Chapter 1 Data Communication Networks and Open System

Chapter 1 Data Communication Networks and Open System Standards

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 4

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 5

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 6

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

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 8

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 9

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) 10

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 11

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 12

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 13

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 14

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 15

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

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 17

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 18

OSI model (cont’d) – Intermediate transport layer (4 layer) • it masks the upper application-oriented layers from the detailed operation of the lower networkdependent layers 19

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 20

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 21

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) 22

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 23

Transport layer • Acts – interface between the higher applicationoriented layers and the underlying networkdependent 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 24

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 25

Network-dependent layers (cont’d) • Two types of service – Connectionless » it treats each information frame as a selfcontained entity that is transferred using a best-try approach – Connection oriented » it endeavors to provide an error-free information transfer facility 26

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 30

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 31

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 32