Chapter 2

Spring 2006

Network Models

Computer Networks 1

Figure 2.1

Sending a letter

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Layered Protocols



Communication tasks are divided into series of layers or levels

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Each layer is responsible for particular task and act on them by using one or more protocols

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Each layer is built upon one bellow it

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The number and name of the layers differ from network to network

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Figure 2.17

The OSI seven layer model

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Application Presentation Session Transport Network Data Link Physical

The Layers of OSI Model

End System R Intermediate System Network Data Link Physical Application Presentation Session Transport Network Data Link Physical Spring 2006 Computer Networks 5

Summary of OSI Layer Functions

To translate, encrypt and compress data To provide reliable end-to end message delivery and error recovery To organize bits into frames, to provide node to-node delivery Spring 2006

Application Presentation

Session Transport Network Data Link Physical Computer Networks To allow access to network resources To establish, manage and terminate sessions To move packets from source to destination; to provide internetworking To transmit bits over a medium; to provide mechanical and electrical specifications 6

Open System Interconnection (OSI)

  

Developed by International Standard Organization (ISO) as a first step towards international standardization



De jure protocol Deals with interconnecting systems that are open for communication with other systems

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Open protocol suite Good as theoretical model, but not widely implemented in practice

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The OSI layers

  

Session layer

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Provides the control structure for communication between applications (dialog control)

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Establishes, manages and terminate connections (sessions) between cooperating applications Presentation layer

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Provides independence to the application processes from differences in data representation Application layer

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Provides access to the OSI environment for users and provides distributed information services

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The OSI layers

 

Physical layer

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Transmission of unstructured bit stream

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Deals with the mechanical, electrical, functional and procedural characteristics to access the physical medium Data link layer

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Provides reliable transfer across the physical link between two ends connected via single link

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Sends blocks of data (frames) with the necessary synchronization, error control and flow control

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Can add header and trailer

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The OSI layers

 

Network layer

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Provides upper layers with independence from the data transmission and switching technologies accross internetwork

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Responsible for source-to-destination delivery, addressing and routing in the internetwork Transport layer

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Provides transparent transport of data between end points that might not be connected via single link

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Provides source-to-destination connection, error recovery and flow control

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Protocol Suites

      

Open System Interconnection (OSI)

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Today used mostly as a reference model

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Prevously used in X.25 based protocols Internet (TCP/IP)

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Most popular suite today Xerox Networking Sysytems (XNS) System Network Architecture (SNA – IBM) Digital Network Architecture (DNA – DEC) NetBIOS (Software interface) AppleTalk

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The TCP/IP five layer model

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TCP/IP-modellen

Exempel

: SMTP, HTTP TCP, UDP IP Ethernet Spring 2006 Computer Networks 13

TCP/IP Protocol Suite

   

De facto (and after that de jure) standards Open (All modification and newly proposed protocols are published in a form of RFC (Request for Comments) RFC as well as drafts are published on the Internet



can be found on many URL (one is

www.rfc editor.org

) RFC becomes a standard when it is:

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Stable and well understood

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Technically competent

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Implemented on multiple independent places

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The TCP/IP Protocol Suite (Cont.)

  

Allows computers of many sizes, vendors and operating systems to communicate with each other History:

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Developed as de facto standard before OSI

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1960’s: started as goverment financed research project

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1990’s: most widely used form of networking Forms the basis for the Internet (capital “I”) (a WAN that spans the globe)

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