Chapter 1

Data Communication Networks

and Open System Standards

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Background

• 컴퓨터– 인간생활의 모든 면에서 사용

• 예 ) 가정 , 사무실 , 여행사 , 학교 , …

– Stand-Alone mode 로만 사용되는 것이 아니고 타 컴퓨터와의 데이터 교환과 상호 공동 작업 필요

컴퓨터 통신망

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Background (cont’d)

• Computer 간 데이터 통신 시 고려사항– 현재 이용 가능한 다른 data 및 computer 망 이해– 장비에 interface 하기 위한 각 computer 내의 H/W 와

S/W 이해– 특수한 분산 응용 기능을 위해 다른 word size 와 문자

set 을 가진 computer 에서 가동되는 응용 program 과 어떻게 협조할 것인가

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Data communication networks

• PSTN– public switched telephone network

– requires a device as a modem

• LAN– local area network

– to enable all the computers to communicate with one another at different times

– all the computers are distributed around a single office of building

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Data communication networks

• WAN– wide area network

– when the computers are located in different sites

– one approach is simply to lease transmission lines from the public carriers

• PSDN– public switched data network

– be interconnected internationally and have been designed specifically for the transmission of data rather than voice

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Data communication networks

• ISDN– integrated services digital networks

– many public carriers are now converting their existing PSTNs to enable data to be transmitted without modems

– in an all-digital mode

• Internet– internetworking

– embraces multiple networks such as LAN-WAN-LAN

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Data communication networks

• Broadband multiservice networks– supports desktop video telephony, videoconfere

ncing, and mode general multimedia services

• ATM– asynchronous transfer mode

• MAN– metropolitan area network

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Standards

• Closed system– computers from other manufactures cannot

exchange information unless they adhere to the standards of a particular manufacturer

– for connecting devices to networks• internationally agreed standards

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Standards (cont’d)

• V-series recommendations– be concerned with the connection of equipment

(DTE) to a modem connected to the PSTN

• X-series recommendations– for connecting a DTE a public data network

• I-series recommendations– for connecting a DTE to the emerging ISDNs

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Open System

• OSIE– Open System Interconnection environment– the equipment from any manufacturer that

adheres to standards can be used interchangeably with equipment from any other manufacturer that complies with the standards

– ISO Reference Model for Open Systems Interconnection(OSI)

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OSI Reference Model

• Aims– to provide a framework for the coordination of

standards development

– to allow existing and evolving standards activities to be set within that framework

– To allow an application process in any computer that supports a particular set of standards to communicate freely with an application process in any other computer that supports the same standards, irrespective of its origin of manufacture

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OSI model (cont’d)

• Examples of application processes– a process(program) executing in a computer and accessing a

remote file system

– a process(server) acting as a central file service to a distributed community of (clients) processes

– a process in an office workstation accessing an electronic mail service

– a process acting as an electronic mail server to a distributed community of processes

– a process in a supervisory computer controlling a distributed community of computer-based instruments or robot controllers associated with a process or automated manufacturing plant

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OSI model (cont’d)

– A process in an instrument or robot controller receiving commands and returning results to a supervisory system

– a process in a bank computer that initiates debit and credit operations on a remote system

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OSI model (cont’d)

• Past– A communication subsystem is a complex piece of hardware and

software– Early attempts at implementing the software for such subsystems

were often based on a single, complex, unstructured program– The resulting software was difficult to test and often very difficult

to modify

• OSI model– To overcome this problem, it adopted a layered approach for the

reference model– The complete comm. Subsystem is broken down into a number of

layers, each of which performs a well-defined function

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OSI model (cont’d)

• Three operational environments– network environment

• it is concerned with the protocols and standards relating to the different types of underlying data communication networks

– OSI environment• it embraces the network environment and adds

additional application-oriented protocols and standards

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OSI model (cont’d)

– Real system environment• it builds on the OSI environment and is concerned

with a manufacturer’s proprietary software and services

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OSI model (cont’d)

• Each layer– It performs a well-defined function– It operates according to a defined protocol by

exchanging messages with peer• Protocol

• Interface

– The implementation of a particular protocol layer is independent of all other layers

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OSI model (cont’d)

• Logical structure of OSI model– three lowest layers(1-3 layers)

• network dependent

• be concerned with the protocols associated with the data communication network being used to link the two communicating computers

– three upper layers(5-7 layers)• application-oriented

• be concerned with the protocols that allow two end-user application processes to interact with each other, normally through a range of services offered by the local operating system

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OSI model (cont’d)

– Intermediate transport layer (4 layer)• it masks the upper application-oriented layers from

the detailed operation of the lower network-dependent layers

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Application layer

• it provides the user interface to a range of network-wide distributed information services

• it includes file transfer access and management, as well as general document and message interchange services such as email

• access to application services– is through a defined set of primitives, each with associated

parameters, which are supported by the local operating system

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Application layer (cont’d)

• Services– information transfer

– identification of the intended communication partner by name or by address

– determination of the current availability of an intended communication partner

– establishment of authority to communicate

– agreement on privacy(encryption) mechanism

– selection of the dialog discipline, including the initiation and release procedures

– identification of constraints on data syntax

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Presentation layer

• It is concerned with the representation of data during transfer between two communicating application processes

• To achieve open systems interconnection– a number of common abstract data syntax forms have been defined

for use by application processes

– The presentation layer negotiates and selects the appropriate syntax

– If necessary, the presentation layer performs the conversion

• It is concerned with data security (encryption)

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Session layer

• It allows two application layer protocol entities to organize and synchronize their dialog and manage their data exchange

• It is responsible for setting up a communication channel between two communicating application layer protocol entities

• Optional services– Interaction management– Synchronization

– Exception reporting

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Transport layer

• Acts– interface between the higher application-

oriented layers and the underlying network-dependent protocol layers

– it provides the session layer with a message transfer facility that is independent of the underlying network type

– it hides the detailed operation of the underlying network from the session layer

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Network-dependent layers

• Network-dependent layers– Network layer

• establishes and clears a network-wide connection between two transport layer protocol entities

• network routing(addressing), flow control

– Link layer• provides the network layer with a reliable

information transfer facility• error detection, transmission error, retransmission of

messages

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Network-dependent layers (cont’d)

• Two types of service– Connectionless

» it treats each information frame as a self-contained entity that is transferred using a best-try approach

– Connection oriented

» it endeavors to provide an error-free information transfer facility

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교환기술• 회선 교환 방식

– 정보 전송의 필요성이 생겼을 때 상대방을 호출하여 연결하고 이 물리적인 연결은 정보 전송이 종료될 때까지 계속됨

– 일단 물리적 연결이 이루어진 후 그 회선은 다른 사람과 공유할 수 없음

– 특징• 전송 중 항상 동일한 경로 사용 ( 전화시스템과 유사 )

• point-to-point 의 전송 구조• 접속에는 긴 시간 소요 , 전송지연은 거의 없음

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교환기술 (cont.)

• 고정적인 대역폭 사용

• 코드의 변환이 불가

• 길이가 긴 연속적인 데이터에 적합

• 패킷 교환 방식– 전송하고자 하는 정보를 작은 단위로 나눈 다음

나누어진 정보단위 즉 패킷마다 발신지와 수신지의 주소를 넣고 패킷 교환기는 그 주소를 보고 최종 목적지까지 패킷을 전달

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교환기술 (cont.)

– 특징• 회선 교환의 단점 최소화

• 메시지를 정해진 비트 수로 자른 다름 정해진 형식에 맞추어 만들어진 데이터 블록

• 패킷 형태로 만들어진 데이터를 패킷 교환기가 목적지 주소에 따라 적당한 통신 경로를 선택하여 전송

• 네트워크를 지나는데 데이터 단위의 길이가 제한

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Network-dependent layers (cont’d)

– Physical layer• it is concerned with the physical and electrical

interfaces between the user equipment and the network terminating equipment

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Open system standards

• Three major international bodies– IEEE (Institution of Electrical and Electronic Engineers)– ITU-T (International Telecommunications Union -

Telecommunications Sector)– CCITT (International Telegraph and Telephone

Consultative Committee)

• ISO and IEEE– standards for use by computer manufacturers

• ITU-T– standards for connecting equipment to the different types of

national and international public network

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Open system standards (cont’d)

• TCP/IP– by DARPA– It includes both network-oriented protocols and

application support protocols– be used as the basis for ISO standards– be used extensively by commercial and public

authorities for creating open system networking environments