Real-Time, Determinism and Ethernet

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1 Real-Time, Determinism and Ethernet Paul Taylor Slide 1

2 Agenda Definitions Real-Time Determinism Control Network Ethernet Evolution Bandwidth Switches Fast Ethernet Full Duplex Prioritisation Redundancy Real-Time / Determinism with Ethernet Mathematical Example Slide 2

3 Definitions Real-Time Adjective :(computer science) of a system, in which dataprocessing occurs as the data is generated. Real-time systems are systems that respond to an input immediately. Any system which uses some kind of shared transmission medium cannot be a real-time system under this definition. Slide 3

4 Definitions - Determinism Determinism. The ability for a system to respond with a consistent, predictable time delay between input and response. This time delay is commonly referred to as a scan time and is usually given a maximum value. Slide 4

5 Definitions Control Network That network which links the controllers of a process to the devices which the process controls. May also link several controllers to one (or more) master controllers. Merges with the automation network and the device network. Information Automation Control Device Slide 5

6 IBM Ethernet Evolution - The ALOHA network Terminal Terminal Pioneered the concept of a shared transmission channel, or the CSMA/CD model. Developed to link the IBM 360 mainframe to the other islands of Hawaii Outbound channel uses one to many transmission All inbound transmissions use same channel frequency with possible contention IBM Mainframe University of Hawaii Terminal Acknowledgements sent to everyone simultaneously Slide 6

7 Ethernet Evolution - The First Ethernet Designed by Bob Metcalf at PARC in 1972/3 Slide 7

8 Ethernet Evolution Ethernet today is not the same as it was when it was invented 30 years ago. Improvements have consisted of : Higher bandwidth Dual duplex communication Switching Prioritisation VLANs Slide 8

9 Ethernet Evolution - Bandwidth 1000 Mbps IEEE 802.3c 1BASE5 StarLAN standard 100 Mbps 10 Mbps ALOHA network runs in Hawaii Bob Metcalf invents Ethernet at PARC DIX Ethernet v2.0 specification IEEE BASE2 standard IEEE 802.3i 10BASE-T standard IEEE 802.3z 1000BASE-T standard 1 Mbps 0.1 Mbps 0.01 Mbps IEEE BASE5 standard IEEE BASE-T standard Slide 9

10 Ethernet Evolution Half, Full and Dual Duplex Half duplex one pair of cables used for communication. Can either be for transmission or reception, but not both at the same time. Full duplex two pairs of cables used for communication. One pair for transmission, one for reception. Dual duplex one pair of cables used for communication. The pair can be used for transmission and reception at the same time. Slide 10

11 Ethernet Evolution - Switching Original Ethernet composed of one collision domain. Only one device at a time can talk. Collision Slide 11

12 F1 F6 F2 F7 F3 F8 F4 F9 F5 F1 Ethernet Evolution - Switching Switched Ethernet has broken up the collision domains Now lots of little collision domains. Still only half duplex. TX Collision TX Slide 12

13 F1 F6 F2 F7 F3 F8 F4 F9 F5 F1 Ethernet Evolution - Switching Switched Ethernet with full duplex communication Now there are no collision domains and hence no collisions! Slide 13

14 Ethernet Evolution Prioritisation (QoS) Original Ethernet standards Maximum frame size 1518 bytes Preamble SFD Destination- Address Source Address Length/ Type Data FCS 64 byte byte Recent change to allow Prioritisation Maximum frame size 1522 bytes (increase of 4 bytes) Preamble SFD Destination- Address Source Address TAG Length/ Type Data FCS TPID TCI TPID = Tag Protocol Identifier Tag Protocol ID User Priority CFI VLAN ID TCI = Tag Control Information 16 bit 3 bit 1 bit 12 bit CFI = Canonical Format Indicator Slide 14

15 Ethernet Evolution Prioritisation (QoS) + Attach File Video Voice Voice Video File Priority Slide 15

16 Ethernet Evolution - VLANs Part of the same development as Prioritisation was VLANs or Virtual Local Area Networks. DTE can be tagged as belonging to a virtual LAN. Any traffic for something outside of that VLAN will not be accepted by the DTE. Switches too can monitor VLAN traffic and will not pass on frames that are not for that VLAN. Slide 16

17 Ethernet Evolution - VLANs Personnel Department Personnel Server Admin Server Admin Department Employees Server Employees Slide 17

18 Ethernet Evolution - Redundancy Spanning Tree? No, Industrial Redundancy! Windows NT, 2000 etc a b Slide 18

19 Bringing it all together By combining all the recent developments of Ethernet together, data throughput is greatly increased. 100Mbit/s switched full duplex 200Mbit/s 100Mbit/s switched 100Mbit/s 100Mbit/s shared 10Mbit/s switched full duplex 10Mbit/s switched 10Mbit/s shared 10Mbit/s 4Mbit/s 20Mbit/s 40Mbit/s Slide 19

20 Real-Time Ethernet So is Ethernet a real-time control network? Consider our original definition of real-time Adjective :(computer science) of a system, in which dataprocessing occurs as the data is generated. No, Ethernet cannot be considered a real-time control network, a control network definitely, but not a real-time one. BUT, what about a deterministic one?? Slide 20

21 Deterministic Ethernet Consider this typical RS485 bus network Bus Termination 500m Bus Termination Master Controller I/O 1 I/O 31 I/O Density site 500m I/O I/O 500m Density Density site site I/O Density site Slide 21

22 Cascading of 10/100 Switches SW1 SW2 SW3 SW4 100 Mbit/s 100 Mbit/s 100 Mbit/s 10 Mbit/s 10 Mbit/s 10 Mbit/s 10 Mbit/s 10 Mbit/s M S7 S8 S1 S8 S15 S16 S23 S24 S31 M M=Master; S= Slave; SW= Switch RS1 Application with one Master, 31 Slaves and 4 RS1-Switches Slide 22

23 Cascading of 10/100 Switches Multicast from Master to all Slaves: Send packet from Master to SW1: 57.6µs Latency in SW1: Receive packet in Slave 1: 57.6µs Send packet S1 to SW2: 5.76µs Transmit time SW1 to SW2 : 2.5µs Latency in SW2: Send packet SW2 to SW3: 5.76µs Transmit time SW2 to SW3 2.5µs Latency in SW3: Send packet SW3 to SW4: 5.76µs Transmit time SW3 to SW4 2.5µs Latency in SW4: Receive packet in Slave 31: 57.6µs Total time: µs Receive packet at slave 1 Receive packet at slave 31 This time is called: Receive variance 36.78µs Slide 23

24 Cascading of 10/100 switches All Slaves sending response to Master: Send packet Slaves to SW4: 57.6µs Latency in SW4: Send packet from SW4 to SW3: 52.8µs Transmit time SW4 to SW3: 2.5µs Latency in SW3: Send packet SW3 to SW2: 52.8µs Transmit time SW3 to SW2 2.5µs Latency in SW2: Send packet SW2 to SW1: 52.8µs Transmit time SW2 to SW1 2.5µs Latency in SW1: Send packet SW1 to Master: 528.0µs Total time: 767.5µs Slide 24

25 Upgrading all links to 100Mbps SW1 SW2 SW3 SW4 100 Mbit/s 100 Mbit/s 100 Mbit/s 100 Mbit/s 100 Mbit/s 100 Mbit/s 100 Mbit/s 100 Mbit/s M S1 S8 S15 S16 S23 S24 S31 M = Master; S = Slave; SW = Switch RS2 Application with one Master, 31 Slaves and 4 RS2-Switches Slide 25

26 Upgrading all links to 100Mbps Multicast from Master to all Slaves: Send packet from Master to SW1: 5.76µs Latency in SW1: Receive packet in Slave 1: 5.76µs Send packet S1 to SW2: 5.76µs Transmit time SW1 to SW2 : 2.5µs Latency in SW2: Send packet SW2 to SW3: 5.76µs Transmit time SW2 to SW3 2.5µs Latency in SW3: Send packet SW3 to SW4: 5.76µs Transmit time SW3 to SW4 2.5µs Latency in SW4: Receive packet in Slave 31: 5.76µs Total time taken: 52.3µs Receive packet at slave 1 Receive packet at slave 31 This time is called: Receive variance 36.78µs Slide 26

27 Upgrading all links to 100Mbps All Slaves sending response to Master: Send packet Slaves to SW4: 5.76µs Latency in SW4: Send packet from SW4 to SW3: 52.8µs Transmit time SW4 to SW3: 2.5µs Latency in SW3: Send packet SW3 to SW2: 52.8µs Transmit time SW3 to SW2 2.5µs Latency in SW2: Send packet SW2 to SW1: 52.8µs Transmit time SW2 to SW1 2.5µs Latency in SW1: Send packet SW1 to Master: 52.8µs Total time: µs Slide 27

28 Scan Times Scan time of a 10Mbps network 312 µs µs = 2505 µs Scan time of 10Mbps network with 100Mbps links between switches 156 µs µs = 924 µs Scan time of 100Mbps network 52 µs + 240µs = 292 µs Slide 28

29 Real-Time, Determinism and Ethernet So can Ethernet cope with the demands of a real-time control network? Scan times are as quick if not quicker than contemporary bus systems. Deterministic? Definitely in a controlled traffic network. Worldwide, open standards means all vendors support Ethernet. Flexibility means it already supports such things as remote video monitoring. Evolutionary history means it can only get better! Slide 29

30 The Future How many disparate networks do you need between the information and device layers? Device Control Automation Information Interbus Loop Interbus ASI Profibus DP Seriplex Profibus FMS CAN CCLink ControlNet DeviceNet SDS ETHERNET WorldFIP / FIP LonWorks HART IEC/SP50 H2 IEC/SP50 H1 Profibus PA Slide 30

Chapter 3: Industrial Ethernet

Chapter 3: Industrial Ethernet 3.1 Introduction Previous versions of this handbook have dealt extensively with Ethernet so it is not our intention to revisit all the basics. However, because Smart Grid protocols are increasingly reliant