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An Approach to Real-Time Support in Ad Hoc Wireless Networks Mark Gleeson firstname.lastname@example.org Distributed Systems Group Dept of Computer Science Trinity College, Dublin Supported by the
Overview • Introduce Real-Time • Wireless Issues and Challenges • Our Proposal, TBMAC • Summary
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
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
Our Wireless Domain • Short range wireless communication • Highly dynamic network • Mobility • High risk of node failure • Limited bandwidth • No central point of control
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
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
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)
802.11 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 802.11 • Low level MAC layer function • Key to 802.11 protocol • Shares same CSMA access method • Beacons contend with data traffic
802.11 Issues • Quality of Service • 802.11e still fundamentally 802.11b • 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 802.11a • Shared control channel • To be used to provide inter vehicle safety information in automotive applications
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
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
Need location service to select cell frequency Emerging trend to fit GPS Cellular Structure Spatial reuse Intercell comunication Transmission range overlaps 4 5 6 3 2 5 1 3 4 0 1 6 4 2 5 6 3 2 1 What is TBMAC?
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
Current Status • Full implementation in NS2 simulator • Fully evaluated • Software implementation using 802.11 • Software implementation is restricted • Still have 802.11 CSMA active on card • Performance overhead • A hardware implementation is planned • Improved performance • Avoid 802.11 issues
In Review • Defined Real-Time • Temporal and computational accuracy requirement • Challenging practical demands • Time triggered protocols best suited • Shown 802.11 unsuitable • Adapting 802.11 not possible • Randomness, Beacons, Carrier Sense • Introduced TBMAC • Deterministic communication to support a wide range of distributed applications