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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 아니고 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 문자
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, videoconfere
ncing, 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 email
• 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-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 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
28
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
29
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
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