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1 intelligent field devices
Office network TCP - IP Ethernet Plant Network Ethernet, ControlNet Fieldbus intelligent field devices FF, PROFIBUS, MVB, LON Sensor Busses simple switches etc. CAN, DeviceNet, SDS, ASI-bus, Interbus-S 3 Industrial Communication Systems 3.1 Field Bus: principles Buses de terreno: principios Bus de terrain: principes Feldbusse: Grundlagen

2 Field bus: principles 3.1 Field bus principles 3.2 Field bus operation Centralized - Decentralized Cyclic and Event Driven Operation 3.3 Standard field busses

3 Location of the field bus in the plant hierarchy
2 12 33 23 4 File Edit Engineering SCADA level Operator Plant bus Programmable Logic Controller Plant Level Field bus fieldbus interconnects field devices and PLCs. In particular the data of IOs which are not directly attached to a PLC are transported to the PLCs with a fieldbus. Field level Sensor/ Actor Bus Sensor / Actor direct I/O

4 - robust and easy installation by skilled people
What is a field bus ? A data network, interconnecting an automation system, characterized by: - many small data items (process variables) with bounded delay (1ms..1s) - transmission of non-real-time traffic for commissioning and diagnostics - harsh environment (temperature, vibrations, EM-disturbances, water, salt,…) - robust and easy installation by skilled people - high integrity (no undetected errors) and high availability (redundant layout) - clock synchronization (milliseconds to microseconds) - low attachment costs ( € €50 / node) - moderate data rates (50 kbit/s - 5 Mbit/s), large distance range (10m - 4 km) requirements from PLCs carry over to the field bus as well (real-time constraints, harsh environments, high integrity and availability). In addition synchronization is necessary to assign precise timestamps to data, attachment cost should be kept low. While data rates are moderate, large distances need to be covered (depending on plant type).

5 - increased modularity of plant (each object comes with its computer)
Expectations - reduce cabling - increased modularity of plant (each object comes with its computer) - easy fault location and maintenance - simplify commissioning (mise en service, IBS = Inbetriebssetzung) - simplify extension and retrofit - off-the-shelf standard products to build “Lego”-control systems

6 The original idea: save wiring
marshalling bar I/O PLC dumb devices (Rangierung, tableau de brassage (armoire de triage) tray capacity Before After PLC COM Marshalling: connects primary and secondary equipment field bus But: the number of end-points remains the same ! energy must be supplied to smart devices

7 Marshalling (Rangierschiene, Barre de rangement)
The marshalling is the interface between the PLC people and the instrumentation people. The fieldbus replaces the marshalling bar or rather moves it piecewise to the process (intelligent concentrator / wiring)

8 Different classes of field busses
One bus type cannot serve all applications and all device types efficiently... Data Networks Workstations, robots, PCs Higher cost Not bus powered Long messages ( , files) Not intrinsically safe Coax cable, fiber Max distance miles 10 100 1000 10,000 Sensor Bus Simple devices Low cost Bus powered Short messages (bits) Fixed configuration Not intrinsically safe Twisted pair Max distance 500m frame size (bytes) High Speed Fieldbus PLC, DCS, remote I/O, motors Medium cost Not bus powered Messages: values, status Not intrinsically safe Shielded twisted pair Max distance 800m Low Speed Fieldbus Process instruments, valves Medium cost Bus-powered (2 wire) Messages: values, status Intrinsically safe Twisted pair (reuse 4-20 mA) Max distance 1200m 10 100 1000 10,000 source: ABB poll time, milliseconds

9 Fieldbus over a wide area: example wastewater treatment
Pumps, gates, valves, motors, water level sensors, flow meters, temperature sensors, gas meters (CH4), generators, etc are spread over an area of several km2. Some parts of the plant have to cope with explosives. Wiring is traditionally mA => long threads of cable (several 100 km).

10 Fieldbus over a wide area: example wastewater treatment
Japan source: Kaneka, Japan Malaysia Motor Control Center Junction Box plant has demanding requirements with respect to large distances and need cope with explosives. A hierarchical structure with H1 Speed Fieldbus on the high level and Hart Fieldbus on the lower levels is used for the water treatment part. The lower levels and higher level are combined with junction box gateways. On the other hand the electrical grid part (substation) uses Ethernet. For the interconnecting of these two parts fieldbus protocol converters are used. Numerous analog inputs/outputs (AI/AO), low speed (37 kbit/s) segments (Hart) merged to 1 Mbit/s links (H1 Speed Fieldbus).

11 Fieldbus Application: locomotives and drives
power line radio cockpit Train Bus diagnosis Vehicle Bus brakes power electronics motors track signals data rate 1.5 Mbit/second delay . Each car of a train has its own vehicle bus to control the brakes, power electronics and motors and gather track signals. In addition there is a train bus that connects the different cars. 1 ms (16 ms for skip/slip control) medium twisted wire pair, optical fibers (EM disturbances) number of stations up to 255 programmable stations, 4096 simple I/O integrity very high (signaling tasks) cost engineering costs dominate

12 Fieldbus Application: automobile
- Electromechanical wheel brakes - Redundant Engine Control Units - Pedal simulator - Fault-tolerant 2-voltage on-board power supply - Diagnostic System The communication network of a car is redundant, there are several engine control units and separate cables connecting them. This allows a highly available and safe operation.

13 Networking busses: Electricity Network Control: myriads of protocols
houses substation Modicom ICCP control center Inter-Control Center Protocol IEC 870-6 HV MV LV High Voltage Medium Low SCADA FSK, radio, DLC, cable, fiber,... RTU COM Remote Terminal Units IEC 870-5 DNP 3.0 Conitel RP 570 serial links (telephone) large number of different fieldbus protocols which need to be interconnected with gateways which convert from one protocol to another. Note that High, Medium and Low Voltage parts have different requirements and thus use different fieldbus protocols low speed, long distance communication, may use power lines or telephone modems. Problem: diversity of protocols, data format, semantics...

14 Engineering a fieldbus: consider data density (Example: Power Plants)
Acceleration limiter and prime mover: 1 kbit in 5 ms Burner Control: 2 kbit in 10 ms For each 30 m of plant: 200 kbit/s Fast controllers require at least 16 Mbit/s over distances of 2 m Data transmitted from periphery or from fast controllers to higher level Slower links to control level through field busses over distances of 1-2 km. The control stations gather data at rates of about 200 kbit/s over distances of 30 m. The control room computers are interconnected by a bus of at least 10 Mbit/s, over distances of several 100 m. plant might require more than one kind of fieldbus. Where which fieldbus protocol is used depends mainly on data density and distances. Field bus planning: estimate data density per unit of length or surface, response time and throughput over each link.

15 How does a field bus support modularity ?
Assessment What is a field bus ? Which of these qualities are required: 1 Gbit/s operation Frequent reconfiguration Plug and play Bound transmission delay Video streaming How does a field bus support modularity ? Which advantages are expected from a field bus ?

16 3 Industrial Communication Systems
3.2 Field bus operation Buses de terreno: modo de trabajo Bus de terrain: mode de travail Feldbus: Arbeitsweise 3 Industrial Communication Systems

17 Fieldbus - Operation 3.1 Field bus principles 3.2 Field bus operation Data distribution Cyclic Operation Event Driven Operation Real-time communication model Time distribution Networking 3.3 Standard field busses

18 Objective of the field bus
Distribute process variables to all interested parties: source identification: requires a naming scheme accurate process value and units quality indication: {good, bad, substituted} time indication: how long ago was the value produced (description) source value quality time description

19 Data format source value quality time description minimum In principle, the bus could transmit the process variable in clear text (even using XML..) However, this is quite expensive and only considered when the communication network offers some 100 Mbit/s and a powerful processor is available to parse the message More compact ways such as ASN.1 have been used in the past with 10 Mbit/s Ethernet Field busses are slower (50kbit/s ..12 Mbits/s) and thus more compact encodings are used. ASN.1: (TLV) type length value

20 Variables may be of different types, types can be mixed.
Datasets Field busses devices have a low data rate and transmit always the same variables. It is economical to group variables of a device in the same frame as a dataset. A dataset is treated as a whole for communication and access. A variable is identified within a dataset by its offset and its size Variables may be of different types, types can be mixed. dataset analog variables binary variables dataset identifier wheel air line time stamp speed pressure voltage Inflexible, but plants change rarely 16 32 48 64 all door closed lights on heat on air condition on 66 70 bit offset size

21 Dataset extension and quality
To allow later extension, room is left in the datasets for additional variables. Since the type of these future data is unknown, unused fields are filled with '1". To signal that a variable is invalid, the producer overwrites the variable with "0". Since both an "all 1" and an "all 0" word can be a meaningful combination, each variable can be supervised by a check variable, of type ANTIVALENT2: dataset 1 check correct variable error undefined variable value var_offset chk_offset 10 = substituted 00 = network error 01 = ok 11 = data undefined A variable and its check variable are treated indivisibly when reading or writing The check variable may be located anywhere in the same data set.

22 hierarchical or peer-to-peer communication
PLC alternate master PLC central master / slave: hierarchical “master” AP AP all traffic passes by the master (PLC); adding an alternate master is difficult (it must be both master and slave) “slaves” input output peer-to-peer: distributed PLC PLC PLC “masters” AP AP AP PLCs may exchange data, share inputs and outputs allows redundancy and “distributed intelligence” devices talk directly to each other separate bus master from application master ! “slaves” input output AP Application

23 Most variables are read in 1 to 3 different devices
Broadcasts Most variables are read in 1 to 3 different devices Broadcasting messages identified by their source (or contents) increases efficiency. application processor application processor application plant processor instances image = = plant plant plant distributed variable image image image database bus Each device is subscribed as source or as sink for some process variables Only one device is source of a certain process variable (otherwise collision) The bus refreshes plant image in the background The replicated traffic memories can be considered as "caches" of the plant state (similar to caches in a multiprocessor system), representing part of the plant image. Each station snoops the bus and reads the variables it is interested in.

24 Transmission principle
The previous operation modes made no assumption, how data are transmitted. The actual network can transmit data cyclically (time-driven) or on demand (event-driven), or a combination of both.

25 Cyclic versus Event-Driven transmission
cyclic: send value strictly every xx milliseconds misses the peak (Shannon-Nyquist!) always the same, why transmit ? time individual period event-driven: send when value change by more than x% of range hysteresis nevertheless transmit: - every xx as “I’m alive” sign - when data is internally updated - upon quality change (failure) how much hysteresis ? - coarse (bad accuracy) - fine (high frequency) limit update frequency !, limit hysteresis

26 Fieldbus: Cyclic Operation mode
3.1 Field bus principles 3.2 Field bus operation Data distribution Cyclic Operation Event Driven Operation Real-time communication model Time distribution Networking 3.3 Standard field busses

27 Traffic Memory: implementation
Bus and Application are decoupled by shared memory, the Traffic Memory, (content addressed memory, CAM, also known as communication memory); process variables are directly accessible by application. Application Processor Traffic Memory Ports (holding a dataset) Associative memory Content-addressable memory (CAM) is a special type of computer memory used in certain very high speed searching applications. Unlike standard computer memory (random access memory or RAM) in which the user supplies a memory address and the RAM returns the data word stored at that address, a CAM is designed such that the user supplies a data word and the CAM searches its entire memory to see if that data word is stored anywhere in it. If the data word is found, the CAM returns a list of one or more storage addresses where the word was found (and in some architectures, it also returns the data word, or other associated pieces of data). Thus, a CAM is the hardware embodiment of what in software terms would be called a map (java) or dictionary (python) Because a CAM is designed to search its entire memory in a single operation, it is much faster than RAM in virtually all search applications, but more expensive. an associative memory decodes the addresses of the subscribed datasets two pages ensure that read and write can occur at the same time (no semaphores !) Bus Controller bus

28 Freshness supervision
Applications tolerate an occasional loss of data, but no stale data, which are at best useless and at worst dangerous. Data must be checked if are up-to-date, independently of a time-stamp (simple devices do not have time-stamping) How: Freshness counter for each port in the traffic memory Reset by the bus or the application writing to that port Otherwise incremented regularly, either by application processor or bus controller. Applications always read the value of the counter before using port data and compare it with its tolerance level. The freshness supervision is evaluated by each reader independently, some readers may be more tolerant than others. Bus error interrupts in case of severe disturbances are not directed to the application, but to the device management. Independent of time stamping (simple devices) Freshness counter Reset by bus or app, incremented regularly App reads value of counter before using data Some readers more tolerant than others

29 Example of Process Variable API (application programming interface)
Simple access of the application to variables in traffic memory: ap_put (variable_name, variable value) ap_get (variable_name, variable value, variable_status, variable_freshness) Optimize: access by clusters (predefined groups of variables): ap_put_cluster (cluster_name) ap_get (cluster_name) Each cluster is a table containing the names and values of several variables. The clusters can correspond to "segments" in the function block programming.

30 Cyclic Data Transmission
address Bus devices (slaves) 1 2 3 4 5 6 Master Poll List plant Principle: master polls addresses in fixed sequence (poll list) Individual period RTD N polls time [µs] read transfer time [ms] The duration of each poll is the sum of the transmission time of address and data (bit-rate dependent) and of the reply delay of the signals (independent of bit-rate). address (i) data (i+1) 10 µs/km 1 2 3 4 5 6 44 µs µs Example Execution

31 Round-trip delay of master-slave exchange
closest data sink most remote data source repeater repeater T_m Master Frame t_repeat t_repeat The round-trip propagation delay T_m (t_pd = 6 µs/km) delay limits the extension t_source access delay t_mm t_ms (t_repeat < 3 µs) of the bus T_s t_repeat Slave Frame t_sm T_m next Master Frame distance

32 Cyclic operation characteristics
1. Data are transmitted at fixed intervals, whether they changed or not. 2. The delivery delay (refresh rate) is deterministic and constant. 3. The bus is under control of a central master (or distributed time-triggered algorithm). 4. No explicit error recovery needed since fresh value will be transmitted in next cycle. Consequence: cycle time limited by product of number of data transmitted and the duration of each poll (e.g. 100 µs / variable x 100 variables => 10 ms) To keep the poll time low, only small data items may be transmitted (< 256 bits) The bus capacity must be configured beforehand. Determinism gets lost if the cycles are modified at run-time.

33 Optimizing Cyclic Operation
Problem: Cyclic operation uses fixed portion of the bus' time => Poll period increases with number of polled items => response time slows down Solution: introduce sub-cycles for less urgent periodic variables length: power of 2 multiple of the base period. 2 ms period 4 ms period 1 2 4a 8 16 1 4b 1 2 3 64 1 4a time 1 ms period 1 ms 1 ms group with period 1 ms (basic period) Notes: The poll cycles should not be modified at run-time (non-determinism) A device exports many process variables with different priorities. If there is only one poll type per device, a device must be polled at the frequency required by its highest-priority data. To reduce bus load, the master polls the process data, not the devices

34 Cyclic Transmission with Decoupled Application
algorithms algorithms algorithms algorithms cyclic application application application application poll 1 2 3 4 bus source master port Ports Ports Ports Ports Traffic Periodic List Memory sink sink port port bus bus bus bus bus controller controller controller controller controller bus Bus and applications are decoupled by a shared memory, the traffic memory, which acts as distributed database updated by the network. The principle of cyclic operation, combined with source-addressed broadcast, has been adopted by most modern field busses. It is currently used for power plant control, rail vehicles, aircrafts, etc... The poll scan list located in the central master (which may be duplicated for availability purposes) determines the behavior of the bus. It is configured for a specific project by a single tool, which takes into account the applications’ requirements. This guarantees that no application can occupy more than its share of the bus bandwidth and gives control to the project leader. port address port data The bus traffic and the application cycles are asynchronous to each other. The bus master scans the identifiers at its own pace. Deterministic behavior, at expense of reduced bandwidth and geographical extension.

35 Example: delay requirement
publisher application instance subscribers application instances device device device applications bus bus instance Worst-case delay for transmitting all time critical variables is the sum of: Source application cycle time 8 ms Individual period of the variable on bus 16 ms Sink application cycle time 8 ms = 32 ms

36 Fieldbus: Event-driven operation
3.1 Field bus types 3.2 Field bus operation Data distribution Cyclic Operation Event Driven Operation Real-time communication model Time distribution Networking 3.3 Standard field busses

37 Event-driven Operation
Events cause transmission only when state changes. Bus load very low on average, but peaks under exceptional situations since transmissions are correlated by process (christmas-tree effect). intelligent event- event- event- stations reporting reporting reporting station station station sensors/ actors plant Transmission when state changes Load low on averge, correlation leads to high peaks Detection needs intelligence (combination of changes, application based, meaning) Detection of an event is an intelligent process: Not every change of a variable is an event, even for binary variables. Often, a combination of changes builds an event. Only the application can decide what is an event, since only the application programmer knows the meaning of the variables.

38 Bus interface for event-driven operation
Each transmission on bus causes an interrupt. Bus controller checks address and stores data in message queues. Driver is responsible for removing messages of queue memory and prevent overflow. Filter decides if message can be processed. application filter driver Application Processor message (circular) queues interrupt Bus Controller bus

39 Response of Event-driven operation
Caller Transport Bus Transport Called Application software software Application request interrupt indication confirm time Since events can occur anytime on any device, stations communicate by spontaneous transmission, leading to possible collisions Interruption of server device at any instant can disrupt a time-critical task. Buffering of events can cause unbounded delays Gateways introduce additional uncertainties

40 Determinism and Medium Access In Busses
Although the moment an event occurs is not predictable, the bus should transmit the event in a finite time to guarantee the reaction delay. Events are necessarily announced spontaneously The time required to transmit the event depends on the medium access (arbitration) procedure of the bus. Medium access control methods are either deterministic or not. Non-deterministic Deterministic Central master, Collision Token-passing (round-robin), (CSMA/CA) Binary bisection (collision with winner)

41 Events and Determinism
Deterministic medium access is necessary to guarantee delivery time bound but it is not sufficient since events messages are queued in the devices. events producers & consumers input and output queues bus acknowledgements data packets The average delivery time depends on the length of the queues, on the bus traffic and on the processing time at the destination. Often, the applications influence the event delay much more than the bus does. Real-time Control = Measurement + Transmission + Processing + Acting

42 Events Pros and Cons In an event-driven control system, there is only a transmission or an operation when an event occurs. Advantages: Can treat a large number of events – but not all at the same time Supports a large number of stations System idle under steady - state conditions Better use of resources Uses write-only transfers, suitable for LANs with long propagation delays Suitable for standard (interrupt-driven) operating systems (Unix, Windows) Drawbacks: Requires intelligent stations (event building) Needs shared access to resources (arbitration) No upper limit to access time if some component is not deterministic Response time difficult to estimate, requires analysis Limited by congestion effects: process correlated events A background cyclic operation is needed to check liveliness

43 Summary: Cyclic vs Event-Driven Operation
decoupled (asynchronous): coupled (with interrupts): application processor application processor events (interrupts) traffic memory (buffer) queues bus controller bus controller sending: application writes data into memory receiving: application reads data from memory the bus controller decides when to transmit bus and application are not synchronized sending: application inserts data into queue and triggers transmission, bus controller fetches data from queue receiving: bus controller inserts data into queue and interrupts application to fetch them, application retrieves data

44 Fieldbus: real-time communication model
3.1 Field bus types 3.2 Field bus operation Centralized - Decentralized Cyclic Operation Event Driven Operation Real-time communication model Time distribution Networking 3.3 Standard field busses

45 represent the state of the plant represent state changes of the plant
Mixed Data Traffic Process Data Message Data represent the state of the plant represent state changes of the plant short and urgent data items infrequent, sometimes long messages reporting events, for: ... motor current, axle speed, operator's commands, emergency stops,... • Users: set points, diagnostics, status • System: initialisation, down-loading, ... -> Periodic Transmission of Process Variables -> Sporadic Transmission of Process Variables and Messages Since variables are refreshed periodically, no retransmission protocol is needed to recover from transmission error. Since messages represent state changes, a protocol must recover lost data in case of transmission errors basic period basic period event time sporadic phase periodic phase sporadic phase periodic phase

46 Mixing Traffic is a configuration issue
Cyclic broadcast of source-addressed variables is the standard solution in field busses for process control. Cyclic transmission takes a large share of the bus bandwidth and should be reserved for really critical variables. The decision to declare a variable as cyclic or event-driven can be taken late in a project, but cannot be changed on-the-fly in an operating device. A message transmission scheme must exist alongside the cyclic transmission to carry not-critical variables and long messages such as diagnostics or network management An industrial communication system should provide both transmission kinds. Broadcast source-addressed variables is standard Cyclic only for REALLY critical variables Decide before deployment Provide both traffic types

47 Real-Time communication stack
The real-time communication model uses two stacks, one for time-critical variables and one for messages time-critical process variables time-benign messages Management Interface 7 Application implicit implicit 6 Presentation Remote Procedure Call 5 Session Logical Link Control connection-oriented 4 Transport (connection-oriented) connectionless 3 Network (connectionless) connectionless 2" Logical Link Control medium access 2' Link (Medium Access) common media 1 Physical

48 Application View Of Communication
Periodic Tasks Event-driven Tasks R1 R2 R3 R4 E1 E2 E3 Variables Services Message Services Traffic Memory Queues Process Data Supervisory Message Data Data (Broadcast) (unicast) node bus controller bus

49 Cyclic or Event-driven Operation For Real-time ?
The operation mode of the communication exposes the main approach to conciliate real-time constrains and efficiency in a control systems. cyclic operation event-driven operation Data are transmitted at fixed intervals, Data are only transmitted when they whether they changed or not. change or upon explicit demand. Deterministic: delivery time is bound Non-deterministic: delivery time vary widely Worst Case is normal case Typical Case works most of the time All resources are pre-allocated Best use of resources (periodic, round-robin) (aperiodic, demand-driven, sporadic) C: controlled by a central master (redundant for availability), number of participants limited by maximum period, call/reply in one bus transfer (fetch principle), cheap connection, dumb devices, limited geographical range E: multi-master arbitration, large number of participants, costly connection, intelligent devices, open systems, bring principle object-oriented bus message-oriented bus Fieldbus Foundation, MVB, FIP, .. Profibus, CAN, LON, ARCnet

50 Fieldbus: Time distribution
3.1 Field bus types 3.2 Field bus operation Data distribution Cyclic Operation Event Driven Operation Real-time communication model Time distribution Networking 3.3 Standard field busses

51 Time: TAI and UTC TAI (Temps Atomique International) is the scientific time scale. It is continuously incrementing and will never be reset or discontinued. It is the base of all other scales. UTC (Universal time coordinated) is the legal time. It is the base for the clocks of all countries. It indicates approximately 12:00:00 at solar noon in Greenwich at the Spring equinox (it was formerly called Greenwich Mean Time). The Bureau International des Poids et Mesures (BIPM), Paris, is responsible for the definition of UTC. Rate: UTC and TAI proceed at exactly the same rate; both were identical back in 1961. Leap Seconds: Since 1961, the earth rotation slightly slowed down, days became longer. When the difference between UTC and solar noon exceed 0,9 s, which happens after some years, the BIMP adjusts UTC by letting all clocks insert a leap second, so the last minute of a day lasts 61 seconds (the reverse case is also possible, but very unlikely). Leap seconds cannot be anticipated, since irregularities of the earth’s rotation are unpredictable. In 2011, UTC lagged behind TAI by 34 seconds: when TAI was :00:00, UTC was :59: One cannot deduce TAI time from UTC time without a table of all elapsed leap seconds, and UTC cannot be predicted for a given TAI, since the introduction of a leap second is a decision of the BIPM. The system of leap seconds is still subject to discussion and could be revised in the future – it could be abolished in 2015. Time expressed in digital form is counted in number of seconds after a reference date, called an epoch. Unix counts seconds since its epoch of 1st January h0s with a signed 32 bits counter which will wrap around in 2038 (Unix Millenium Bug). Unix time is based on Coordinated Universal Time (UTC) and is subject to leap seconds. In IEEE 1588 (PTP) uses the same epoch as Unix, but PTP is based on International Atomic Time (TAI) and moves forward monotonically. GPS uses an epoch of January 6, 1980. Therefore, epoch and time reference are independent.

52 Time-stamping and synchronisation
In many applications, e.g. disturbance logging and sequence-of-events, the exact sampling time of a variable must be transmitted together with its value. => Devices equipped with clock recording creation time of value (not transmission time). To reconstruct events coming from several devices, clocks must be synchronized. considering transmission delays and failures. input input input processing t1 t2 t3 t4 bus t1 val1

53 Example: Phasor information
Phasor transmission over the European grid: a phase error of 0,01 radian is allowed, corresponding to +/- 26 µs in a 60 Hz grid or 31 µs in a 50 Hz grid.

54 Time distribution In master-slave busses, the master distribute the time as a bus frame. The slave can compensate for the path delays. Time is relative to the master In demanding systems, time is distributed over separate lines as relative time (e.g. PPS = one pulse per second) or absolute time (IRIG-B), with accuracy of 1 µs. In data networks, a reference clock (e.g. GPS or atomic clock) distributes the time. A protocol evaluates the path delays to compensate them. NTP (Network Time Protocol): about 1 ms is usually achieved. PTP (Precision Time Protocol, IEEE 1588), all network devices collaborate to estimate the delays, an accuracy below 1 µs can be achieved without need for separate cables (but hardware support for time stamping required). (Telecom networks typically do not distribute time, they only distribute frequency)

55 NTP (Network Time Protocol) principle
client network server time t1 time request t2 network delay time response t3 t4 time request t’1 t’2 time response t’3 network delay  t’4 distance Measures delay end-to-end over the network (one calculation) Problem: asymmetry in the network delays, long network delays

56 IEEE 1588 principle (PTP, Precision Time Protocol)
Pdelay-request Pdelay-response TC OC MC residence time calculation peer delay calculation MC = master clock TC = transparent clock OC = ordinary clock Grand Master Clock Two calculations: residence time and peer delay All nodes measure delay to peer TC correct for residence time (HW support)

57 peer delay calculation residence time calculation
IEEE 1588 – 1 step clocks bridge bridge time 1-step transparent clock 1-step transparent clock ordinary (slave) clock grand master clock t1 Pdelay_Req t2 Pdelay_Req t1 Pdelay_Req t2 peer delay calculation t1 t2 Pdelay_Resp link delay  t3 t4 Pdelay_Resp t4 t3 Pdelay_Resp (contains t3 – t2) t4 t3 Sync residence time t5 residence time calculation residence time Sync t6 t5 Sync (contains all  + ) distance Grandmaster sends the time spontaneously. Each device computes the path delay to its neighbour and its residence time and corrects the time message before forwarding it

58 References To probe further

59 Fieldbus: Networking 3.1 Field bus types 3.2 Field bus operation Data distribution Cyclic Operation Event Driven Operation Real-time communication model Time distribution Networking 3.3 Standard field busses

60 Networking field busses
Networking field busses is not done through bridges or routers, because normally, transition from one bus to another is associated with: - data reduction (processing, sum building, alarm building, multiplexing) - data marshalling (different position in the frames) - data transformation (different formats on different busses) Only system management messages could be threaded through from end to end, but due to lack of standardization, data conversion is not avoidable today.

61 Networking: Printing Example
MPS = Master Printing System LS = Leitstand (section supervision) PM = Print Master SS =Section Steuerung (section control) Production MPS Plant-bus (Ethernet) Operator bus (Ethernet) Console, Section Supervision LS LS LS PM LS LS LS PM LS LS LS PM LS LS LS PM Printing Towers Section Busses (AF100) B E C D Section Control SSB SSC SSD SSE Line bus (AF100) Reelstand-Gateways RPB RPC RPD RPE Reelstand bus (Arcnet) Reelstands multiplicity of field busses with different tasks, often associated with units. main task of controllers: gateway, routing, filtering, processing data. most of the processing power of the controllers is used to route data

62 Assessment What is the difference between a centralized and a decentralized industrial bus ? What is the principle of source-addressed broadcast ? What is the difference between a time-stamp and a freshness counter ? Why is an associative memory used for source-addressed broadcast ? What are the advantages / disadvantages of event-driven communication ? What are the advantages / disadvantages of cyclic communication ? How is time transmitted ? How are field busses networked ?

63 3 Industrial Communication Systems
3.3 Field bus: standards Buses de terreno estándar Bus de terrain standard Standard-Feldbusse

64 Field busses: Standard field busses
3.1 Field bus types 3.2 Field bus operation Centralized - Decentralized Cyclic and Event Driven Operation 3.3 Field bus standards International standard(s) HART ASI Interbus-S CAN Profibus LON Ethernet Automotive Busses

65 Different classes of field busses
One bus type cannot serve all applications and all device types efficiently... Data Networks Workstations, robots, PCs Higher cost Not bus powered Long messages ( , files) Not intrinsically safe Coax cable, fiber Max distance miles 10 100 1000 10,000 Sensor Bus Simple devices Low cost Bus powered Short messages (bits) Fixed configuration Not intrinsically safe Twisted pair Max distance 500m frame size (bytes) High Speed Fieldbus PLC, DCS, remote I/O, motors Medium cost Not bus powered Messages: values, status Not intrinsically safe Shielded twisted pair Max distance 800m Low Speed Fieldbus Process instruments, valves Medium cost Bus-powered (2 wire) Messages: values, status Intrinsically safe Twisted pair (reuse 4-20 mA) Max distance 1200m 10 100 1000 10,000 source: ABB poll time, milliseconds

66 Which field bus ? • A-bus • IEEE 1118 • Partnerbus • Arcnet • Instabus
(Bitbus) • Partnerbus • Arcnet • Instabus • Arinc 625 • Interbus-S • Profibus-FMS * • ASI • ISA SP50 • Profibus-PA • Batibus • IsiBus • Profibus-DP • Bitbus • IHS • PDV * • CAN • ISP * • SERCOS • ControlNet • J-1708 • SDS • DeviceNet • J-1850 • DIN V 43322 • LAC • Sigma-i • DIN 66348 • Sinec H1 (Meßbus) • LON • FAIS • MAP • Sinec L1 • EIB • Master FB • Spabus * • Ethernet • MB90 • Suconet • Factor • MIL 1553 • VAN • Fieldbus Foundation • MODBUS • WorldFIP • FIP • MVB • ZB10 * • Hart • P13/42 • ... • IEC 61158 • P14

67 Worldwide most popular field busses
*source: ISA, Jim Pinto (1999) Bus User* Application Sponsor CANs 25% Automotive, Process control CiA, OVDA, Honeywell Profibus (3 kinds) 26% Process control Siemens, ABB LON 6% Building systems Echelon, ABB Ethernet 50% Plant bus all Interbus-S 7% Manufacturing Phoenix Contact Fieldbus Foundation, HART 7% Chemical Industry Fisher-Rosemount, ABB ASI 9% Building Systems Siemens Modbus 22% obsolete point-to-point many ControlNet 14% plant bus Rockwell Sum > 100%, since many companies use more than one bus European market in 2002**: 199 Mio €, 16.6 % increase (Profibus: 1/3 market share) **source: Elektronik, Heft

68 Field device: example differential pressure transducer
4..20 mA current loop fluid The device transmits its value by means of a current loop

69 4-20 mA loop - the conventional, analog standard (recall)
The 4-20 mA is the most common analog transmission standard in industry sensor transducer reader reader voltage source 10V..24V 1 2 i(t) = f(v) flow RL1 R1 RL2 R2 RL3 R3 RL4 RL4 conductor resistance i(t) = 0, mA The transducer limits the current to a value between 4 mA and 20 mA, proportional to the measured value, while 0 mA signals an error (wire break) The voltage drop along the cable and the number of readers induces no error. Simple devices are powered directly by the residual current (4mA), allowing to transmit signal and power through a single pair of wires. 4-20mA is basically a point-to-point communication (one source)

70 3.3.2 HART Data over mA loops

71 HART - Principle HART (Highway Addressable Remote Transducer) was developed by Fisher-Rosemount to retrofit 4-to-20mA current loop transducers with digital data communication. HART modulates the 4-20mA current with a low-level frequency-shift-keyed (FSK) sine-wave signal, without affecting the average analogue signal. HART uses low frequencies (1200Hz and 2200 Hz) to deal with poor cabling, its rate is 1200 Bd - but sufficient. HART uses Bell 202 modem technology, ADSL technology was not available in 1989 (when HART was designed) Transmission of device characteristics is normally not real-time critical

72 Hart frame format (character-oriented, not bit-oriented):
HART - Protocol Hart communicates point-to-point, under the control of a master, e.g. a hand-held device Master Slave command Indication Request time-out response Response Confirmation Hart frame format (character-oriented, not bit-oriented): preamble start address command bytecount [status] data data checksum 5..20 (xFF) 1 1..5 1 1 [2] (slave response) 0..25 (recommended) 1

73 total: 44 standard commands, plus user-defined commands
HART - Commands Universal commands (mandatory): identification, primary measured variable and unit (floating point format) loop current value (%) = same info as current loop read current and up to four predefined process variables write short polling address sensor serial number instrument manufacturer, model, tag, serial number, descriptor, range limits, … Common practice (optional) time constants, range, EEPROM control, diagnostics,… total: 44 standard commands, plus user-defined commands Transducer-specific (vendor-defined) calibration data, trimming,…

74 HART - Importance Practically all 4..20mA devices come equipped with HART today About 40 Mio devices are sold per year. more info:

75 3.3.5 CAN Automotive bus

76 CAN (1) - Data Sheet Supporters Automotive industry, Intel/Bosch, Honeywell, Allen-Bradley Standard SAE (automotive), ISO11898 (only drivers), IEC x (?) Medium dominant-recessive (fibre, open collector), ISO 11898 Medium redundancy none Connector unspecified Distance 1 Mb/s (A); 100kb/s (B); 25kb/s (B) Repeaters unspecified (useless) Encoding NRZ, bit stuffing User bits in frame 64 Mastership multi-master, 12-bit bisection, bit-wise arbitration Mastership redundancy none (use device redundancy) Link layer control connectionless (command/reply/acknowledgement) Upper layers no transport, no session, implicit presentation Application Protocols CAL, SDS, DeviceNet (profiles) Chips comes free with processor (Intel: 82527, 8xC196CA; Philips: 82C200, 8xC592; Motorola: 68HC05X4, 68HC705X32; Siemens: SAB-C167

77 + - CAN (2) - Analysis ”Unix" of the fieldbus world.
strong market presence, Nr 1 in USA (> 12 Mio chips per year) limited product distance x rate (40 m x Mbit/s) sluggish real-time response (2.5 ms) supported by user organisations ODVA, Honeywell... non-deterministic medium access numerous low cost chips, come free with many embedded controllers several incompatible application layers (CiA, DeviceNet, SDS) application layer definition strongly protected by patents (Bosch) interoperability questionable (too many different implementations) application layer profiles bus analyzers and configuration tools available small data size and limited number of registers in the chips. no standard message services. Market: industrial automation, automobiles

78 "The Dawn of Fast Ethernet"
The universal bus To probe further: "Switched LANs", John J. Roese, McGrawHill, ISBN b "The Dawn of Fast Ethernet"

79 Ethernet - another philosophy
classical Ethernet + Fieldbus SCADA switch Ethernet PLC PLC PLC Fieldbus cheap field devices decentralized I/O cyclic operation simple devices Ethernet as Fieldbus SCADA switch Ethernet costlier field devices Soft-PLC as concentrators Event-driven operation Soft-PLC Soft-PLC Soft-PLC Soft-PLC This is a different wiring philosophy. The bus must follow the control system structure, not the other way around

80 The Ethernet consortia
Ethernet/IP (Internet Protocol), Rockwell Automation IAONA Europe (Industrial Automation Open Networking Alliance, (www.iaona-eu.com) ODVA (Open DeviceNet Vendors Association, CIP (Control and Information Protocol) DeviceNet, ControlNet ProfiNet Siemens (www.ad.siemens.de), PNO (www.profibus.com) « Industrial Ethernet » new cabling: 9-pin D-shell connectors «  direct connection to Internet (!?) » Hirschmann (www.hirschmann.de) M12 round IP67 connector Fieldbus Foundation (www.fieldbus.org): HSE FS 1.0 Schneider Electric, Rockwell, Yokogawa, Fisher Rosemount, ABB IDA (Interface for Distributed Automation, - Jetter, Kuka, AG.E, Phoenix Contact, RTI, Lenze, Schneider Electric, Sick .. there are currently more than 25 standardized industrial Ethernets

81 The Ethernet „standards“
IEC SC65C „standardized“ 22 different, uncompatible "Industrial Ethernets“, driven by „market demand“. 2 EtherNet/IP (Rockwell. OVDA) 3 Profibus, Profinet (Siemens, PNO) 4 P-NET (Denmark) 6 INTERBUS (Phoenix) 10 Vnet/IP (Yokogawa, Japan) 11 TCnet (Toshiba, Japan) 12 Ethercat (Beckhoff, Baumüller) 13 Powerlink (BR, AMK) 14 EPA (China) 15 Modbus-RTPS (Schneider, IDA) 16 SERCOS (Bosch-Rexroth / Indramat) In addition to Ethernets standardized in other committees: FF's HSE, (Emerson, E&H, FF) IEC61850 (Substations) ARINC (Airbus, Boeing,..) Compatibility: practically none Overlap: a lot

82 The "real-time Ethernet"
The non-determinism of “normal” Ethernet makes it usuitable for the real-time world. Several improvements have been made, but this is not anymore a standard solution. Method 1: Common clock synchronisation: return to cyclic. Master clock Method 2: IEEE 1588 (Agilent) PTP precision time protocol Method 3: Powerlink B&R, Kuka, Lenze, Technikum Winterthur Method 4: Siemens Profinet V3 synchronization in the switches

83 Ethernet and fieldbus roles
Traditionally, ethernet is used for the communication among the PLCs and for communication of the PLCs with the supervisory level and with the engineering tools Fieldbus is in charge of the connection with the decentralized I/O and for time-critical communication among the PLCs. local I/O CPU fieldbus Ethernet

84 Future of field busses Non-time critical busses are being displaced by LANs (Ethernet) and cheap peripheral busses (Firewire, USB) These "cheap" solutions are being adapted to the industrial environment and become a proprietary solution (e.g. Siemens "Industrial Ethernet") The cost objective of field busses (less than 50$ per connection) is out of reach for LANs. The cabling objective of field busses (more than 32 devices over 400 m) is out of reach for the cheap peripheral busses such as Firewire and USB. Fieldbusses tend to live very long (10-20 years), contrarily to office products. There is no real incentive from the control system manufacturers to reduce the fieldbus diversity, since the fieldbus binds customers. The project of a single, interoperable field bus defined by users (Fieldbus Foundation) failed, both in the standardisation and on the market.

85 Fieldbus Selection Criteria
Installed base, devices availability: processors, input/output Interoperability (how likely is it to work with a product from another manufacturer Topology and wiring technology (layout) Power distribution and galvanic separation (power over bus, potential differences) Connection costs per (input-output) point Response time Deterministic behavior Device and network configuration tools Bus monitor (baseline and application level) tools Integration in development environment

86 Which are the selection criteria for a field bus ?
Assessment Which are the selection criteria for a field bus ? Which is the medium access and the link layer operation of CAN ? Which is the medium access and the link layer operation of Industrial Ethernet? What makes a field bus suited for hard-real-time operation ? How does the market influence the choice of the bus ?

87 3 Field busses 3.4 Industrial Wireless

88 Motivation for Industrial Wireless
Reduced installation and reconfiguration costs Easy access to machines (diagnostic or reprogramming) Improved factory floor coverage Eliminates damage of cabling Globally accepted standards (mass production) Wireless technology is increasingly being used in several fields because of the tremendous advantages it is capable of offering. Even for industrial networks, its usage can offer numerous benefits such as reduced installation, reconfiguration and maintenance costs. avoiding adverse effects due to damage of cables and providing improved coverage of the factory floor Standards leading to mass production and reduced prices

89 Wireless Landscape There are many frequencies and protocols for a number of good reasons. There are many different application requirements for wireless. Some requirements are for high bandwidth – and long distance other are for low power short range…. These different needs dictate different technologies.

90 Wireless IEEE Numbers Wireless communications is hardly new. Here are a few examples.

91 Requirements for Industrial Wireless
An application is said to be real-time if the total correctness of an operation depends not only upon its logical correctness, but also upon the time in which it is performed. Based on this, Industrial applications can be classified into three different categories namely non real-time, soft real-time and hard real-time. In case of hard real-time applications such as control loops, tasks need to completed within the deadline, if not, it could lead to a critical failure of the complete system. A soft real-time application on the other hand will tolerate such lateness but may respond with decreased service quality (e.g., dropping frames while displaying a video). By contrast, a non-real time application is one for which there is no deadline, even if fast response or high performance is desired or even preferred.

92 Wireless for Non Real-Time Applications
Remote Control: Used for remote control of overhead cranes High security requirements Long code words to initiate remote control action Machine health monitoring: Accurate information about status of a process Local on demand access: PDA or laptop that connects to sensors or actuators Control room: access point / gateway

93 Wireless for Soft Real-Time Applications
Measurements: For physical process, timestamp values Ability to reconstruct course of events Requires clock synchronization; precision dictated by granularity of measurement E.g. geological or industrial sensors collecting data and transmitting them to base station or control room Media: Delay and loss rate constraints for user comfort E.g. voice and video transfer Control loops: Slow or non-critical operations Low sample rate Not affected by a few samples being lost Delay constraint based on comfort demands E.g. heat control and ventilation system

94 Wireless Hard Real-Time Applications
Late transmission cannot be tolerated E.g. control loops Assumes fault-free communication channel Wireless: Error probability cannot be neglected Sporadic and bursty errors

95 Challenges and Spectrum of Solutions
Wireless Challenges Attenuation Fading Multipath dispersion Interference High Bit Error rate Burst channel errors Existing Solutions Application Requirements Reliable delivery Meet deadlines Support message priority face inherent characteristics of the wireless medium that conflict with the application requirements Properties hamper the reliable delivery and stringent deadline requirements demanded by industrial applications. A number of schemes have been developed to that exploit the spatial and temporal diversity of the wireless channel to overcome the bursty error conditions. Antenna redundancy uses multiple spatially seperated antennas to overcome the bursty channel error conditions co-operative diversity scheme uses other nodes in the medium to provide efficient delivery of messages. Several error correction codes are also being used to improve the reliability of the wireless medium.

96 Reliability for wireless channel
Radio wave interferes with surrounding environment creating multiple waves at receiver antenna, they are delayed with respect to each other. Concurrent transmissions cause interference too. => Bursts of errors Forward Error Correction (FEC): Encoding redundancy to overcome error bursts Automated Repeat ReQuest (ARQ): Retransmit entire packets when receiver cannot decode the packet (acknowledgements)

97 Deadline Dependent Coding
Uses FEC and ARQ to improve Bit Error Rate: Re-transmissions before deadline Different coding rate depending on remaining time to deadline Tradeoff between throughput and how much redundancy is needed Additional processing such as majority voting Decoder keeps information for future use (efficiency)

98 Existing protocols- comparison
Feature 802.11 Bluetooth Zigbee / Interference from other devices -- Avoided using frequency hopping Dynamic channel selection possible Optimized for Multimedia, TCP/IP and high data rate applications Cable replacement technology for portable and fixed electronic devices. Low power low cost networking in residential and industrial environment. Energy Consumption High Low (Large packets over small networks) Least (Small packets over large networks) Voice support/Security Yes/Yes No/Yes Type of Network / Channel Access Mobile / CSMA/CA and polling Mobile & Static / Polling Mostly static with infrequently used devices / CSMA and slotted CSMA/CA Bit error rate Low Real Time deadlines ??? This slide shows the comparison between the different wireless standards contrasting different characteristics such as the applications type each is optimized for their but error rate, energy consumption and their mode of channel access. As you can see, none of these standards provide hard real-time guarantees as required by industrial applications. Hence there arises a need to develop protocols specifically targeted towards the message characteristics and requirements of industrial applications

99 10 km 3G 1 km 100 m 802.11b,g 802.11a Bluetooth 10 m ZigBee ZigBee UWB
Range 10 km 3G 1 km 100 m 802.11b,g 802.11a Bluetooth 10 m ZigBee ZigBee UWB 1 m UWB 0 GHz 1GHz 2 GHz 3 GHz 4 GHz 5 GHz 6 GHz

100 Legal Frequencies www.fcc.gov
In each country organizations similar to the FCC exist and establish which EM frequencies can be used for communication. Some bands require a license, others are for everyone’s use.

101 Industrial Example: WirelessHART
Not part of exam 2015 HART (Highway Addressable Remote Transducer) fieldbus protocol Supported by 200+ global companies Since 2007 Compatible WirelessHART extension

102 WirelessHART Networking Stack
Not part of exam 2015 PHY: 2,4 GHz Industrial, Scientific, and Medical Band (ISM-Band) Transmission power dBm 250 kbit/s data rate MAC: TDMA (10ms slots, static roles) Collision and interference avoidance: Channel hopping and black lists Network layer: Routing (graph/source routing) Redundant paths Sessions and broadcast encryption (AES) eine TDMA genutzt. Dazu wird die zur Verfügung stehende Zeit in Für die Steuerung des Zugriffverfahrens auf das Medium wird Kommunikationsbeziehung zugeordnet. Dazu werden zwei jeweils 10ms Slots eingeteilt und jeder Senke. Geräten feste Rollen zugewiesen. Eines als Quelle und eines als Quelle mit dem Senden einer Nachricht nach einer definiertes Zeit Empfänger Synchron auf das Medium zugreifen. Dazu beginnt die Um eine Kommunikation zu gewährleisten müssen Sender und nach dem Start des Slots. Daraus kann sich die Senke den übermittelt. Ein Sonderfall ist der Broadcast, bei diesem wird kein ACK Slotstart errechnen und darauf einstellen und antwortet mit ACK. Da WirelessHART nicht nur in dem ISM-Band sendet und andere Teilnehmer stören kann, sondern auch in diesem Band Empfängt Verfügung stehenden Kanäle gemacht. Wenn auf Kanälen zu werden. Um dieses zu minimieren wird ein Hopping über die zur kann die Kommunikation von anderen Teilnehmern gestört dauerhaften Störungen kommt oder diese nicht genutzt werden werden beim Hopping ausgelassen. sollen können diese auf Blacklisten eingetragen werden und 16 channels jeden Teilnehmer dass er nicht nur Quelle wie auch Senke sein kann Da WirelessHART als Mesh-Netzwerk betrieben wird bedeutet das für redundante Pfade vorgehalten um so das Routing gegenüber den Ausfall Zur Sicherung der Netzwerkinfrastruktur werden im Netz meistens sondern auch die jeweiligen Pakete weiter routen muss. eines oder mehrerer Konten zu sichern. Dieses kann eher mit Knoten in eine Sterntopologie dabei ist jeder Knoten einen Hop vom Gateway dienen und meist mehr beansprucht werden. Eine mögliche Alternative ist der Nähe des Gateways passieren da diese als Dienstzugangspunkt entfernt, was die Kommunikation sehr beschleunigt. Jedoch ist der große Nachteil, dass es nur zu einer geringen räumlichen Ausdehnung des Verfügung. Bei dem Graph-Routing für den Austausch von alarms, Als Routing-Verfahren stehen das Graph- und das Source-Routing zur Netzes kommt. request/reponse, publishing, etc. kennt jedes Gerät einen Ausschnitt des Information über das Routing hinzugefügt. Dies hat zur Folge das die Bei dem Source-Routing wird in dem zu übertragenden Paket die Netzwerkes, damit ist ein redundante und sichere Kommunikation möglich. Redundanz möglich ist. Diese Verfahren wird nur beim Routentest der Routing-Liste durch die Konten abgearbeitet wird und bei Fehlern keine Jedes Gerät hat mindestens zwei Sessions mit dem Netwerk Manager Die End-zu-End Kommunikation wird mit Hilfe von Sessions gesichert. Fehlersuche und dem Aufbau einer Ad-Hoc-Verbindung genutzt. eine für die paarweise Kommunikation und eine für den Broadcast. Nachbar welche es weiter gibt. Für die Wahrung der Redundanz wird das Dazu führt jedes Gerät Statistiken über die Kommunikation mit dem Die Netzwerkperformance wird kontinuierlich beobachtet und angepasst. Energieverbrauch zu senken wird versucht das Netzwerk möglichst .flach. Netzwerk regelmäßig durch den Netzwerk Manager aktualisiert. Um den Zur Steuerung des Datenflusses wurde dieser in 4 Kategorien eingeteilt. Die zu halten. Command-Pakete enthalten Payload zur Netzwerk-Diagnose, Konfiguration den Prozess-Daten und Netzwerk-Statistiken. Jedoch werden diese und Kontroll-Informationen. Die Process-Data-Pakete enthalten Payload zu ist. Von Alarm-Pakete die Alarme und Events enthalten wird nur eines Informationen verworfen wenn schon ¾ des Buffers eines Gerätes belegt gebuffert. Alle Pakete die in keiner der drei Kategorien passen werden als halber Buffer beim Feldgeräte zur Verfügung steht. Sind verschiedene Normale-Pakete bezeichnet und werden solange gebuffert wie noch ein Priorität der höchsten im Paket enthaltenen Priorität gegeben. Um die Befehle verschiedener Priorität in einem Paket so wird dem Paket die Als Erweiterung dieser Priorisierung wird vorgeschrieben das Keep-Alive, Schwellenwerte anpassen. Flusssteuerung fein granularer zu gestalten kann der Netzwerk Manager die einem ACK beantwortet werden. Für das Empfangen und Weiterleiten von Advertise und Disconnect Pakete empfangen werden müssen und mit Command-Paketen sollte immer Buffer zur Verfügung stehen.

103 WirelessHART Networking Stack
Not part of exam 2015 Transport layer: Segmentation, flatten network Quality of Service (QoS): (Command, Process-Data, Normal, Alarm) Application layer: Standard HART application layer Device Description Language Smart Data Publishing (lazy) Timestamping Events Command aggregation Boot-strapping: Gateway announcements (network ID and time sync) Device sends join request Authentication and configuration via network manager Die Netzwerkperformance wird kontinuierlich beobachtet und angepasst. Dazu führt jedes Gerät Statistiken über die Kommunikation mit dem Nachbar welche es weiter gibt. Für die Wahrung der Redundanz wird das Netzwerk regelmäßig durch den Netzwerk Manager aktualisiert. Um den Energieverbrauch zu senken wird versucht das Netzwerk möglichst .flach. zu halten. Zur Steuerung des Datenflusses wurde dieser in 4 Kategorien eingeteilt. Die Command-Pakete enthalten Payload zur Netzwerk-Diagnose, Konfiguration und Kontroll-Informationen. Die Process-Data-Pakete enthalten Payload zu den Prozess-Daten und Netzwerk-Statistiken. Jedoch werden diese Informationen verworfen wenn schon ¾ des Buffers eines Gerätes belegt ist. Von Alarm-Pakete die Alarme und Events enthalten wird nur eines gebuffert. Alle Pakete die in keiner der drei Kategorien passen werden als Normale-Pakete bezeichnet und werden solange gebuffert wie noch ein halber Buffer beim Feldgeräte zur Verfügung steht. Sind verschiedene Befehle verschiedener Priorität in einem Paket so wird dem Paket die Priorität der höchsten im Paket enthaltenen Priorität gegeben. Um die Flusssteuerung fein granularer zu gestalten kann der Netzwerk Manager die Schwellenwerte anpassen. Als Erweiterung dieser Priorisierung wird vorgeschrieben das Keep-Alive, Advertise und Disconnect Pakete empfangen werden müssen und mit einem ACK beantwortet werden. Für das Empfangen und Weiterleiten von Command-Paketen sollte immer Buffer zur Verfügung stehen. Zur Sicherung der Abwärtskompatibilität nutzt WirelessHART den Standard HART-Application Layer. Mit der Device Description Language können Transmitter, Steuerventile und analyse Instrumenten die entsprechenden Paramter übergeben und Funktionen aufgerufen werden. Zur Verbesserung Kommunikation in HART7 und zur Erweiterung zu WiredHART wurde das .Smart Data Publishing. eingeführt. Dabei werden Daten erst übertragen wenn sie gebraucht werden oder ein bestimmter Schwellenwert erreicht wurde. Da durch die Latenz im Netzwerk unterschiedliche Pakete unterschiedlich lange bei der Übertragung benötigen können, wurde ein Zeitstempel eingeführt, um zu verhindern das Latenz die Messergebnisse beeinflussen kann. Da Messungen nicht kontinuierlich laufen müssen sondern nur wenn sich bestimmte Umwelteinflüssen ändern soll es möglich sein Messungen zum Beispiel bei bestimmten Vibrationen zu starten. Für die Beschleunigung der Lese-Kommandos ist es möglich diese zusammen zu fassen um diese so schneller hoch zu laden. Um ein Netzwerk aufzubauen gibt der Netzwerk Manager dem Gateway ein Netzwerk bekannt zu machen, zu annoncieren. Dazu sendet der Gateway regelmäßig Informationen über die Netzwerk-ID und die Zeitsynchronisation aus. Im späteren Verlauf wird dieses auch durch die Teilnehmer gemacht um Feldgeräte zu erreichen die außerhalb des Empfangsradius des Gateways liegen zu erreichen. Diese Phase wird als Annoncierungs-Zyklus bezeichnet. Das Gerät hört die Netzwerk-ID und sendet über den Gateway eine Anfrage zur Teilnahme an den Netzwerk Manager. Zur Authentifizierung des Feldgerätes nutzt dieser den Security Manager. Wenn das Feldgerät von diesen authentifiziert wird sendet der Netzwerk Manager ein Aktivierungsdatenpaket und definiert die Route zum Feldgerät. Abschließend werden die Konfigurationsparameter, z.B. wie oft Prozessdaten gesendet werden, ausgelesen und im Superframe eingetrage

104 Design Industrial Wireless Network
Existing wireless in plant; frequencies used? Can the new system co-exist with existing? How close are you to potential interferences? What are uptime and availability requirements? Can system handle multiple hardware failures without performance degradation? What about energy source for wireless devices? Require deterministic power consumption to ensure predictable maintenance. Power management fitting alerting requirements and battery replacement goals Availability is uptime assurance - maximizing uptime so you can access the information when you need it, especially in abnormal situations. Reliability is message delivery assurance - are my messages consistently getting through when needed. that they’re accurate when they arrive. Analogy of airline – availability is like having a private jet – it’s always there. Reliability is that it always get you there – but not guaranteeing a time window.

105 Why is a different wireless system deployed in a factory than at home?
Assessment Why is a different wireless system deployed in a factory than at home? What are the challenges of the wireless medium and how are they tackled? How can UWB offer both a costly and high bandwidth and a cheaper and high bandwidth services? Which methods are used to cope with the crowded ISM band? Why do we need bootstrapping in Wireless HART?

106 References Wireless Communication in Industrial Networks, Kavitha Balasubramanian, Cpre 458/558: Real-Time Systems, WirelessHART, Christian Hildebrand, _Seminar_EDS_Hildebrand.pdf WirelessHARTTM Expanding the Possibilities, Wally Pratt HART Communication Foundation, Industrial Wireless Systems, Peter Fuhr, ISA,

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