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Chap. 2 Protocol

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Communication model

• Simplified communication model

source node

gather data from sensor or switch using ADC (analog-to-digital

converter) or external interrupt

generate digital binary data

generate frame by adding source/destination address and error

detection code

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transmitter

covert into electro-magnetic signal to transmit frame via transmission

medium

transmission system

transmission medium

point-to-point link vs. network link or multi-point link

receiver

covert into binary data

destination node

check error

transmit acknowledgement

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Communication function

• Segmentation and reassembly

• Message formatting

• Signal generation and encoding

• Transmission medium

• Interface

• Topology

• Addressing

• Flow control

• Error detection and error control

• Medium access control

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Protocol and standard

• Need for protocol

Rules for communications between communication nodes

Communication nodes

computer

controller, sensor, actuator

robot, NC machine

Must speak the same language for successful communications

successful transmission unsuccessful transmission

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• Standard

Required to allow for interoperability between communication nodes

de facto standard vs. de jure standard

de facto standard or voluntary standard ( 임의 표준 ) : TCP/IP

de jure standard or regulatory standard ( 규제적 표준 ) : IEEE 802.3

unique standard vs. multiple standard

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International Standards Organization

ISO (International Organization for Standards)

ISO8802

IEEE (Institute of Electrical and Electronics Engineers)

IEEE 802.3, IEEE 802.15

EIA (Electronics Industries Association)

EIA-232, EIA-485

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Layer architecture

• Why?

Communications is a complex task, too

complex for single unit

Better if task broken into subtasks

implemented separately in layers in stack

Structured design breaks down

problem into smaller units; modules

each layer provides functions needed

to perform communication for layers

above

using functions provided by layers

below

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• Advantage of layer architecture

If each layer’s functions are well defined

each layer’s standardization is performed separately and rapidly

If boundary between layers is well defined

one layer does not affect the other layers

development of new protocol for specific layer is possible without

change of the other layers

If service between services

vendor can develop software for specific layer without consideration

of the other layers

interface between upper and lower layers is not problem

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• Logical connection between layers

connection

physical connected between lowest layer (layer 1)

logical connected between upper layer (layer N)

PDU (protocol data unit)

segmentation

overhead : header, trailer

frame

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• Consideration of the number of layer

advantage of layer architecture

many !

disadvantage of layer architecture

overhead per each layer

waste of transmission capacity

the number of layer

to simplify protocol

to minimize overhead of frame

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OSI RM (Open System Interconnection, Reference Model

• Developed by the International Organization

for Standardization (ISO)

• Seven layer Architecture

• Each layer performs a subset of the required

communication functions

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• Physical

Transmission of electromagnetic signal via transmission medium

transmission medium, connector

encoding

interface : mechanical, electrical, functional, procedural specification

• Data Link

Activating, maintaining, and deactivating a reliable data transmission

between point-to-point link

message formatting, addressing, flow control, error control, medium

access control

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• Network

If two node are connected to different network, network & transport

layer are necessary

Transmission of frame from source to destination

connection control, routing

• Transport

Exchange of data for error free

transmission between end systems

flow control, error control between

end systems

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• Session

Control of dialogues between applications

dialog control, synchronization

• Presentation

Information such as encryption, compression

• Application

Means for applications to access network services

Service

read service, write service, notification service

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• 3 layer architecture for embedded network system