inter-cluster communications challenge. The modification I made is using CDMA combined with TC-MAC for inter-cluster communications. With the use of the CDMA assigned code and TC-MAC inside the cluster, intra-cluster collisions should not happen. Also, this will support broadcast inside the cluster without the risk of the interference with other vehicles from other clusters. Code Assignment protocol is used to assign a CDMA code to the cluster
  • Code Assignment protocol is used to assign a CDMA code to the cluster 14 codes CDMA/TCMAC considers vehicles in opposite directions, as well as opposite highway.
  • The Recovery Protocol works when a vehicle in a cluster detects another vehicle from another cluster using the same CDMA code When the gap between the two clusters is less than 300 m, at least one vehicle from each cluster will detect the present of the other cluster, vehicles X and Y. The vehicle for the approaching cluster will inform its CH The CH will pick another CODE
  • the GWs of both clusters need to get engaged in the process. the GWs of both clusters need to get engaged in the process. Let us assume that GW_tail of cluster A is sending and GW_head of cluster B At time t : GW_ tail of cluster A will send an advertisement on the mini-slot 0 on the CCH using CDMA code Code 1 . This advertisement will include the cluster size of A and the CDMA code used by cluster A (3, and Code 2 ). GWhead of cluster B will receive the advertisement from GWtail of cluster A. Then, it will pass the advertisement to the CH of B during its own time in cluster B using CDMA code Code 3. Note, this process could be done within 100 msec, before GWhead of cluster B sends back its own advertisement acknowledgement to GWtail of cluster A on the CCH. At time t +100 msec : GW head of cluster B will send an advertisement-acknowledgement to G Wtail of cluster A . Beside its cluster’s CDMA code and size, GW head of cluster B indicates that it has received the previous advertisement from GW tail of cluster A , and in the process of assigning a local ID in cluster B to GW tail of cluster A . The GWtail of cluster A will receive the advertisement-acknowledgement from GWhead of cluster B, and then pass its own CH. For the next transmission cycle, GWtail of cluster A should listen to the GWhead of cluster B on CCH during mini-slot 0 to get its own local ID in cluster B. At time t + 200 msec: By this time, GWhead of cluster B has already received from the CH of cluster B the local ID assigned to GWtail of cluster A. GWhead of cluster B sends the local ID to GWtail of cluster A. By the end of this cycle, GWtail of cluster A should receive the local ID of GWhead of cluster B in cluster A, and it can be sent to GWhead of cluster B during its own time slot in cluster B. After finishing the setup for the inter-cluster communications, vehicles GWtail of cluster A and GWhead of cluster B will be treated as cluster members of both clusters A and B.
  • I ran simulations using ns-3 network simulator [53], which is a follow-on to the popular ns-2 simulator. For VANET, I used modules [which is developed by Dr Arbabi and Weigle from our research lab] that added well-known traffic mobility models, the Intelligent Driver Model (IDM) [55] and the MOBIL lane change model [56]. The goal was to create a vehicular network on highways with different number of lanes and different number of vehicles.
  • Make sure to say that my algorithm is the traffic flow
  • Make sure to say that my algorithm is the traffic flow
  • Based on the network setting we have, The slots sizes are
  • What is Communication Density? Communication Density is used to measure the channel load in vehicular communications. This is done by calculating the number of carrier sensible events per unit of time. In our evaluation, the CD is the number of messages per second.
  • This aims to develop a P2P file sharing application using TC-MAC to improve the file downloading time between neighbouring vehicles.
  • This aims to develop a P2P file sharing application using TC-MAC to improve the file downloading time between neighbouring vehicles.
  • the granted time slots on the SCHs in the first half of the TDMA frame, and switch to the CCH during the second half of the TDMA frame. In the following TDMA frame, vehicles i and j will keep listening to the CCH, and then switch to the granted time slots on the SCHs in the second half of the TDMA frame. This process will continue until the file transmission is completed, or interrupted by the CH due to changes in the availability of the unused time slots
  • TDMA Slot Reservation in Cluster-Based VANETs

    1. 1. TDMA Slot Reservation inCluster-Based VANETSMohammad S. AlmalagDissertation DefenseApril 3, 2013Advisor: Michele C. Weigle
    2. 2. Research QuestionCan VANETs accommodate non-safetyapplications’ use of the spectrum whilestill meeting the requirements of safetyapplications?04/03/20132
    3. 3. Outline• Background• Related Work• Thesis Statement• TC-MAC• Evaluation• P2P File Sharing using TC-MAC• Conclusion• Future Work304/03/2013
    4. 4. BackgroundVANETVehicular Ad-Hoc Network (VANET):▫ Increase traveler safety▫ Decrease traveling time and fuel consumption▫ Increase on board luxury04/03/20134
    5. 5. BackgroundApplications04/03/20135Application categories:• SafetyEmergency Electronic Brake LightCo-operative Collision warningIntersection Collision WarningApproaching Emergency Vehicle• Non-safetyFile sharingTraffic InformationParking Lot PaymentElectronic Toll Collection
    6. 6. BackgroundMessages• Update messages (periodic)▫ Broadcast every 100 msec▫ Speed, position, direction, etc.• Safety messages▫ Event-driven▫ Very high priority▫ High reliability• Non-safety message▫ High transmission rate▫ Low priority04/03/20136
    7. 7. BackgroundDSRC/WAVEDedicated Short Range Communication (DSRC) channelsWireless Access in Vehicular Environments (WAVE)04/03/20137
    8. 8. The problem• Scalability: Poor channel reliability• Hidden terminal04/03/20138
    9. 9. The problem• Safety message requirements▫ High reliability▫ Lifetime 100 msec[Wang, ACM VANET 2008]04/03/20139Non-Safety messagesalmost shut downLow Medium HighTraffic Density
    10. 10. Related WorkMAC PROTOCOLS FOR VANETClassified into three different categories1.Channel partitioning:▫ Time Division Multiple Access (TDMA), Frequency-DivisionMultiple Access (FDMA) and Code-Division Multiple Access(CDMA)1.Random access:▫ Contention based protocols, CSMA1.Taking turns:▫ Polling (master-slave) or token ring04/03/201310
    11. 11. Related WorkMAC PROTOCOLS FOR VANETExamples:1.VeSOMAC (Yu and Biswas, 2007)▫ TDMA + location based▫ Does not considerV2I communication1.VeMAC (Omar et al., 2011)▫ TDMA + 2 transceivers▫ Includes vehicles in both directions + RSU in one TDAM frame▫ 2 transceivers + Overhead1.PP-CSMA (Yang et al., 2005)1. CSMA + priority scheme + polling scheme2. Does not consider Broadcasting!2.MCTRP (Bi et al., 2009)▫ Token ring + 2 transceivers + road conditions▫ Uses CSMA CA mechanism for inter-ring communication04/03/201311
    12. 12. Thesis StatementTDMA slot reservation in cluster-basedVANETs will improve the performance ofdelivering non-safety messages with littleimpact on the delivery of safety messages.04/03/201312
    13. 13. Outline• Background• Related Work• Thesis Statement• TC-MAC• Evaluation• P2P File Sharing using TC-MAC• Conclusion• Future Work1304/03/2013
    14. 14. My Approach1. Solves network scalability and hidden terminalproblems2. Fair transmission for safety and non-safetymessages3. Lightweight TDMA assignment scheme4. Efficiently manages cluster with changes oftopology5. Develop TC-MAC cluster-based MAC protocol6. Evaluate TC-MAC vs. WAVE04/03/201314
    15. 15. TC-MACStructure• Based on multi-channel DSRC layout▫ 1 CCH and 6 SCHs• TDMA technique using local IDs for accessingthe channels• Cluster-based protocol▫ Clusterhead (CH) scheme04/03/201315
    16. 16. TC-MACTDMA• Time Division Multiple Access (TDMA) is a method usedto enable multiple nodes to transmit on the samefrequency channel04/03/201316
    17. 17. TC-MACTDMA Frame04/03/201317
    18. 18. TC-MACTDMA Frame04/03/201318
    19. 19. TC-MACLocal IDs04/03/201319• Each vehicle in the cluster will receive a local ID (0 toNmax -1):▫ CH will always have ID 1▫ ID 0 is reserved for a virtual vehicle
    20. 20. TC-MACTDMA Slot Assignment04/03/201320
    21. 21. TC-MACIntra-cluster Communication• Lightweight communication protocol• Each vehicle uses its own mini-slot to disseminatesafety/update messages• First byte, 28= 128 different situations. Examples:▫ 0: vehicle is not communicating at the moment▫ 1: vehicle is involved in communicating with some other vehiclein the cluster. (Unicast)▫ 2: vehicle is involved in communicating with some other vehiclesin the cluster. (multicast group)▫ 3: vehicle is involved in communicating with a vehicle or RSUoutside the cluster▫ 4: the CH is leaving the cluster04/03/201321
    22. 22. TC-MACIntra-cluster Communication• Disseminating intra-cluster non-safety messages:▫ Unicast: Set up without CH intervention. Using the local ID for channel and slot time.▫ Multicast: Small group of vehicles (<5): Set up without CH intervention. Large group of vehicles (>=5): Set up with CH intervention.04/03/201322
    23. 23. TC-MACIntra-cluster Communication04/03/201323
    24. 24. ClusteringClusterhead Election• Based on the traffic flow:▫ Lane Weight▫ Little overhead▫ Network Connectivity Level (NCL)▫ Average Distance Level (ADL)▫ Average Velocity Level (AVL)▫ Clusterhead Level (CHL):04/03/201324iiii AVLADLtNCLCHL ++= )(
    25. 25. ClusteringCluster Maintenance• The behavior of many vehicles may change thetopology of the cluster:1. A Clusterhead leaving the cluster2. A new vehicle joining the cluster3. A cluster member leaving the cluster4. Two clusters come in range of each other5. A multi-hop cluster shrinks to a one-hop cluster04/03/201325
    26. 26. ClusteringCluster MaintenanceA Clusterhead leaving the cluster :04/03/201326
    27. 27. ClusteringCluster MaintenanceA new vehicle joining the cluster :04/03/201327
    28. 28. ClusteringCluster MaintenanceTwo clusters come in range of each other:04/03/201328
    29. 29. CDMA/TC-MACInter-cluster communication• CDMA/TC-MAC hybrid protocol for inter-clustercommunication.• CDMA code for each cluster▫ Intra-cluster communication• Two protocols:1.Code assignment protocol1.Recovery protocol04/03/201329
    30. 30. CDMA/TC-MACCode assignment protocol• Select a CDMA code to the cluster Done by the CH• 18 different CDMA codes:▫ Based on the highway and the cluster’s directions▫ 2 codes are global for each highway direction▫ 4 codes for each traffic direction 1 code is temp▫ Two clusters with two different codes between clusters sharing the samecode04/03/201330
    31. 31. CDMA/TC-MACRecovery protocol• Solve the issue of having two clusters in range ofeach other and sharing the same code04/03/201331
    32. 32. CDMA/TC-MACDISSEMINATING INTER-CLUSTER MESSAGES• Setup:04/03/201332
    33. 33. Outline• Background• Related Work• Thesis Statement• TC-MAC• Evaluation• P2P File Sharing using TC-MAC• Conclusion• Future Work3304/03/2013
    34. 34. EvaluationExperiment Plan04/03/201334• NS-3 network simulator▫ For VANET, (Arbabi and Weigle, 2010): Intelligent Driver Model (IDM) MOBIL lane change model• Network settings:▫ Transmission range = 300 m▫ Max. safety message size = 200 bytes▫ Max. non-safety message size = 1200 bytes▫ Date rate = 6 Mbits/sec▫ CCHI = SCHI = 0.050 sec
    35. 35. EvaluationExperiment Plan04/03/201335• Clusterhead Election:▫ Scenarios: Implemented for a highway Different number of lanes, 2, 3, and 4 lanes 2 exits, on the right 25% of vehicles on the rightmost lane take the exit▫ Evaluation metrics: The number of CH changes▫ Comparison: Lowest-ID, Highest-degree, and Utility Functionalgorithms
    36. 36. EvaluationClusterhead Election (Results)04/03/201336Exit 1Traffic Flow
    37. 37. EvaluationClusterhead Election (Results)04/03/201337Exit 2Traffic Flow
    38. 38. EvaluationExperiment Plan04/03/201338• TC-MAC:▫ Scenarios: Implemented for a highway Different number of lanes, 2, 3, and 4 lanes Different vehicles densities Single-hop and two-hop clusters▫ Evaluation metrics: Reliability of safety messages Direct safety/update messages Indirect safety messages Throughput of non-safety messages
    39. 39. EvaluationTC-MAC (Frame)04/03/201339
    40. 40. EvaluationTC-MAC (Frame)04/03/201340
    41. 41. EvaluationTC-MAC (Results)04/03/201341• Single-hop cluster, all vehicles in the cluster are engaged incommunications during their own slot time on the SCHs
    42. 42. EvaluationTC-MAC (Results)04/03/201342• Single-hop cluster, half vehicles in the cluster are engagedin communications during their own slot time on the SCHs
    43. 43. EvaluationWAVE (Results)04/03/201343• Percentage of collisions during the CCHI for WAVEusing a single-hop cluster
    44. 44. EvaluationWAVE (Results)04/03/201344• Reliability of Safety messages in WAVE using a single-hop cluster
    45. 45. EvaluationTC-MAC (Results)04/03/201345• The Throughput of non-safety messages using TC-MAC in asingle-hop cluster
    46. 46. EvaluationWAVE (Results)04/03/201346• The Throughput of non-safety messages using WAVE ina single-hop cluster
    47. 47. Outline• Background• Related Work• Thesis Statement• TC-MAC• Evaluation• P2P File Sharing using TC-MAC• Conclusion• Future Work4704/03/2013
    48. 48. ApplicationP2P File Sharing Using TC-MAC04/03/201348• Large-scale file sharing between two vehicles• Using their slots on the SCHs• Using un-used slots on the SCHs▫ Needs CH permission• Single-hop cluster
    49. 49. ApplicationExample04/03/201349• Assume vehicle A with local ID=4 wants to share a 4MB MP3 file with vehicle B with local ID=15▫ A will make the handshake with B▫ A will request time slots on the SCHs from the CH
    50. 50. ApplicationExample04/03/201350• To ensure that vehicles A and B are still receivingupdate messages from other vehicles nearby duringthe transmission of the shared file▫ Switching on the SCH is required
    51. 51. ApplicationResults04/03/2013510 12 32 58# of Slots Borrowed
    52. 52. ApplicationResults04/03/2013520 12 32 58# of Slots Borrowed
    53. 53. ApplicationResults04/03/2013530 12 32 58# of Slots Borrowed
    54. 54. Conclusion• TC-MAC performs better than WAVE▫ Collision free▫ Higher reliability of safety messages▫ High throughput of non—safety messages▫ Support high densities▫ Support topology changes, through the local IDs• CH election algorithm:▫ Fewer CH head changes compared to otheralgorithms04/03/201354
    55. 55. My Contributions• Summary of existing MAC protocols for VANETs▫ "MAC Protocols for VANETs", in Mobile Ad Hoc Networking, Cutting EdgeDirections, 2nd ed., 2013.• A multi-channel cluster-based TDMA MAC protocol tocoordinate intra-cluster communications (TC-MAC)▫ [IEEE VTP, 2012], [MP2P, 2013]• A CH election and cluster formation algorithm based on thetraffic flow and a cluster maintenance algorithm.▫ [ON-MOVE, 2010]• A multi-channel cluster-based CDMA/TDMA hybrid MACprotocol to coordinate inter-cluster communications▫ [IEEE Transactions on Intelligent Transportation Systems, in preparation]04/03/201355
    56. 56. Future Work• Further Analysis:▫ The effect of guard intervals▫ Three-hop clusters▫ Different data rates• Design a numbering scheme for cluster members▫ Missed update messages• Enhance the utilization on the SCHs▫ By using the un-used slots on the SCHs.• Speed-based clustering▫ Highways with different speed for each lane• CDMA/TDMA evaluation using simulation04/03/201356
    57. 57. Publications• Mohammad Almalag, Michele C. Weigle, Stephan Olariu, and Samy El-Tawab, Peer-to-Peer File Sharing in VANETs Using TC-MAC, Proceedings of theInternational Workshop on Mobile Peer-to-Peer Computing (MP2P) San Diego, CA,March 2013.• Mohammad Almalag, Stephan Olariu, and Michele C. Weigle, TDMA Cluster-based MAC for VANETs (TC-MAC), Proceedings of the IEEE Workshop onVANETs From Theory to Practice (VTP). San Francisco, CA, June 2012.• Mohammad S. Almalag and Michele C. Weigle. Using Traffic Flow for ClusterFormation in Vehicular Ad-hoc Networks. Proceedings of the Workshop OnUser Mobility and Vehicular Networks (ON-MOVE), Denver, CO, October 2010, pp.631-636.• Book Chapter: Mohammad Almalag, Stephan Olariu, and Michele C. Weigle. MACProtocols for VANETs. In S. Basagni, M. Conti. S. Giordano, I. Stojmenovic, eds.,editors, Mobile Ad Hoc Networking: Cutting Edge Directions, 2nd Edition. Wiley-IEEE Press, March 2013.• Book Chapter: Mohammad S. Almalag. Safety-related vehicular applications.In Stephan Olariu and Michele C. Weigle, editors, Vehicular Networks From Theoryto Practice. Chap- man & Hall/CRC, 2009.ACM (Association for ComputingMachinery).04/03/201357
    58. 58. Thank youQuestionsMohammad Salem AlmalagDepartment of Computer ScienceOld Dominion University04/03/201358

    ×