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
http://www.dsg.cs.tcd.ie2
Overview• Introduce Real-Time
• Wireless Issues and Challenges
• Our Proposal, TBMAC
• Summary
http://www.dsg.cs.tcd.ie3
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
http://www.dsg.cs.tcd.ie4
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
http://www.dsg.cs.tcd.ie5
Our Wireless Domain• Short range wireless communication
• Highly dynamic network– Mobility
• High risk of node failure
• Limited bandwidth
• No central point of control
http://www.dsg.cs.tcd.ie6
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
http://www.dsg.cs.tcd.ie7
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
http://www.dsg.cs.tcd.ie9
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)
http://www.dsg.cs.tcd.ie10
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
http://www.dsg.cs.tcd.ie11
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
http://www.dsg.cs.tcd.ie12
Chaos in 802.11CSMA round trip times, location 120ms interval between requests
6.00
7.00
8.00
9.00
10.00
11.00
12.00
13.00
14.00
15.00
16.00
0 100 200 300 400 500 600 700 800 900 1000
Packet Sequence Number
Ro
un
d T
rip
Tim
e (
msec)
Station 2
Station 3
http://www.dsg.cs.tcd.ie13
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
http://www.dsg.cs.tcd.ie14
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
http://www.dsg.cs.tcd.ie15
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
56
03
45
6
123
4
6
1
123
2
http://www.dsg.cs.tcd.ie16
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
http://www.dsg.cs.tcd.ie17
Stability with TDMARound Trip Times (location 1)
Static TDMA 8 Slots, 3 stations, 3.5ms slot time
6.00
7.00
8.00
9.00
10.00
11.00
12.00
13.00
14.00
15.00
16.00
0 100 200 300 400 500 600 700 800 900 1000
Packet Sequence Number
Ro
un
d T
rip
Tim
e (
ms
ec
)
Station 2
Station 3
http://www.dsg.cs.tcd.ie18
Chaos in 802.11CSMA round trip times, location 120ms interval between requests
6.00
7.00
8.00
9.00
10.00
11.00
12.00
13.00
14.00
15.00
16.00
0 100 200 300 400 500 600 700 800 900 1000
Packet Sequence Number
Ro
un
d T
rip
Tim
e (
msec)
Station 2
Station 3
http://www.dsg.cs.tcd.ie19
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
http://www.dsg.cs.tcd.ie20
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