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					November 2008                                           doc.: IEEE 802.11-08/1273r0



                Hybrid MAC for VANET
                               Date: 2008-11-11

Authors:
Name          Affiliations   Address              Phone         email
Sangwoo Lee   ETRI           161 Gajeong-dong,    +82-42-860-   swlee@etri.re.kr
                             Yuseong-gu,          5254
                             Daejeon, Korea
Woong Cho     ETRI           161 Gajeong-dong,    +82-42-860-   woongcho@etri.re.kr
                             Yuseong-gu,          5289
                             Daejeon, Korea
Hyunseo Oh    ETRI           161 Gajeong-dong,    +82-42-860-   hsoh5@etri.re.kr
                             Yuseong-gu,          5659
                             Daejeon, Korea
Dongyong Kwak ETRI           161 Gajeong-dong,    +82-42-860-   dykwak@etri.re.kr
                             Yuseong-gu,          5148
                             Daejeon, Korea


Submission                              Slide 1                               Sangwoo Lee, ETRI
November 2008                         doc.: IEEE 802.11-08/1273r0



                       Contents

                1   Introduction

                2   Proposed MAC scheme

                3   Performance Analysis

                4   Conclusions




Submission                  Slide 2                 Sangwoo Lee, ETRI
November 2008                                                doc.: IEEE 802.11-08/1273r0

                                    Introduction
• V2V/V2I communications
      – V2V application : Vehicle Multi-hop Networking for Vehicle Safety
      – V2I application : Bi-directional Packet Communication for ITS/Telematics

                 TSP Server                         Cellular/WiBro



                                                                            GP
              Internet                                                      S



                                        Emergency
                                        Message
        RSE                   RSE
                                                        Warning                  Warning
         Probe
                  V2I
                                 Traffic
                               Information
                                                        Message    V2V           Message
         Data




                                             Accident             Hopping

Submission                                   Slide 3                              Sangwoo Lee, ETRI
 November 2008                                                                     doc.: IEEE 802.11-08/1273r0

                   Proposed MAC Scheme: key idea
• Hybrid MAC scheme
     –     TDMA : channel access based on time slot
     –     CSMA/CA : contention with other nodes within the time slot
     –     Time frame consists of multiple time slots
     –     Each node can select its own time slot


                                   frame                                            frame

                              Service             service                       Service           service
         Beacon    Control                                  Beacon    Control
                               slot                 slot                         slot               slot
          slot      slot                                     slot      slot
                                #1                   #n                           #1                 #n
                                                                                                                 time




                  RTS        CTS           DATA   ACK          RTS        CTS        DATA   ACK
                                                                                                                 time




 Submission                                                 Slide 4                                 Sangwoo Lee, ETRI
 November 2008                                           doc.: IEEE 802.11-08/1273r0

    Proposed MAC Scheme: slot management
• Slot owner vs non-slot owner
     –   Each node can select its own time slot in a frame
     –   Node which selects its own transmission slot become slot owner of the slot.
     –   The other nodes become non-slot owner of the slot
     –   Channel access with priority: based on slot ownership
     –   No back-off time increment




 Submission                                Slide 5                        Sangwoo Lee, ETRI
 November 2008                                          doc.: IEEE 802.11-08/1273r0

      Proposed MAC Scheme: synchronization
• Time slot allocation
     – Each node can select its own time slot in a frame randomly
     – Slot owner vs non-slot owner
     – No central coordination is required for assigning time slot
     – Time slot
         – Not a unique assignment for one node
         – Have a channel access priority
     – Does not require strict synchronization


    Distinguished from the original TDMA




 Submission                               Slide 6                     Sangwoo Lee, ETRI
 November 2008                                            doc.: IEEE 802.11-08/1273r0
                      Performance Simulation
• Simulation setup
     – PHY : 802.11a OFDM, 6Mbps fixed
     – MAC : 802.11 DCF, hybrid MAC
     – Number of slot : 4 slots for hybrid MAC(control, beacon slots are not
       considered, only service slots are considered)
     – Slot duration : 10 ms
     – Application : CBR Traffic (Unicast, Broadcast)
     – Simulation tool : Qualnet 4.0 version




                  20 unicast flows                  10, 20. 30, 40, 40, 50, 60, 70, 80
                                                             broadcast flows


 Submission                               Slide 7                            Sangwoo Lee, ETRI
                              November 2008                                                          doc.: IEEE 802.11-08/1273r0
                                           Performance Analysis: unicast flows
                                            Throughput and delay depending on # of flows
                          •      5, 10, 15 and 20 unicast CBR flows generated every 10 ms periodically
                          •      Packet size : 512 bytes(400Kbps)
                          •      802.11 DCF vs. hybrid MAC
                     4
                                                                                            2
                    3.8
                                                                                           1.8
                    3.6                                                                    1.6
                    3.4                                                                    1.4
throughput (Mbps)




                                                                                           1.2




                                                                               delay (s)
                    3.2
                                                                                            1
                     3
                                                                                           0.8                                             hybrid MAC
                                                                 hybrid MAC
                    2.8
                                                                                           0.6                                             802.11
                                                                 802.11
                    2.6                                                                    0.4
                    2.4                                                                    0.2
                    2.2                                                                     0
                                                                                                 5        10          15        20
                     2
                                  5        10          15   20                                             No. of flows

                                            No. of flows


                                Two schemes show almost same performance in throughput & delay
                                    – May be because of the usage of back-off time

                              Submission                                  Slide 8                                          Sangwoo Lee, ETRI
    November 2008                                                               doc.: IEEE 802.11-08/1273r0
                 Performance Analysis: unicast flows
                 # of retransmission of RTS depending on # of flows
•     5, 10, 15 and 20 Unicast CBR flows generated every 10 ms periodically
•     Packet size : 512 bytes(400Kbps)
•     802.11 DCF vs. hybrid MAC
                                          16000

                                          14000

                                          12000
                     No. of ReTx of RTS




                                          10000

                                          8000
                                                                                         hybrid MAC
                                          6000                                           802.11

                                          4000

                                          2000

                                              0
                                                  5   10                  15   20
                                                           No. of flows

           Hybrid MAC has small number of retransmission of RTS
                – Due to the distribution effect of multiple time slots

    Submission                                                   Slide 9                          Sangwoo Lee, ETRI
    November 2008                                                        doc.: IEEE 802.11-08/1273r0
                 Performance Analysis: unicast flows
                 Total # of packet drop depending on retransmit limit
•     20 Unicast CBR flows generated every 5 ms periodically
•     802.11 DCF vs. hybrid MAC

                                          Number of Packet Drops (5ms)
                                  50000
                                  45000
                                  40000
                                  35000
                         Amount




                                  30000
                                  25000
                                  20000                                      802.11
                                  15000
                                                                             hybrid MAC
                                  10000
                                   5000
                                      0
                                            1      2           3     4
                                                  Retransmit Limit



As number of retransmit limit decreases
    – Packet drop difference hybrid MAC and 802.11 is increased
    – If number of retransmit is small, hybrid MAC has less retransmission related to
       802.11 DCF

    Submission                                           Slide 10                         Sangwoo Lee, ETRI
                          November 2008                                                               doc.: IEEE 802.11-08/1273r0
                                       Performance Analysis: unicast flows
                                       Throughput and delay depending on retransmit limit
                     •      20 Unicast CBR flows generated every 5 ms periodically
                     •      802.11 DCF vs. hybrid MAC

                                       Throughput (5ms)                                                         Delay (5ms)
                    137                                                                     2.5
                    135                                                                     2.3
                                                                                            2.1
Throughput (Kbps)




                    133
                                                                                            1.9




                                                                              Delay (sec)
                    131                                                                     1.7
                    129                                                                     1.5
                    127                                        802.11                       1.3                                               802.11
                                                                                            1.1
                    125                                        hybrid MAC                                                                     hybrid MAC
                                                                                            0.9
                    123                                                                     0.7
                    121                                                                     0.5
                              1          2           3     4                                      1         2           3          4
                                        Retransmit Limit                                                   Retransmit Limit



                    As number of retransmit limit decreases
                        – Throughput difference hybrid MAC and 802.11 is increased
                        – If number of retransmit is small, hybrid MAC has good performance related to
                           802.11 DCF

                          Submission                                    Slide 11                                              Sangwoo Lee, ETRI
                    November 2008                                                                                   doc.: IEEE 802.11-08/1273r0
                                 Performance Analysis: broadcast flows
                                       Throughput and delay depending on # of node
      •                   Broadcast frame sent every 100ms periodically from each nodes
      •                   802.11 DCF vs. hybrid MAC

                                      Throughput (100ms)                                                                  Delay (100ms)
                    1.6                                                                                0.025
                    1.4
Throughput (Mbps)




                                                                                                        0.02
                    1.2




                                                                                         Delay (sec)
                      1                                                                                0.015
                    0.8
                    0.6                                                     802.11                      0.01                                                802.11
                    0.4                                                     hybrid MAC                                                                      hybrid MAC
                                                                                                       0.005
                    0.2
                      0                                                                                   0
                            10   20    30      40   50       60   70   80                                      10    20   30   40   50   60    70   80
                                            Number of Node                                                                 Number of Node




As number of node increases
    – Hybrid MAC’s throughput is better than 802.11’s throughput
    – Hybrid MAC’s delay is larger than 802.11 DCF due to slot access

                    Submission                                                 Slide 12                                                     Sangwoo Lee, ETRI
                          November 2008                                                                            doc.: IEEE 802.11-08/1273r0

                                 Performance Analysis: broadcast flows
                                              Throughput and delay depending on # of node
                      •     Broadcast frame sent every 10ms periodically from each nodes
                      •     802.11 DCF vs. hybrid MAC


                                     Throughput (10ms)                                                                        Delay (10ms)
                    4.5                                                                                  12
                     4                                                                                   10
Throughput (Mbps)




                    3.5                                                                                  8




                                                                                           Delay (sec)
                     3                                                                                   6
                                                                            802.11                                                                                 802.11
                    2.5                                                                                  4
                                                                            hybrid MAC                                                                             hybrid MAC
                     2                                                                                   2
                    1.5                                                                                  0
                           10   20     30      40   50       60   70   80                                     10   20   30      40   50       60     70   80
                                            Number of Node                                                                   Number of Node




                    As number of node increases
                        – network throughput is decreased(802.11 DCF, hybrid MAC)
                        – Hybrid MAC’s throughput is better than 802.11’s throughput

                          Submission                                                 Slide 13                                                      Sangwoo Lee, ETRI
  November 2008                                     doc.: IEEE 802.11-08/1273r0

                            Conclusions
• Hybrid MAC for VANET
    – TDMA + CSMA
    – Different contention window (slot owner or non slot owner)
    – Random selection of time slot in a frame


• Performance analysis: throughput and delay
    − In case of unicast, hybrid MAC shows similar performance to 802.11
    − In case of broadcast, hybrid MAC shows better throughput than 802.11


• Most safety applications need point-to-multipoint communication
    : 30 out of 34 safety applications( DOT HS 809 859, Identify intelligent
       vehicle safety applications by DSRC)


• Hybrid MAC is well suited for safety application in VANET

 Submission                            Slide 14                    Sangwoo Lee, ETRI

				
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