RT-WiFi: Real-Time High-Speed
Communication Protocol
for Wireless Cyber-Physical Control
Applications
Ramyaa & Malak
Control System
• Enhance the mobility.
• Reduced the cost of maintenance and deployment.
WirelessHART
ISA100.11a
MBStar
Bluetooth
ZigBee
WiFi
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Not suitable for high-speed real-time wireless control.
Cannot provide high enough sampling rate.
Can provide real-time communication, but the maximum supported
sampling frequency is only 100Hz
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Not suitable for high-speed real-time wireless control.
Can provide real-time communication, but the maximum supported
sampling frequency is only 100Hz
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Not suitable for high-speed real-time wireless control.
Cannot provide high enough sampling rate
It can support up to 400Hz sampling rate, which is still much lower
than the requirement in many high-speed control applications.
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Not suitable for high-speed real-wireless control.
Cannot guarantee real-time data delivery, except for voice time
packets
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Not suitable for high-speed real-time wireless control.
Cannot provide high enough sampling rate
Can provide real-time data delivery in beacon-enabled mode, but the
data rate is up to 250kbps.
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High-speed wireless local area network and support data rates up to
150Mbps.
In DCF  the delivery time of data is not deterministic
In PCF  cannot provide real-time data delivery guarantee & it is
nondeterministic when the subsequent packets will be sent.
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- Complicate the design
- Increase development time & maintenance cost
RT-WiFi - Goals
 Real-time Data Delivery and High Sampling Rate
• Requires sampling rate >1KHz  IEEE802.11 physical layer.
• Time deterministic  TDMA mechanism.
 Flexible Data Link Layer Configuration
• Design trade-offs: sampling rate, communication reliability, realtime data delivery, and co-existence with regular WiFi networks.
 Transparent System Design
• Reuse hardware & software available & run existing
applications with minimum modifications.
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RT-WIFI Design and Implementation
Performance Evaluation
Case Study
Conclusion
Future Work
Control System based on RT-WiFi
RT-WiFi Design
A. Timer
• Global synchronization.
• Achieves high sampling rate.
• Deterministic timing behavior Node access the
channel in its pre-assigned time slots.
• Based on (Timing Synchronization Function) TSF.
RT-WiFi Design
B. Link Scheduler
Link
• Defines the
communication
behavior
Superframe
• Sequence of
consecutive
time slots
Device Profile
• Each node has
a profile that
used by the
manger
RT-WiFi
C. Flexible Data Link Layer Design
Out-of-slot
retrysize and data rate Co-existence
Packet
Sampling Rate
Reliability
with regular WiPacket fails
be transmission
transmitted intime
oneof
time
slot,
RT-is larger
 Iftothe
the
packet
Fi
network:
WiFi node can
retransmit.
than
the time slot size then RT-WiFi cannot
Requirement
of retransmit
depends
the
transmit
the on
packet.
Performing Carrier
Sense successfully
Packet size
Performing
application.
 IEEEIn-slot
802.11
 Uses multiple transmission rates
retry
and data rate
Carrier Sense
that
utilize
different
modulation
and coding
Absence of WiFi  RT-WiFi node does not perform
carrier sense. In-slot retry scheme.
Computational
resource
 Higher
transmission
rate Shortening
higher throughput
Presence
of
WiFi

Possibility
that
the
two
types
Synchronization
accuracy
Out-of-slot
Computational
 less resilient to noise. Inter-frame
of traffic could
collide.
resource
retry rate  reduce the time slot

Higher
sampling
 Flexible
data link layer designSpacing
 Selection of
Solution: RT-WiFi nodes perform
clear
The
minimum
slot
size
is
influenced
by the
size.
the
date
rate
that
best
fits
the
current
channel
channel assessment (CCA)
at theInter-frame
start of the
Shortening
Spacing
synchronization
accuracy

the
size
of
time
slot

Task
Execution
in
the
shorter
time
interval

condition
and
the
desired
time
slot
size.
transmission.
has
to computational
be larger than the
guard. Interval.
Synchronization
more
resource
Interval
defined
between
the transmission
accuracy
Solution
: reduce
the guard
interval sizeofbytwo
utilizing
consecutive
Wi-Fi packets.
more accurate
clock.
Higher priority for RT-WiFi  shorter IFS.
D. Association Process
 RT-WiFi node has no information about TDMA schedule
before it joins the network.
 It works like a regular Wi-Fi node and follows same
authentication and association process.
 RT-WiFi node waits for the next beacon frame.
 Operates according to the TDMA schedule.
 Schedule information is attached to the beacon frame in the
vendor specific field.
 Thus, a regular Wi-Fi station can easily associate with a RTWiFi AP without any modification.
Platform
Hardware Platform
 Port to any IEEE 802.11 compatible hardware.
 Atheros AR9285 Wi-Fi chip.
 Ubuntu 12.04 as the operating system, which runs
Linux kernel 3.2.0.
Software Platform
 Two software modules from compat-wireless driver,
mac80211 and ath9k are incorporated with the
TDMA design to build the MAC layer of RT-WiFi.
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RT-WIFI Design and Implementation
Performance Evaluation
Case Study
Conclusion
Future Work
Performance Evaluation
Testbed Setting
 Performance comparison between RT-WiFi and regular Wi-Fi.
• Interference free environment
• office environment
 Compare the MAC layer to MAC
layer performance between
RT-WiFi and regular Wi-Fi
in two test scenarios.
 UDP socket program is installed on
each Device.
 Sensor data with a fixed size payload
are transmitted from each station
to the AP.
 AP transmits control data with
the same packet size back to
each station.
Performance Evaluation
Latency and packet loss ratio comparison in an interference-free environment
 The data link layer transmission latency is calculated as the difference of a
frame’s TSF timestamps between the receiver side and the sender side.
 The packet loss ratio measures the percentage of packets lost by tracking
the sequence number of each packet.
 Standard deviation of the latency in RT-WiFi network is less than 5.3µs.
 Regular Wi-Fi network has a higher average delay and a larger transmission
variation.
Performance Evaluation
Latency comparison between Wi-Fi and RT-WiFi in an
interference-free environment
Performance Evaluation
Latency and packet loss ratio comparison in an office environment
 Deployed the testbed on the 5th floor of the building.
 10 Wi-Fi Aps
 Latency of regular Wi-Fi network is increased because the office
environment has more inference from existing Wi-Fi networks.
 The maximum latency of RT-WiFi is increased up to 4.2ms.
• Uncontrolled mobile devices in the office environment.
 The packet loss ratio of RT-WiFi network increases to 10%.
• Collision with background traffic.
 Regular wifi - maximum delay – 100ms, standard deviation – 2800µs.
Performance Evaluation
Latency comparison between Wi-Fi and RT-WiFi in an office
environment
Performance Evaluation
Flexible Channel Access Controller
Testbed setting:
Network A:
– Regular WiFi network
– 10 Mbps UDP traffic generator.
Network B:
– UDP program.
configured Network-B by using four settings:
• Regular WiFi
• RT-WiFi baseline
• RT-WiFi with co-existence enabled
• RT-WiFi with co-existence enabled and
one in-slot retransmission enabled
Performance Evaluation
Flexible Channel Access Controller
 Mean delay of regular Wi-Fi is increased to 580µs.
• Interference from Network-A.
 The packet loss ratio of baseline RT-WiFi network is increased to 50.21%
• Interference from Network-A.
 RT-WiFi network in the co-existence mode, the packet loss ratio is
decreased to 10.92%.
• Enable the carrier sense mechanism
• Totally not eliminated because of hidden terminal problem.
 Packet loss ratio is further decreased to 4.96% when we enable the in-slotretry.
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RT-WIFI Design
Implementation
Performance Evaluation
Case Study
Conclusion
Future Work
Case study
Mobile gait rehabilitation system
• Smart shoes with
embedded air pressure
Sensors.
• Multiple IMU motion
Sensors a robotic device.
• Host computer running
control applications.
Two types of wireless
Links.
• Transmit sensing signals
from sensing devices to the
control applications.
• Controlling the robotic
assistive device.
Case study
Integration of smart shoes with a RT-WiFi station
 Mobile gait rehabilitation system periodically requests sensing signals from
air pressure sensors for abnormal gait detection.
 The real-time sensor data were first collected from an analog-input module
NI 9221 on an NI 9116 and then sent through a UDP socket from an
Ethernet port of the NI 9022.
 The RT-WiFi station then forward the sensor data through the RT-WiFi
wireless communication link to the RT-WiFi AP on which the controller was
running.
Case study
Emulation of a Wireless Control System:
 Numerous emulations based on the data traces collected from the smart
shoes hardware.
 Step 1: Data from the smart shoes to acquire the network dynamics
(latency and packet loss ratio).
 Step 2: Emulation to evaluate the performance of the wireless control
system.
 Probability to achieve small tracking errors is always higher for RT-WiFi
than regular Wi-Fi.
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RT-WIFI Design
Implementation
Performance Evaluation
Case Study
Conclusion
Future Work
Conclusion
 RT-WiFi supports real-time high-speed wireless
control systems.
 High sampling rate.
 Timing guarantee on packet delivery.
 Configurable components: sampling rate, real-time
performance, communication reliability.
 It is compatibility to existing Wi-Fi networks.
Future Work
 Fault tolerance.
 Dynamic resource management.
 Energy-efficient power management.
 Extend the network topology to mesh structure.
Thank you
Why TDMA
NOT CSMA/CA ?
• CSMA/CA helps to increase throughput, but
not support real time traffic.
• Wi-Fi packet with a hard deadline may be
blocked for a nondeterministic time interval
because of carrier sense OR delayed by the
random backoff access.
• TDMA access the channels according to a
strict time schedule. One node can access a
certain channel in a given time slot.
Types of Link
broadcast link Used for transmit a data frame to all the
neighbor nodes
transmit link
used for dedicated data frame
transmission to the given destination
receive link
used for receiving a data frame from the
given source
shared link
is for multiple nodes to compete for
transmitting data frames
Packet size & data rate
• IEEE802.11 allows to use multible transmission
rates at phusical layerRT-WiFi can deal with
different modulation and coding schemes.
• Higher transmission rate  provides higher
throughput & less resilient to noise and easy
prone to error.
• Our flexible data link layer design allows the
selection of the date rate that best fits the
current channel condition and the desired time
slot size.
Computational resource
• Increasing the sampling rate of the TDMA data
link layerreduce the time slot size.
• For executing tasks in a shorter time interval,
more computational resource is required.
• The maximum sampling rate supported by a
RT-WiFi node is limited by its computation
capability.
Synchronization accuracy
• The guard interval is reserved for the drift
between two RT-WiFi devices.
• The minimum slot size is influenced by the
synchronization accuracy  because the size
of time slot has to be larger than the guard
interval.
• We can reduce the guard interval size by
utilizing more accurate clock or decreasing the
synchronization interval.
Reliability
• RT-WiFi applies two retransmission
mechanisms to improve the reliability of a
communication link.
• Used either independently or in combination.
• Depends on the available computation
resource and specific control applications 
choose the mechanism.
In-slot retry
• If the sender does not receive an ACK immediately message
from the receiver The retransmission is invoked
• The retransmission time of an in-slot retry should not exceed
the length of a time slot.
Out-of-slot retry
• If a packet fails to be transmitted in one time
slot, RT-WiFi node can retransmit it on the
next available link.
• Notice that the retransmission heavily
depends on the desired application behavior.
• For example, on the next available link, if new
control/sensor data is available, then it does
not make sense to retransmit the old data.
Co-existence with regular Wi-Fi network
• Performing Carrier Sense:
• RT-WiFi node does not perform carrier sense, because the manager
will make sure that there is no temporal or spatial reuse in the
operation environment on the channel specified in that time slot.
• co-existence performance between RT-WiFi and regular Wi-Fi could
be poor, because of the high possibility that the two types of traffic
could collide with each other.
• Solution: RT-WiFi nodes should perform clear channel assessment
(CCA).
Co-existence with regular Wi-Fi network
• Shortening Inter-frame Spacing (IFS):
• IFS: interval between the transmission of two consecutive Wi-Fi
packets.
• Wi-Fi nodes wait for a pre-defined IFS before start to transmit the
next frame.
• higher priority for the RT-WiFi node  a shorter IFS can be assigned
for the RT-WiFi node.