1. MAC for Dedicated Short Range Communications (DSRC) in Intelligent Transport Systems IEEE Communications Magazine, December 2003 Cpre 592 – Wireless Networks Seminar, (Session 1) Iowa State University, Fall 2004 Presented by: Zak Abichar, abicharz@iastate.edu September 1, 2004

2. 2 Presentation Objective Present a tutorial on Dedicated Short Range Communications (DSRC) for Intelligent Transportation Systems (ITS) Several MAC issues are considered –Basic access, fairness, QoS, data transmission efficiency Checking the classic solutions to the needs of DSRC –Immature research area

3. 3 Outline Introduction The IEEE 802.11 protocol DSRC environments Limitations of 802.11 in DSRC environments Discussion of solutions and approaches

4. 4 Introduction Motivation for DSRC –Safety, information, entertainment DSRC, an interesting problem to industry –Bandwidth allocation 75 MHz band in the 5.9 GHz range –ASTM endorsed of a variant of 802.11a for Roadside Applications Borrowing from the 802.11 protocol Interesting research topic –Many questions remain open

5. 5 The 802.11 Protocol – Relevant Details First standard document approved in 1997 Widely used in WLANs Distributed access scheme –DCF, based on CSMA/CA, BEB Centralized access scheme: PCF Hidden/exposed nodes can significantly hinder the performance

6. 6 DCF Operation A station defers for a duration of DIFS A station chooses a random value for BO timer from the CW When BO timer expires a station transmits of the channel is idle Colliding stations double their CW windows and defer Successful stations reset their CW to the initial value

7. 7 PCF Operation PC maintains a list of active nodes PC knows the requirements of the contracted nodes PC polls the nodes to meet their QoS requirements

8. 8 DSRC Environments Distributed mobile multi-hop network – v2v –Only DCF is allowed –Relative speed between cars is low

9. 9 DSRC Environments Centralized network – v2r –Both DCF and PCF may operate –Cars passing at 100 km/h roadside unit with 200 m coverage radius Connection duration: 15 seconds

10. 10 Limitations of 802.11 in DSRC Environments The blocking problem: exposed receiver C –D enters into an inhibition period –cannot communicate with other nodes –Problem was addressed at TCP level – however, it is rooted at the MAC level –Fairness issue

11. 11 DCF/PCF over High Mobility Short v2r connection time –System does not reach steady state Need for a very efficient protocol with low overhead –Overhead of RTS/CTS is not adequate PCF maintains a list of active nodes –Too many updates for a highly mobile environment

12. 12 Multi-rate Connections 802.11 supports multiple rates as a function of distance Under- or over-estimation of NAV in v2r cases

13. 13 DSRC Problems in Summary Hidden/exposed nodes –Blocking problem Short-lived connections –Network operating in transient state –Need for a protocol with low overhead Several updates for PCF states Correctness of NAV value Approaches for –Fairness –QoS support Previously tackled problems Open questions

14. 14 Efficient Data Transmission in DSRC Experiments indicate that RTS/CTS should be used in most of the cases –High overload when used on a per-packet basis –Use RTS/CTS handshake for multiple packets –Reduce overhead in RTS/CTS procedure A stations hearing an ACK initiates a CTS Problem arises if no other station has ready frames

15. 15 Fairness in DSRC Colliding stations in 802.11 double their CW –Leads to unfairness –MACAW resolves to two new packet types to propagate information about CW –High overhead: not suitable for DSRC Estimation-based fairness approaches –Collect network information, i.e., channel status, active neighboring nodes, BW share to adjust the CW –Network in transient status: not suitable for DSRC A stateless protocol is suitable to address fairness in DSRC

16. 16 QoS in DSRC Real-time support –Shorter CW to low-delay traffic –Several proposed schemes –Works in DSRC Black-burst scheme –Jamming the medium with bursts of different lengths –Longer bursts means stations has higher priority –Burst length determined in function of delay and priority –Station with longest burst accesses the channel –Works in DSRC Blackburst scheme

17. 17 QoS in DSRC DIME (DiffServ MAC Extension) –A MAC framework for QoS support –Two modules Expedited forwarding (EF)- reuses PIFS Assured forwarding (AF) –Works for DSRC (PCF no longer available)

18. 18 Conclusions DSRC environments (high mobility, multi- hop) –New challenge to MAC design High mobility in ad hoc networks poses challenges –Scarcely investigated: open problems include Multi-rate environments Shortened connection times Frequent updating of stations in coverage area

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