1. November 2008
Hybrid MAC for VANET
Date:
Authors:

2. Contents Introduction 1 Proposed MAC scheme 2 3 Performance Analysis 4
November 2008
Contents
Introduction 1
Proposed MAC scheme 2 3
Performance Analysis 4
Conclusions

3. Introduction V2V/V2I communications November 2008 V2V V2I
V2V application : Vehicle Multi-hop Networking for Vehicle Safety
V2I application : Bi-directional Packet Communication for ITS/Telematics
Accident
GPS
Emergency
Message
Warning Message
Cellular/WiBro
Probe Data
TSP Server
Internet
RSE
Traffic
Information
V2V
V2I
Hopping

4. Proposed MAC Scheme: key idea
November 2008
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

5. Proposed MAC Scheme: slot management
November 2008
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

6. Proposed MAC Scheme: synchronization
November 2008
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

7. Performance Simulation
November 2008
Performance Simulation
Simulation setup
PHY : a OFDM, 6Mbps fixed
MAC : 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
10, , 40, 40, 50, 60, 70, 80
broadcast flows
20 unicast flows

8. Performance Analysis: unicast flows
November 2008
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)
DCF vs. hybrid MAC
Two schemes show almost same performance in throughput & delay
May be because of the usage of back-off time

9. Performance Analysis: unicast flows
November 2008
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)
DCF vs. hybrid MAC
Hybrid MAC has small number of retransmission of RTS
Due to the distribution effect of multiple time slots

10. Performance Analysis: unicast flows
November 2008
Performance Analysis: unicast flows
Total # of packet drop depending on retransmit limit
20 Unicast CBR flows generated every 5 ms periodically
DCF vs. hybrid MAC
As number of retransmit limit decreases
Packet drop difference hybrid MAC and is increased
If number of retransmit is small, hybrid MAC has less retransmission related to DCF

11. November 2008
Performance Analysis: unicast flows Throughput and delay depending on retransmit limit
20 Unicast CBR flows generated every 5 ms periodically
DCF vs. hybrid MAC
As number of retransmit limit decreases
Throughput difference hybrid MAC and is increased
If number of retransmit is small, hybrid MAC has good performance related to DCF

12. November 2008
Performance Analysis: broadcast flows Throughput and delay depending on # of node
Broadcast frame sent every 100ms periodically from each nodes
DCF vs. hybrid MAC
As number of node increases
Hybrid MAC’s throughput is better than ’s throughput
Hybrid MAC’s delay is larger than DCF due to slot access

13. 2008Sep. 2008
doc.: IEEE xx/xxxxr0
November 2008
Performance Analysis: broadcast flows Throughput and delay depending on # of node
Broadcast frame sent every 10ms periodically from each nodes
DCF vs. hybrid MAC
As number of node increases
network throughput is decreased( DCF, hybrid MAC)
Hybrid MAC’s throughput is better than ’s throughput

14. Conclusions Hybrid MAC for VANET
November 2008
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
In case of broadcast, hybrid MAC shows better throughput than
Most safety applications need point-to-multipoint communication
: 30 out of 34 safety applications( DOT HS , Identify intelligent vehicle safety applications by DSRC)
Hybrid MAC is well suited for safety application in VANET