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Industrial Automation Automation Industrielle Industrielle Automation

Application. 7. Presentation. 6. Industrial Automation Automation Industrielle Industrielle Automation. Session. 5. Transport. 4. Network. 3. Link. 2. 3 Industrial Communication Systems Open System Interconnection (OSI) model 3.3.1 Modèle OSI d’interconnexion OSI-Modell.

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Industrial Automation Automation Industrielle Industrielle Automation

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  1. Application 7 Presentation 6 Industrial AutomationAutomation IndustrielleIndustrielle Automation Session 5 Transport 4 Network 3 Link 2 3 Industrial Communication Systems Open System Interconnection (OSI) model 3.3.1 Modèle OSI d’interconnexion OSI-Modell Physical 1 Prof. Dr. H. Kirrmann ABB Research Center, Baden, Switzerland

  2. The OSI model The Open System Interconnection (OSI) model is a standard way to structure communication software that is applicable to any network. • was developed to structure telecommunication protocols in the ‘70(Pouzin & Zimmermann) • standardized by CCITT and ISO as ISO / IEC 7498 • all communication protocols (TCP/IP, Appletalk or DNA) can be mapped to the OSI model. • is a model, not a standard protocol, but a suite of protocols with the same name • has been standardized by UIT / ISO / IEC for open systems data interconnection • (but with little success) • mapping of OSI to industrial communication requires some additions

  3. OSI-Model (ISO/IEC standard 7498) All services directly called by the end user(Mail, File Transfer,...) e.g. Telnet, SMTP Application 7 "Application" protocols Definition and conversion of the data formats (e.g. ASN 1) Presentation 6 Management of connections(e.g. ISO 8326) Session 5 End-to-end flow control and error recovery (e.g. TP4, TCP) Transport 4 Routing, possibly segmenting (e.g. IP, X25) Network 3 "Transport" protocols Error detection, Flow control and error recovery,medium access (e.g. HDLC) Link 2 Coding, Modulation, Electrical andmechanical coupling (e.g. RS485) Physical 1

  4. Application Presentation Session Transport Network Link Physical OSI Model with two nodes node 1 node 2 7 7 6 6 5 5 4 4 3 3 2 2 1 1 Physical Medium

  5. Repeater 500m To connect a workstation of department A to the printer of department B, the cable becomes too long and the messages are corrupted. department A server workstations The repeater restores signal levels and synchronization. It introduces a signal delay of about 1..4 bits Ethernet repeater printer 500m server department B Physically, there is only one bus carrying both department’s traffic, only one node may transmit at a time. Ethernet 500m

  6. Node 1 bridge Node 2 Application Presentation Session Transport Network Link Physical OSI model with three nodes (bridge) 7 7 6 6 5 5 4 4 3 3 2 2 2 2 1 1 1 1 physical medium (0) physical medium (0) e.g. Ethernet 100 MBit/s e.g. ATM The subnet on both sides of a bridge have: • the same frame format (except header), • the same address space (different addresses on both sides of the bridge) • the same link layer protocol (if link layer is connection-oriented) Bridges filter the frames on the base of their link addresses

  7. Bridge example In this example, most traffic is directed from the workstations to the department server, there is little cross-department traffic department A department A server server workstations Ethernet 1 Ethernet repeater Bridge printer printer server server department B department B Ethernet Ethernet 2 There is only one Ethernet which carries both department’s traffic There are now two Ethernets and only the cross-department traffic burdens both busses

  8. port port Networking with bridges LAN port port port port port Spanning-tree-Algorithmen port avoid loops and ensures LAN redundancy port port LAN port LAN LAN

  9. Switch nodes queues crossbar- switch (or bus) full-duplex a switch is an extension of a hub that allows store-and-forward.

  10. Node 1 Router Node 2 Application 7 7 Presentation 6 6 Session 5 5 Transport 4 4 Network 3 3 3 Link 2 2 2 2 Physical 1 1 1 1 OSI Model with three nodes (router) physical medium (0) The router routes the frames on the base of their network address. The subnets may have different link layer protocols Frames in transit are handled in the network layer .

  11. Repeater, Bridge, Router, Gateway: Topography application- dependent backbone (e.g. FDDI) different subnetworks,same address spacesame transport protocol,segmentation/reassembly routers are initially opaque gateway end-to-end transport protocol Router connects different speed,different medium access by store-and-forward same speed same medium access Bridge same frames same frames and addresses initially transparent in both ways. can limit traffic by filtering Repeater segment subnet (LAN, bus, extended link) devices (nodes, stations) have different physical addresses devices (nodes, stations) have different link addresses

  12. Repeaters, Bridges, Routers and Gateways: OSI model intelligent linking devices can do all three functions (if the data rate is the same) gateway Apl Apl Apl Apl Pre Pre Pre Pre RPC Ses Ses Ses Ses bridge Trp Trp Trp Trp TCP ( "switch") Layer 3 router (store-and-forward) Net Net Net IP repeater Layer 2 LLC LLC LLC LLC LLC or hub MAC MAC MAC MAC MAC MAC Layer 1 MIS MIS MIS MIS MIS MIS MDS MDS MDS MDS MDS MDS MDS MDS MDS 10 Mbit/s coax 10 Mbit/s fibre 100 Mbit/s Ethernet Fibre ATM 155 Mbit/s

  13. To which level does a frame element belong ? presentation application transport session network link physical phy link LLC preamble destination source LLC final origin NC TRP SES PRE APL CRC ED repeater, hub bridge router application (gateway) Network Control A frame is structured according to the ISO model

  14. INFO TrpCrt NetAdr LinkCrt LinkAdr CRC Encapsulation User information Transport header Network address Link control (Acknowledge, Token,etc.) Link-address size Error detection Flag Flag Frame Signal Each layer introduces its own header and overhead

  15. Example: OSI-Stack frame structure IEEE 802.4 ISO 8802 ISO 8473 ISO 8073 token bus logical link control connectionless network control class 4 transport control MAC_header NET_header TRP_header DATA LNK_hdr 13 3 >48 5 MA. frame control L_destination SAP Protocol Identifier (81) LI L_source SAP Header Length TPDU (CDT) L_PDU Version/Protocol ID (01) Destination Reference Lifetime LSAP = DSAP MA. destination FIXED FE = network layer address PART DT/ER Type N(S) SP MS ER ET 18 = Mini-MAP Object (6 octets) Dictionary Client DATA (DT) TPDU 19 = Network Management PDU Segment Length (normal format) 00 = own link layer L_PDU = UI, XID, TEST address length IDP Checksum (initial AFI = 49 domain Destination Address part) IDI, Area ID (18 octets) (7 octets) MA. source Source Address address (18 octets) (6 octets) PSI Segmentation ADDRESS DSP (0 or 6 octets) PART Physical Address (domain (6 octets) specific Options part) (priority = 3 octets) LSAP = FE NSAP = 00

  16. Protocol Data Units and Service Data Units (n+1)-layer entity (n+1)-layer entity N+1- Layer Service- Data Unit Protocol (SDU) Data Unit (PDU) N - Layer (n)-layer entity (n)-layer entity Protocol Data Unit Service- Data Unit (PDU) (SDU) N-1 Layer (n-1)-layer entity (n-1)-layer entity Layer N provides services to Layer N+1; Layer N relies on services of Layer n-1

  17. Service Access Points user of user of service N service N Service Access Points (SAP) functions in layer N Service Access Points (SAP) provider of service (N-1) provider of service (N) Service Access Points represent the interface to a service (name, address, pointer,...)

  18. Address and SAPs in a device ASAP Application (z.B. File transfer, Email,....) z.B. TCP/IP z.B. ISO 8073 TSAP Transport-SAP Transport ISO 8473 Network-SAP NSAP Network (not Network address) ISO-stack Logical Address or link address LSAP Link Physical PhSAP Physical Address

  19. Procedure call conventions in ISO Service User Service User Service Provider (Network Transmission) request indication confirm (local) confirm (network) response confirm (user) time

  20. OSI implementation OSI should be considered as a model, not as an implementation guide The idea of independent layers is a useful as a way of thinking, not the best implementation. Even if many claim to have "OSI"-conformant implementation, it cannot be proven. IEC published about 300 standards which form the "OSI" stack, e.g.: ISO/IEC 8473-2:1996 Information technology -- Protocol for providing the connectionless-mode network service -- ISO/IEC 8073:1997 Information technology -- Open Systems Interconnection -- Protocol for providing the connection-mode transport service ISO/IEC 8327-1:1996 Information technology -- Open Systems Interconnection -- Connection-oriented Session protocol: Protocol specification ISO/IEC 8649:1996 Information technology -- Open Systems Interconnection -- Service definition for the Association Control Service Element ISO 8571-2:1988 Information processing systems -- Open Systems Interconnection -- File Transfer, Access and Management Former implementations, which implemented each layer by an independent process, caused the general belief that OSI is slow and bulky. OSI stack has not been able to establish itself against TCP/IP

  21. OSI protocols in industry Theory: ISO-OSI standards should be used since they reduce specification and conformance testing work and commercial components exist Reality: • the OSI model is a general telecommunication framework - implementations considers feasibility and economics. • the overhead of the ISO-OSI protocols (8073/8074) is not bearable with low data rates under real-time conditions. • the OSI-conformant software is too complex: simple devices like door control or air-condition have limited power. • the OSI model does not consider transmission of real-time data Therefore: • industrial busses use for real-time data a fast response access and for messages a simplified OSI communication stack • the devices must be plug compatible: there are practically no options. • Communication is greatly simplified by adhering to conventions negotiating parameters at run-time is a waste in closed applications.

  22. TCP / IP structure Applications FTP SMTP HTTP SNMP Files Transport TCP UDP Network IP routing ICMP Link & Physical Ethernet ATM modem radio The TCP/IP stack is lighter than the OSI stack, but has about the same complexity TCP/IP was implemented and used before being standardized. Internet gave TCP/IP a decisive push

  23. Conclusions The OSI model is the reference for all industrial communication Even when some layers are skipped, the concepts are generally implemented Real-Time extensions to OSI are under consideration TCP/IP however installs itself as a competitor to the OSI suite, although some efforts are made to integrate it into the OSI model TCP/IP/UDP is becoming the backbone for all non-time critical industrial communication TCP/IP/UDP is quickly displacing proprietary protocols. Next generation TCP/IP (V6) is very much like the OSI standards. Many embedded controllers come with an integrated Ethernet controller, an the corresponding real-time operating system kernel offers TCP/IP services Like OSI, TCP protocols have delays counted in tens or hundred milliseconds, often unpredictable especially in case of disturbances. For further reading: Motorola Digital Data Communication Guide

  24. Assessment 1) Name the layers of the OSI model and describe their function 2) What is the difference between a repeater, a bridge and a router ? 3) What is encapsulation ? 4) By which device is an Appletalk connected to TCP/IP ? 5) How successful are implementations of the OSI standard suite ?

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