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.

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 100HzWirelessHART

Not suitable for high-speed real-time wireless control. Can provide real-time communication, but the maximum supported

sampling frequency is only 100Hz ISA100.11a

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.MBStar

Not suitable for high-speed real-wireless control. Cannot guarantee real-time data delivery, except for voice time

packets Bluetooth

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.ZigBee

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.

WiFi

- 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, real-

time data delivery, and co-existence with regular WiFi networks.

Transparent System Design• Reuse hardware & software available & run existing

applications with minimum modifications.

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

Sampling Rate

Packet size and data rate

Computational resource

Synchronization accuracy

Reliability

In-slot retry

Out-of-slot retry

Co-existence with regular Wi-

Fi network:

Performing Carrier Sense

Shortening Inter-frame

Spacing

Packet size and data rate

If the transmission time of the packet is larger than the time slot size then RT-WiFi cannot successfully transmit the packet.

IEEE 802.11 Uses multiple transmission rates that utilize different modulation and coding scheme.

Higher transmission rate higher throughput less resilient to noise.

Flexible data link layer design Selection of the date rate that best fits the current channel condition and the desired time slot size.

Computational resource

Higher sampling rate reduce the time slot size. Task Execution in the shorter time interval

more computational resource.

Synchronization accuracy

The minimum slot size is influenced by the synchronization accuracy the size of time slot has to be larger than the guard Interval.

Solution : reduce the guard interval size by utilizing more accurate clock.

Out-of-slot retry

Packet fails to be transmitted in one time slot, RT-WiFi node can retransmit.

Requirement of retransmit depends on the application.

Performing Carrier Sense

Absence of WiFi RT-WiFi node does not perform carrier sense.

Presence of WiFi Possibility that the two types of traffic could collide.

Solution: RT-WiFi nodes perform clearchannel assessment (CCA) at the start of the transmission.

Shortening Inter-frame Spacing

Interval defined between the transmission of two consecutive Wi-Fi packets.Higher priority for RT-WiFi shorter IFS.

In-slot retry

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 RT-

WiFi AP without any modification.

PlatformHardware 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.

RT-WIFI Design and Implementation Performance Evaluation Case Study Conclusion Future Work

Performance EvaluationTestbed 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 EvaluationLatency 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 EvaluationLatency 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 EvaluationFlexible Channel Access ControllerTestbed 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 EvaluationFlexible 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-slot-retry.

RT-WIFI Design Implementation Performance Evaluation Case Study Conclusion Future Work

Case studyMobile gait rehabilitation system• Smart shoes withembedded air pressure Sensors.• Multiple IMU motion Sensors a robotic device.• Host computer runningcontrol applications.Two types of wirelessLinks.• Transmit sensing signalsfrom sensing devices to the control applications.• Controlling the robotic assistive device.

Case studyIntegration 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 studyEmulation 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.

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 frametransmission 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 layerRT-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 layerreduce 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.