Chapter 1 Data Communication Networks and Open System Standards

2 Background 컴퓨터 – 인간생활의 모든 면에서 사용 예 ) 가정, 사무실, 여행사, 학교, … –Stand-Alone mode 로만 사용되는 것이 아니 고 타 컴퓨터와의 데이터 교환과 상호 공동 작업 필요  컴퓨터 통신망

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

4 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

5 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

6 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

7 Data communication networks Broadband multiservice networks –supports desktop video telephony, videoconferencing, and mode general multimedia services ATM –asynchronous transfer mode MAN –metropolitan area network

8 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

9 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

10 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)

11 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

12 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

13 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

14 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

15 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

16 OSI model (cont’d) –Real system environment it builds on the OSI environment and is concerned with a manufacturer’s proprietary software and services

17 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

18 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

19 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

20 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 access to application services –is through a defined set of primitives, each with associated parameters, which are supported by the local operating system

21 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

22 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)

23 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

24 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

25 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

26 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

27 교환기술 회선 교환 방식 – 정보 전송의 필요성이 생겼을 때 상대방을 호출하 여 연결하고 이 물리적인 연결은 정보 전송이 종료 될 때까지 계속됨 – 일단 물리적 연결이 이루어진 후 그 회선은 다른 사 람과 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

28 교환기술 (cont.) 고정적인 대역폭 사용 코드의 변환이 불가 길이가 긴 연속적인 데이터에 적합 패킷 교환 방식 – 전송하고자 하는 정보를 작은 단위로 나눈 다음 나 누어진 정보단위 즉 패킷마다 발신지와 수신지의 주소를 넣고 패킷 교환기는 그 주소를 보고 최종 목 적지까지 패킷을 전달

29 교환기술 (cont.) – 특징 회선 교환의 단점 최소화 메시지를 정해진 비트 수로 자른 다름 정해진 형식에 맞추 어 만들어진 데이터 블록 패킷 형태로 만들어진 데이터를 패킷 교환기가 목적지 주 소에 따라 적당한 통신 경로를 선택하여 전송 네트워크를 지나는데 데이터 단위의 길이가 제한

30 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

31 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

32 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