10.- osi model i inst… · osi model: structure the osi reference model is composed of seven...

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Motion Control for Electric Drives1

Engineering Electronic Department

Technical University of Catalonia, UPCCampus Terrassa, SPAIN

Lecturer: Dr. Luis Romeral


Outline Block 3:

Digital Communications and OSI Model

Subject 1.- OSI Model: definitions - General Concepts- Seven layers OSI - Model- Protocols and Service data Units- Repeaters, Bridges, Routers, and Gateways

Subject 2.- Physical Layer- Functions of the Physical Layer- Interfaces RS 232 / RS 485 / Ethernet- Physical media: cables and optical fibers- Modulation - Coding and Synchronization

Subject 3.- Aircraft Buses- TCP/IP- ARINC- CSDB / FDDI

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Engineering Electronic Department

Technical University of Catalonia, UPCCampus Terrassa, SPAIN

Lecturer: Dr. Luis Romeral


OSI Model: origin

Over the past couple of decades many of the networks that were built used different hardware and software implementations, as a result they were incompatible and it became difficult for networks using different specifications to communicate with each other.

To address the problem of networks being incompatible and unable to communicate with each other, the ISO (International StandardizationOrganization) recognized there was a need to create a NETWORK MODEL that would help vendors to create interoperable network implementations:

In 1984 in order to aid network interconnection without necessarily requiring complete redesign, the Open Systems Interconnection (OSI) reference model was approved as an international standard for communications architecture.

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OSI Model: concept

The Open System Interconnection (OSI) model is a standard way tostructure communication software that is applicable to any network.

• was developed to structure telecommunication protocols in the ‘70(Pouzin & Zimmermann), and standardized by CCITT and ISO as ISO / IEC 7498 in 1984

• all communication protocols (TCP/IP, Appletalk or DNA) can be mapped to the OSI model.

• mapping of OSI to industrial communication requires some additions

• OSI should be considered as a model, not as an implementation guide


OSI Model: concept

• The Open Systems Interconnection (OSI) reference model is a descriptive network scheme. It ensures greater compatibility andinteroperability between various types of network technologies.

• The OSI model describes how information or data makes its way from application programmes (such as spreadsheets) through a network medium (such as wire) to another application programme located on another network.

• The OSI reference model divides the problem of moving data between computers over a network medium into SEVEN smallerand more manageable problems .

• This separation into smaller more manageable functions is known as layering.

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OSI Model: structure

The OSI Reference Model is composed of seven layers, each specifying particular network functions.

The process of breaking up the functions or tasks of networking into layers reduces complexity.

Each layer provides a service to the layer above it in the protocol specification.

Each layer communicates with the same layer’s software or hardware on other computers.


OSI-Model (ISO/IEC standard 7498)

Physical

Link

Network

Transport

Session

Presentation6

5

4

3

2

1

Application7

"Transport"protocols

"Application" protocols

Definition and conversion of the dataformats (e.g. ASN 1)

All services directly called by the end user(Mail, File Transfer,...) e.g. Telnet, SMTP

Management of connections(e.g. ISO 8326)

End-to-end flow control and error recovery(e.g. TP4, TCP)

Routing, possibly segmenting(e.g. IP, X25)

Error detection and recovery, Flow control and medium access (e.g. HDLC)

Coding, Modulation, Electrical andmechanical coupling (e.g. RS485)

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Protocol Data Units and Service Data Units

Protocol Data Unit

(PDU)

N - Layer

N+1- Layer

N-1 Layer

Protocol Data Unit

(PDU)

Service-Data Unit

(SDU)

Service-Data Unit

(SDU)

Layer N provides services to Layer N+1; Layer N relies on services of Layer n-1

(n)-layer entity(n)-layer entity

(n+1)-layer entity(n+1)-layer entity

(n-1)-layer entity(n-1)-layer entity


OSI Model with two nodes

PhysicalLinkNetworkTransport

SessionPresentationApplication

PhysicalLinkNetworkTransport

SessionPresentationApplication

Physical Medium

node 1 node 2

7

6

5

4

3

2

1

7

6

5

4

3

2

1

6


LAYER 7: APPLICATION

• The application layer is the OSI layer that is closest to the user. It Implements the functionality of the application

• It provides network services to the user’s applications.

• It differs from the other layers in that it does not provide services to any other OSI layer, but rather, only to applications outside the OSI model.

• Examples of such applications are spreadsheet programs, word processing programs, and bank terminal programs.

• The application layer establishes the availability of intended communication partners, synchronizes and establishes agreement on procedures for error recovery and control of data integrity.


LAYER 6: PRESENTATION

• The presentation layer ensures that the information that the application layer of one system sends out is readable by the application layer of another system.

• Provides a common format of data exchange between different types of peers.

• If necessary, the presentation layer translates between multiple data formats by using a common format.

• Provides encryption and compression of data.

• Examples :- JPEG, MPEG, ASCII, EBCDIC, HTML.

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LAYER 5: SESSION

• The session layer defines how to start, control and endconversations (called sessions) between applications.

• This includes the control and management of multiple bi-directional messages using dialogue control.

• It also synchronizes dialogue between two hosts' presentation layers and manages their data exchange.

• Creates and manages a name space used to link different transports considered part of a single application.

• The session layer offers provisions for efficient data transfer.

• Examples :- SQL, ASP(AppleTalk Session Protocol).


LAYER 4: TRANSPORT

• Establishes a process-to-process channel and regulates information flow to ensure end-to-end connectivity between host applications reliably and accurately.

• The transport layer segments data from the sending host's system and reassembles the data into a data stream on the receiving host's system.

• The boundary between the transport layer and the session layer can be thought of as the boundary between application protocols and data-flow protocols. Whereas the application, presentation, and session layers are concerned with application issues, the lower four layers are concerned with data transport issues.

• Layer 4 protocols include TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

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LAYER 3: NETWORK

• Performs routing among nodes within a packetswitchednetwork.

• Defines end-to-end delivery of packets.

• Defines logical addressing so that any endpoint can be identified.

• Defines how routing works and how routes are learned so that the packets can be delivered.

• The network layer also defines how to fragment a packet into smaller packets to accommodate different media.

• Routers operate at Layer 3.

• Examples :- IP, IPX, AppleTalk.


LAYER 2: DATA LINK

• The data link layer provides access to the networking media and physical transmission across the media.

• The data link layer provides reliable transit of data across a physical link by using the Media Access Control (MAC) addresses.

• The data link layer uses the MAC address to define a hardware or data link address in order for multiple stations to share the same medium and still uniquely identify each other.

• Aggregates a stream of bits into a larger data unit called a frame, which is concerned with network topology, network access, error notification, ordered delivery of frames, and flow control.

• Examples :- Ethernet, Frame Relay, FDDI.

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LAYER 1: PHYSICAL

• The physical layer deals with the physical characteristics of the transmission medium.

• It handles the transmission of raw bits.

• It defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.

• Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other similar attributes are defined by physical layer specifications.

• Examples :- EIA/TIA-232, RJ45, NRZ.


Repeater Concept

repeaterEthernet

server

Ethernet

server

To connect a workstation of department A to the printer of department B, a cable becomes too long and the messages are corrupted.

workstationsdepartment A

department B

Physically, there is only one bus carrying both department’s traffic, only one node may transmit at a time.

printer

The repeater restores signal levels and synchronization.It introduces a signal delay of about 1..4 bits

500m

500m

500m

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OSI model with three nodes (bridge)

7

6

5

4

3

2

1

2

1

physical medium (0)

2

1

7

6

5

4

3

2

1Physical

Link

Network

Transport

Session

Presentation

Application

Node 1 bridge Node 2

The subnet on both sides of a bridge have:• the same frame format (except header),• 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

physical medium (0)

e.g. Ethernet 100 MBit/s e.g. ATM


Bridge example

RepeaterEthernet

server

Ethernet

server

BridgeEthernet 1

server

Ethernet 2

In this example, most traffic is directed from the workstations to the department server, there is little cross-department traffic

workstationsdepartment A

department B

There is only one Ethernet which carries both department’s traffic

department A

There are now two Ethernets and only the cross-department traffic burdens both busses

printer

serverdepartment B

printer

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OSI Model with three nodes (router)

physical medium (0)

Frames in transit are handled in the network layer .

The router routes the frames on the base of their network address.

The subnets may have different link layer protocols

Node 1 Router Node 2

PhysicalLink

Network

TransportSessionPresentation

Application

3

2

1

2

1

7

6

5

4

3

2

1

7

6

5

4

3

2

1


Repeater, Bridge, Router, Gateway: Topography

same speedsame medium accesssame framesBridge

Router

backbone (e.g. FDDI)

segment

Repeater

subnet (LAN, bus, extended link)

end-to-endtransport protocol

gateway

application-dependent

connects different speed,different medium accessby store-and-forward

same frames and addressesinitially transparent in both ways.

can limit traffic by filtering

devices (nodes, stations) have different link addresses

devices (nodes, stations) have different physical addresses

different subnetworks,same address spacesame transport protocol,segmentation/reassemblyrouters are initially opaque

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Repeaters, Bridges, Routers and Gateways: OSI model

NetTrpSesPreApl

TrpSesPreApl

MDS

LLCNetTrpSesPreApl

MAC

10 Mbit/s coax

MISMDS

Layer 1 MDS

repeateror hub

10 Mbit/s fibre

MDSMIS

MDSMIS

Layer 2

100 Mbit/s Ethernet

bridge( "switch")

(store-and-forward)

MDSMIS

LLCMAC

Layer 3

MDSMIS

LLCMAC

ATM 155 Mbit/s

MDSMIS

LLCMAC

NetTrpSesPre

Apl

MAC MAC

router

MDS

LLCIP

TCP

RPC

gateway

intelligent linking devices can do all three functions

(if the data rate is the same)

Fibre


To which level does a frame element belong

destination source final originpreamble

physical link

bridge

LLC NC

network

router

TRP SES PRE APL

application (gateway)

repeater, hub

CRC

A frame is structured according to the ISO model

ED

link

LLC

Net

wor

k C

ontro

l

trans

port

sess

ion

pres

enta

tion

appl

icat

ion

phy

Frame: a frame is data that is transmitted between network points as a unit complete with addressing and necessary protocol control information.

A frame is usually transmitted serial bit by bit and contains a header field and a trailer field that "frame" the data. (Some control frames contain no data).

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Data Encapsulation

Frame

Signal

Error detection

Flag Flag

Link - address

Link control(Acknowledge, Token,etc.)

Network address

Transport header

size

User information

CRC

LinkAdr

LinkCrt

NetAdr

INFO

TrpCrt

Each layer introduces its own header and overhead

Data is Encapsulated with the necessary protocol information as it movesdown the layers before network transit


Example: OSI-Stack frame structure

>48

ISO 8473 connectionless network control

5

ISO 8073 class 4 transport control

MA. frame control

MA. destinationaddress(6 octets)

MA. sourceaddress(6 octets)

L_destination SAP

L_source SAP

L_PDU

L_PDU = UI, XID, TEST

LI

TPDU

Protocol Identifier

Header Length

Version/Protocol ID (01)

Lifetime

DT/ER TypeSP MS ER

Checksum

PDU Segment Length

Destination Address(18 octets)

Source Address(18 octets)

ADDRESS PART

Segmentation(0 or 6 octets)

Options(priority = 3 octets)

(CDT)

N(S)ET

MAC_header LNK_hdr NET_header TRP_header

DestinationReference

FIXEDPART

13 3DATA

AFI = 49

IDI, Area ID(7 octets)

PSI

Physical Address(6 octets)

LSAP = FE

NSAP = 00

IDP(initial

domainpart)

DSP(domainspecific

part)

DATA (DT) TPDU (normal format)

LSAP = DSAPFE = network layer18 = Mini-MAP Object Dictionary Client19 = Network Management00 = own link layer

(81)

IEEE 802.4token bus

ISO 8802logical link control

address length

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OSI protocols in industry

ISO-OSI standards should be used since they reduce specification and conformance testing work and commercial components exist

the OSI model is a general telecommunication framework -implementations considers feasibility and economics.

industrial busses use for real-time data a fast response access andfor messages a simplified OSI communication stack

the OSI model does not consider transmission of real-time data

the overhead of the ISO-OSI protocols (8073/8074) is not bearablewith low data rates under real-time conditions.

communication is greatly simplified by adhering to conventionsnegotiating parameters at run-time is a waste in closed applications.

the OSI-conformant software is too complex:simple devices like door control or air-condition have limited power.

•

•

•

•

•

•

Theory:

Reality:

Therefore:

the devices must be plug compatible: there are practically no options.•

•


TCP / IP structure

OSI stack has not been able to establish itself against TCP/IP

TCP UDP

IP routing ICMP

FTP SMTP HTTPFiles SNMP Applications

Transport

Network

Ethernet ATM radiomodem Link & Physical

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

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Conclusions

The OSI model is the reference for all industrial communicationEven when some layers are skipped, the concepts are generally implementedReal-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

Many embedded controllers come with an integrated Ethernet controller, an the corresponding real-time operating system kernel offers TCP/IP services

TCP/IP/UDP is quickly displacing proprietary protocols.

Like OSI, TCP protocols have delays counted in tens or hundred milliseconds, often unpredictable especially in case of disturbances.

Next generation TCP/IP (V6) is very much like the OSI standards.


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) How is the TCP/IP structure?

5) How successful are implementations of the OSI standard suite ?

Assessment

10.- osi model i inst… · osi model: structure the osi reference model is composed of seven...

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