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04/19/23 1
• Describing a street testbed we recently built for studying the use of wireless mesh network for adaptive traffic control system
• Discuss some initial measurement results regarding link characteristics of– 802.11– 900Mhz– Ethernet over powerline– and Unwired (a WiMax variant)
• Discuss some of our experience in building a testbed in a real-life environment
A case study
• Describing a street testbed we recently built for studying the use of wireless mesh network for adaptive traffic control system
• Discuss some initial measurement results regarding link characteristics of– 802.11– 900Mhz– Ethernet over powerline– and Unwired (a WiMax variant)
• Discuss some of our experience in building a testbed in a real-world environment
04/19/23 2
Adaptive Traffic Control
• How it works– Road-side sensors detect the states of
vehicle/road• e.g loop detector under the pavement for vehicle
counting
– Sensor data is fed to traffic light controller• Sensor data can be also fed to variable speed limit
sign
– the controller uses the sensor data to make decision about the duration of green/red lights
04/19/23 3C=C+1C=8 for the last 10 sec
Sensor info from other intersections
Turn green at t1 for 30sec
Traffic server (Regional Computer)
Traffic controller
loopdetector
04/19/23 4
Communication for traffic control system
• Traditionally rely on wired connections– Private or leased lines
• High operating cost, inflexibility
• People have started looking at using public shared network – eg. ADSL, GPRS– Inconsistent delay jitter and reliability issues
• e.g. GPRS can have high RTT (>1sec), fluctuating bandwidth and occasional outage
Sydney Coordinated Adaptive Traffic System (SCATS)
• A popular traffic management system (used by >100 cities)
• Created by Sydney RTA (Road and Transport Authority)
• Serial point-to-point communication over voice-grade telephone line, using 300bps modem
• Hierarchical structure– TMC (Traffic Management Center)
• Regional Computers (RC)– Traffic controllers
TMC
RC RC RC
controller controller controller
Second-by-second SCATS messages
Loop detector
Controller
Regional Computer
SCATS protocol
• Periodic message exchanges: every sec• If RC does not receive ACK within 1 sec: retry• If the ACK fails to arrive the 2nd time: link failure
– Controller enters ‘self-controlling’ mode and stays in this mode for 15 min
• Uncoordinated traffic control• extend by another 15 min if another
communication failure happens in this mode
RC controllercontrol command
Sensor data + ACK
04/19/23 8
Summary for communication layer of traffic control
• Wired connections are typically used– private or leased from public
telecommunications operators
• Traffic signal data demand is light– Low-bandwidth dial-up network is commonly
used
• But reliability and latency are critical issues
What are the problems?
• High cost– High front-end cost
• RTA pays 14 millions each year to Telstra for the leased lines
– High maintenance cost • Installation or relocation is expensive
– Very inflexible • installation/relocation incur long delays
• Low bandwidth– RTA uses 300 bps dial-up lines!– Difficult to integrate other sensors/equipment (e.g.
video cameras)
Wireless mesh network
• Getting increasing popularity recently• Trial deployment in several major cities
– Strix, Tropos, LocustWorld, etc
• A competitive ‘last-mile’ solution– Application: residential broadband, public safety
• Our research– Using mesh network for a mission-critical system such as
traffic control• Can we use low-cost, standard-based wireless technology (such
as 802.11, 802.16) to build a dedicated RTA wireless network?
– Different requirement from prior work• Trade throughput for latency and reliability
Research Challenges
• Scalability– Connecting numerous road-side devices to SCATS
• Reliability– Mission-critical data (e.g. accident detection, traffic
signal control, etc)– Requires timely routing that is robust against faults
in nodes or links
• Low latency– SCATS is a real-time traffic control system (< 1 sec)
04/19/23 12
Today’s talk: The testbed
• Collaborate with New South Wales Road and Traffic Authority (NSW RTA)
• Study the feasibility of using wireless mesh network for traffic control
04/19/23 13
Outline
• Background• Site survey for the testbed
– What is typical node distance?• Traffic controller map
– Feasibility of using off-the-shelf hardware?• Intersection-pair measurements
• Wireless mesh testbed• Preliminary results• Experience we learned and conclusion
Typical distance between two adjacent traffic lights?
• Q: What is the degree of connectivity between traffic controller for a given radio range?
• Data source: traffic controller map for Sydney CBD area (2787 traffic controllers)
• 354 traffic controllers have their closest neighbors within 100m
• 2407 traffic controllers have their closest neighbors within 500m
• 2701 traffic controllers have their closest neighbors within 1000m
Degree of connectivity between traffic controllers
to ensure that 90% (2500) nodes each has at least 3 neighbours (e.g. for fault tolerance) requires a radio range of at least 1km.
04/19/23 16
Wireless survey
• Building a testbed in real world can involve lots of $$– ~$NTD 500K for only 7 nodes– Not to mention the numerous man-hours
• To understand the feasibility of using off-the-shelf 802.11 radios products– What is the performance of 802.11 with
different parameter settings?
Experiment setup
• 20 intersection pairs• Two linux laptop• External antennas
– 8 dBi omni-directional– 14 dBi directional
• Two wireless interfaces: Intel Centrino (RFMON) and Senao SL-2511CD (200mW)
• Antenna height: 4m– signal loss over the coaxial cable: 2.7dB
• Duration of each experiment: 5 min (3 times for consistency)• Use GPS to measure distance between intersections
04/19/23 18
Factors that might affect the performance of 802.11
• Effect of – Distance
– Transmission rate
– Number of MAC-layer retry
– Type of antenna
04/19/23 19
Effect of the distance
• Pathloss: attenuation experienced by a wireless signal as a function of distance
• Shadowing: amount of variations in pathloos between similar propagation scenarios
• prior work suggested pathloss can range from 2 to 5 for outdoor urban environment
• Using linear regression, we find our environment has a pathloos 3.1 and shadowing 7.2– significantly lower than the suggested urban pathloss
of 4 in the literature
04/19/23 20
Effect of transmission rate
• Higher transmission rates– allow high quality links to transmit more data– but have a higher loss probability on lossy links.
• throughput is a function of transmission rate and the delivery probability.
• We tried 1Mbps, 2Mbps, 5.5Mbps,11Mbps
• Most of our links have a higher throughput when using a higher transmission rate
21
Effect of maximum retries
• MAX-RETRY is one of the wireless card parameters• A higher retry limit
– Decrease the probability that a packet is dropped due to a link error
– potentially increase the probability of network interface buffer overflow and the latency
• A optimal setting depends on the channel conditions and flow rate
• MAX-RETRY=10 seems to work best in our case• MAC-layer re-transmissions is a norm
– our links have intermediate quality
More than 50% are retransmitted at the MAC layer
Link distance: 200m
MAX-RETRY=10
04/19/23 23
Omni-directional vs. directional
• Directional antenna+ increased spatial reuse and improved signal
quality
+ less power consumption while maintaining a similar link quality
- higher cost- Deployment- Opportunistic forwarding
04/19/23 24
13 pairs: no loss
Intersection selection (omni-directional, 11Mbps)
9 pair > 5Mbps
04/19/23 25
Intersection selection (directional, 11Mbps)
19 pairs: no loss
14 pair > 5Mbps
04/19/23 26
Outline
• Background
• Site survey for the testbed
• Wireless mesh testbed– Hardware and software
• Preliminary results
• Experience we learned and conclusion
04/19/23 27
Street testbed
NICTA
CBD
200mUniv. ofSydney
200m 200m
300m200m
500m400m
04/19/23 28
STaRCOMM testbed
• Cover 7 intersections in Sydney CBD (Central Business District)– Inter-node distance 200m ~ 500m– 500m x 1000m area
• Currently extending to 15-20 nodes • Nodes are custom-build embedded PCs• NLOS for all the nodes• Three types of nodes
– mesh nodes – gateway node – Curbside node
04/19/23 29
Node
• mesh nodes – Each node has 3 radio interfaces
• Two 2.4GHz (802.11) or one 2.4GHz + one 900MHz• One 3.5GHz (WiMax variant) for backhaul
– Connect to traffic controller via powerline communication• gateway node
– located at Sydney U.– Connect to mesh nodes via 802.11– Connect to Regional Computer (at NICTA) via AARNet
• Curbside node– Located in traffic controller housing– One serial interface (to traffic controller) and one IP interface (to
mesh node via ethernet-over-powerline)– Encapsulate SCATS data into IP packet and decapsulate IP
packet into serial data
Usyd NetUsyd Net Unwired Net
Unwired Net
Wired
3.5GHz
2.4GHz
900MHz
Motherboard
Ethernet switch
Unwired modem
Trafficcontroller
Power line
521 522 523 524
413 414
curbside node
Gateway node
m0
c6
RC Testbedmanagement
NICTA
415
(Internet)
Mesh node
04/19/23 31
Mesh node
• VIA MB770F motherboard• Ubiquity SR2 (400mW)
– w/ 8dBi omni-directional ant.
• Ubiquity SR9 (700mW)– w/ 6dBi omni-directional ant.
• Uniwred modem• Diamond digital router• Netgear powerLAN adapter• Fault recovery
– Remote switch– Watchdog timer
• Roof for water/heat proof• Mosquito mesh for insect proof
04/19/23 32
04/19/23 33
Gateway Node
04/19/23 34
Curbside Node
Power-over-ethernet adapter
04/19/23 35
Software
• custom-built Linux OS image
• watchdog timer– A daemon periodically update the timer to
keep system from rebooting
• Software from Orbit project– Including OML for measurement collection
04/19/23 36
Outline
• Background
• Site survey for the testbed
• Wireless mesh testbed
• Preliminary results
• Experience we learned and conclusion
37
Effect of hop numbers on losses (2.4GHz)
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
1 2 3 4 5
consecutive loss
pro
bab
ilit
y
one hop
two hops
One hop: 521-522Two hop: 521-523
• Consecutive loss increases as the number of hops increase•On the same link or from different links?
04/19/23 38
Effect of distance on losses (with 2.4GHz)
0
0.01
0.02
0.03
0.04
0.05
0.06
1 2 3 4 5
consecutive losses
pro
bab
ilit
y
521-522
521-413
521-522: 200m521-523: 400m
losses become burstier as the distance increases
39
Effect of number of hops on latency
Latency and its variation increase as the number of hops increase
40
Effect of distance on latency
Latency is not strongly correlated with distance
Effect of distance on loss
Loss is not completely correlated with distance: location-dependant
04/19/23 42
Effect of antenna location
Antenna location makes a difference
04/19/23 43
900MHz vs. 2.4GHz
900MHz has a lower loss rate but higher latency: due to retry?
04/19/23 44
Power-line communication
Powerline communication works pretty well when distance is withinIts operation region
04/19/23 45
Throughput from different technologies
•Larger variation for 900 Hz• powerline does better than radio when the distance is short
04/19/23 46
Latency of Unwired link(round-trip delay from mesh node to unwired gateway)
•High latency •Large variation•Outage is common
521 522523 524
413 414
NICTA
415
Unwired
A
B
A
Latency of backhaul link(round-trip delay from nicta to mesh node)
Almost half of the delay happens on the Unwired wireless link
A+BA
A+B
Clear Diurnal Pattern
• More interference?Other user traffic causing network congestion?
04/19/23 49
Outline
• Background
• Site survey for the testbed
• Wireless mesh testbed
• Preliminary results
• Experience we learned and conclusion
04/19/23 50
Deployment
• Protection of antenna connectors is necessary – Connectors often held on by weak glue or crimp. – Gradual stress (e.g. vibration) could eventually loosen
the plug – degrade the signal before it is transmitted into the air
• Make sure that your wireless cards comply to the specification before starting using them. – E.g. some of our Senao wireless cards does not
output 200mW as they should
Deployment
• while the hardware can be identical, different firmwares and drivers could introduce inaccuracy in the measurement results.
• compare against with a spectrum analyzer if you can!
• Antenna locations matter!– At 2.4GHz, a quarter wavelength is approximately
30cm– when multiple antennas are deployed, it is essential to
have a means for independently adjusting their position.
Maintenance
• Remote management is important for an outdoor testbed
• Access the node– Unwired link– 802.11 link
• Ethernet port• Serial port
• Reboot the node– Remote switch– Watchdog timer– PXE network reboot (configured in BIOS)
• DHCP server by default does not provide PXE boot info
• Second image for fallback (via Grub)
53
Security
• A major concern to to any wireless network– Anybody can sniff the air– Connected to the Internet via Unwired– It’s real!! Two nodes were hacked.
• integrated with the traffic control system security model– segmentation to contain the damage of a attack– multiple levels of fallback to local control
54
Interference
• 2.4GHz/900MHz are shared channels
• We saw an average of 50+ external APs at any time of the day
• A serious problem when WiFi becoming more and more pervasive
55
Conclusion
• It is feasible to build a wireless network with off-the-shelf hardware/software to control traffic lights
• Signal quality and losses are location-dependent (but not strongly correlated with distance)
• For a good link, losses are in general uniformly distributed
• Larger variation in 900MHz than in 2.4GHz• Powerline communication is excellent for a short
distance• Issues with using public shared network
– Large variations and outages is a norm– Diurnal patterns
04/19/23 56
Future work..
• By collaborating with NICTA and department of transportation @ NCKU, we plan to a build a similar testbed around NCKU campus– Vehicle-infrastructure communication– Multimedia (Video/Audio) over mesh– Hierarchical mesh-sensor networks
04/19/23 57
..Future work
• Wireless data mining– Loss Model for mesh links – Outage prediction
• Dynamic channel assignment
• Multi-path routing
04/19/23 58
Thank you!
Questions?
Why WMN for traffic control?
– Low installation cost• Low front-end investment
– Easy maintenance– Robust and reliable
• Reliability increases as the number of nodes increase
Source
destination
wireless
Multi-path
Self-forming and self-healing
04/19/23 60
Effect of antenna
• directional antenna exhibits similar performance as omni-directional antenna for most of the links in our environment
• But directional antenna does help for challenging links
04/19/23 61
Testbed location
• A typical suburban area with lots of traffic, foliages, pedestrians and high-rise residential buildings.
• The 200-500m range is representative of 90% of the distance between traffic controllers in the Sydney CBD area
• Close to NICTA (for on-site maintenance)