1 An Approach to Real-Time Support in Ad Hoc Wireless Networks Mark Gleeson Distributed Systems Group Dept of Computer Science Trinity College, Dublin Supported by the

2 Overview Introduce Real-Time Wireless Issues and Challenges Our Proposal, TBMAC Summary

3 Real-time what do we mean? Much confusion exists over the meaning of “real-time” The IEEE define a real-time system as: “A real-time computer system is a computer system in which the correctness of the system demands not only on the logical results of the computations but also the physical instant at which these results are produced” We have both a temporal and computational accuracy requirement

4 Real Time Systems Event Triggered –Processing initiated for each event Asynchronous –Need to cope with varying load Deterministic execution? Time Triggered –Events processed at predetermined intervals Synchronous –System specified for a known bounded load –Can reserve communication bandwidth in advance

5 Our Wireless Domain Short range wireless communication Highly dynamic network –Mobility High risk of node failure Limited bandwidth No central point of control

6 Media Access with CSMA/CA Carrier sense media access collision avoidance –Two methods of carrier sensing Physical – Actually listen Virtual – Have prior knowledge that someone else (may) be transmitting –Can’t listen and transmit at same time Avoidance not detection

7 Fair Vs Time Bounded Under CSMA must contend for medium for every transmission –Promotes fairness –No state information to maintain –Unsuitable for time bounded traffic Capture –A station which accesses the medium first can lock out other stations for a significant duration

8 Channel Access in

9 4 Key Observations Contention is at the receiver not the transmitter Congestion is location dependant. Each node should propagate congestion information. There is a need for synchronisation to ensure effective use of the contention windows. Bharghavan, V et al, MACAW: A media mac protocol for wireless LAN’s. SIGCOMM Comput. Commun. Rev. 24, 4 (1994)

Issues Contention resolved non deterministically –Optimistic media access –Exponential random back off function Beacon Frames –Sent by stations to seek out other stations Provides the “Plug n’Play” element of Low level MAC layer function Key to protocol –Shares same CSMA access method Beacons contend with data traffic

Issues Quality of Service –802.11e still fundamentally b Adds separate queues for traffic types Some changes in media access priority Fixed access point only –802.11p for Distributed Short Range Communication (DSRC) Basically a Shared control channel To be used to provide inter vehicle safety information in automotive applications

12 Chaos in

13 Time Division Multiple Access TDMA, Time Division Multiple Access A slotted medium access control protocol Only one station can transmit in each slot –Packet transmission is collision and contention free –Deterministic transmission delays

14 What is TBMAC? Real Time Medium Access Control –Deterministic communication to support a wide range of distributed applications Supports time triggered protocols Based on TDMA Distributed co-ordination –No single point of failure –Reliable atomic multicast protocol

15 What is TBMAC? Need location service to select cell frequency –Emerging trend to fit GPS Cellular Structure –Spatial reuse Intercell comunication –Transmission range overlaps

16 What is TBMAC ? Solves TDMA bootstrapping problem –To transmit you need a slot –If you can’t transmit you can’t request a slot !!! Divides the TDMA cycle into: –A contention free period using TDMA –A contention period using slotted CSMA Each packet contains a slot bitmap –Data structure with brief details of slot allocations

17 Stability with TDMA

18 Chaos in

19 Current Status Full implementation in NS2 simulator –Fully evaluated Software implementation using –Software implementation is restricted Still have CSMA active on card Performance overhead A hardware implementation is planned –Improved performance –Avoid issues

20 In Review Defined Real-Time –Temporal and computational accuracy requirement –Challenging practical demands –Time triggered protocols best suited Shown unsuitable –Adapting not possible –Randomness, Beacons, Carrier Sense Introduced TBMAC –Deterministic communication to support a wide range of distributed applications

21

22 Chaos in