Http:// 1 An Approach to Real-Time Support in Ad Hoc Wireless Networks Mark Gleeson [email protected] Distributed Systems Group Dept.

http://www.dsg.cs.tcd.ie1

An Approach to Real-Time Support in Ad Hoc Wireless

Networks

Mark [email protected]

Distributed Systems GroupDept 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


Channel Access in 802.11


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


Chaos in 802.11CSMA round trip times, location 120ms interval between requests

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Packet Sequence Number

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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


What is TBMAC?

• Need location service to select cell frequency– Emerging trend to fit GPS

• Cellular Structure– Spatial reuse

• Intercell comunication– Transmission range overlaps

45

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03

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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


Stability with TDMARound Trip Times (location 1)

Static TDMA 8 Slots, 3 stations, 3.5ms slot time

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Chaos in 802.11CSMA round trip times, location 120ms interval between requests

6.00

7.00

8.00

9.00

10.00

11.00

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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



Chaos in 802.11Round Trip Times

CSMA, 3 stations 20ms between requests

6.00E+06

7.00E+06

8.00E+06

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1.10E+07

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Http:// 1 An Approach to Real-Time Support in Ad Hoc Wireless Networks Mark Gleeson [email protected] Distributed Systems Group Dept.

Documents

advance slide

detection slide

data traffic slide

realtime system

channel access

tbmac summary slide

significant duration

realtime wireless issues