Chair of Network Architectures and Services Department of Informatics Technical University of Munich Time Sensitive Networking for Wireless Networks Alexander Mildner advised by Fabien Geyer Thursday 11 th October, 2018 Chair of Network Architectures and Services Department of Informatics Technical University of Munich
Outline • Introduction to Time Sensitive Networking (TSN) • History of TSN • Use-Cases • TSN Components & Standards • Example: 802.1Qbv - Time Based Scheduling • Towards TSN for Wireless Networks • Challenges • Biref introduction to PTP • Accurate Clock Synchronization • Bounded Low Latency • Providing High Reliability • Resource Management • Summary & Future Outlook A. Mildner — TSN for Wireless Networks 2
Time Sensitive Networking History of Time Sensitive Networking • IEEE Audio/Video Bridging (AVB) Task Group (founded 2005): • addressed real-time capabilities of Ethernet for Audio/Video market, consumer electronics and automotive infotainment • introduced several standards/extensions to IEEE 802.3 (Ethernet) • e.g. a Credit Based Shaper (CBS) in 802.1Qav and a Stream Reservation Protocol (SRP) in 802.1Qat • IEEE 802.1AS-2011 - a generalized Precision Time Protocol (gPTP) • since 2012: IEEE Time Sensitive Networking Task Group: • more interest on AVB features from other market segments e.g. Automotive and Industrial • Renaming of AVB TG to Time Sensitive Networking TG • Task to adapt and extend the set of standards, i.o. to provide real-time capabilities for IEEE 802.1 in general • NOTE: In this work, we are focusing on TSN on Layer 2 only, but there is more: • For Layer 3 there is the IETF Deterministic Netwroking (DetNet) [7] standardization • For Application Layer: e.g. OPC-UA 1 , which is being widely used 1 https://opcfoundation.org/about/opc-technologies/opc-ua/ A. Mildner — TSN for Wireless Networks 3
Time Sensitive Networking Use-Cases • Automotive • Professional Audio/Video • Industrial Automation Figure 1: Example Industrial Ethernet Network 2 2 http://www.veryxtech.com/wp-content/uploads/2014/06/industrial-ethernet-network.jpg A. Mildner — TSN for Wireless Networks 4
Time Sensitive Networking Figure 2: TSN components [4] A. Mildner — TSN for Wireless Networks 5
Time Sensitive Networking Example: 802.1Qbv - Time Based Scheduler Figure 3: 802.1Qbv - Time Based Scheduler [1] A. Mildner — TSN for Wireless Networks 6
Towards TSN for Wireless Networks Challenges towards TSN for Wireless Networks • Advantages of Wireless Networks over Ethernet Networks: • increased flexibility & mobility of devices • better cost efficiency (e.g. in regard to wiring cost) • reduced complexity • Challenges for enabling TSN for Wireless Networks: • unreliability of wireless links • asymmetric path delay • channel interference • signal distortions from the environment • Accurate Clock Synchronization not trivial (e.g. absence of a PHC in wireless NICs) • currently no time triggered packet sending for Wireless NICs available ⇒ Need for a careful design of technologies towards TSN for Wireless, including solutions for the above mentioned challenges (e.g. dynamic channel quality measurements) A. Mildner — TSN for Wireless Networks 7
Towards TSN for Wireless Networks Brief introduction to PTP • Precision Time Protocol (PTP) defined in IEEE 1588(-v2) • Protocol for accurate and precise time/clock synchronization for LAN (e.g. Ethernet) • Most common Network Interface Cards (NICs) have a built-in Physical Hardware Clock (PHC) • Grand Master: Reference Clock, Best Master Clock Algorithm (BMCA) for determining the Grand Master Clock • Ordinary Clock: also referred as Slave (or Endstation), synchronizes its local time to a Master Clock • Boundary Clocks: Interface between separate PTP domains, acts as Master & Slave • Transparent Clocks: • End-to-End (E2E): forwards PTP messages and includes the residence time for the message to traverse the bridge/router • Peer-to-Peer (P2P): same as E2E, but also measures the local link delays A. Mildner — TSN for Wireless Networks 8
Towards TSN for Wireless Networks Brief introduction to PTP Figure 4: Precision Time Protocol Synchronization Messaging PathDelay = [( t 4 − t 1) − ( t 3 − t 2)] / 2 OffsetFromMaster = ( t 2 − t 1) − PathDelay A. Mildner — TSN for Wireless Networks 9
Towards TSN for Wireless Networks Accurate Clock Synchronization • IEEE 802.1AS (gPTP) contains a proposal for Time Synchronization over Wireless Net- works, in particular for IEEE 802.11 (Wi-Fi), in section 12 • The proposed solution uses 802.11v Timing Measurement (TM), for taking link delay asym- metry into account and can achieve sub-microsecond accuracy • IEEE 802.1AS-Rev - Revision of the 802.1AS standard, will contain a new synchronization method for Wireless Networks using 802.11mc Fine Timing Measurement (FTM) • Other approaches [3], propose the use of 802.11 Beacon Frames and the Time Synchro- nization Function (TSF) for clock synchronization - microsecond accuracy, no dependency on PTP A. Mildner — TSN for Wireless Networks 10
Towards TSN for Wireless Networks Accurate Clock Synchronization Figure 5: Accurate Clock Synchronization using TM ′ − t 1) / ( t 2 ′ − t 2) neighborRateRatio = ( t 1 linkDelay = [( t 4 − t 1) − ( t 3 − t 2)] / 2 timeOffset = [( t 2 − t 1) − ( t 4 − t 3)] / 2 A. Mildner — TSN for Wireless Networks 11
Towards TSN for Wireless Networks Bounded Low Latency • For Ethernet: Time Based Packet Transmission using SO_TXTIME Socket Option, Earli- est TxTime First (ETF) and TAPRIO scheduler 3 • For Wireless NICs: yet no similar functionality exists • The Latency heavily depends on the signal quality and possible signal distortions from the environment ⇒ Need for dynamic link quality measurements/monitoring • Link delay asymmetries need to be monitored, as they are dynamically changing • Delays from Automatic Repeat Request (ARQ) mechanisms also need to be taken into account • Possible transition solution: Hide the unreliable Wireless link behind a 802.1Qbv Time Based Scheduler 3 https://lwn.net/Articles/758592/ A. Mildner — TSN for Wireless Networks 12
Towards TSN for Wireless Networks Bounded Low Latency Figure 6: Hiding the wireless channel behavior behind 802.1Qbv [2] A. Mildner — TSN for Wireless Networks 13
Towards TSN for Wireless Networks Providing High Reliability • The Majority of time-critical traffic uses UDP as transport protocol ⇒ there is no built-in recovery scheme for lost packets on the Application Layer • Some Wireless Network technologies, have a built-in ARQ scheme on Layer 2, such as 802.11 • But: The additional latency overhead needs to be taken into account as a maximum bounded latency • TSN provides 802.1CB for Seamless Redundancy and Stream Identification, which also is supported by e.g. 802.11 • 802.1CB : Replicate each frame and send them on two (or more) disjoint paths to the target, duplicate/extra frames will be eliminated on the way to the target A. Mildner — TSN for Wireless Networks 14
Towards TSN for Wireless Networks Providing High Reliability Figure 7: Stream duplication over Wireless [2] A. Mildner — TSN for Wireless Networks 15
Towards TSN for Wireless Networks Resource Management • fully centralized vs. fully decentralized model • Centralized User Configuration (CUC): • Applications can request their needed network resources ate the CUC • Has a global view of the whole network • Triggers actions/resource reservations at the CNC (using e.g. RESTCONF/NETCONF Protocol) • Centralized Network Configuration (CNC): • Takes care of the actual network device configuration (e.g. schedules) • Has an overview of the currently available network resources • A proper Configuration Model needs to be specifically defined for Wireless Nodes, which is even for Ethernet Networks still under development • Possible transition solution: Using Wireless Networking for the Control connections be- tween CUC and CNC and the network nodes for configuration only A. Mildner — TSN for Wireless Networks 16
Towards TSN for Wireless Networks Resource Management Figure 8: TSN Resource Management [2] A. Mildner — TSN for Wireless Networks 17
Summary • TSN for Wireless Networks is in an early development stage • Not all challenges have been resolved or even been approached • Accurate Clock Synchronization is already possible on 802.11, using different approaches • Providing Bounded Low Latencies and high Reliability still needs further research • Some Open Topics: • Sophisticated Comparison of current methods for Accurate Clock Synchronization, using mea- surements • Improved Hardware & Software support for PTP over Wireless • Development of a time based packet transmission mechanism for Wireless NICs • Dynamic, standardized monitoring & measurements for quality parameters of Wireless Links • and many more ... A. Mildner — TSN for Wireless Networks 18
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