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Computer Networks OSI

Jun 02, 2018




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    Protocol Reference Model ofOS

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    Introduction to the OSI layerHow OSI was created and whyComparison with TCP/IPLayers : Application layer Presentation layer Session layer Transport layer Network layer Data link layer Physical layerConclusionResources

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    Open Systems Interconnection (OSI)Each layer support the layers above it andoffers services to the layers belowEach layer performs unique and specific taskA layer only has knowledge of its neighbourlayers only

    A layer service is independent of theimplementation

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    An attempt for a framework for developingnetworking technologiesOSI became a tool for explaining theNetworking in generalBefore OSI was created people created theirSoftware/Hardware as they wanted it to be.There was not any compatibility. Now OSI is

    used as a rule set for all vendors to createtheir Software/Hardware by using thestandards.

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    II OSI Overview

    1. OSI - layered framework forthe design of network systems

    that allows communicationacross all types of computersystems.

    2. The OSI 7 Layers. ( Brieffunctional overview. )

    3. Vertical and horizontalcommunication between thelayers using interfaces. (defineswhat information and servicesshould the layer provide to thelayer above it. )

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    Each layer contains a logical grouping offunctionsEach function receive an input(one or more)and produces an output

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    I History1. The need for standardization

    - many vendors, nointeroperability- no common framework

    2. ISO and CCITT came up with

    OSI (Open SystemIntercommunication) in 1984.

    3. OSI Protocol Suite unaccepted by vendors andusers. (TCP won)

    4. OSI a standard, which allowscommunication betweendifferent systems withoutrequiring changes to the logicof the underlying hardware andsoftware.

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    Layer abstraction and the path of themessage

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    OSI Overview

    4. Data Encapsulationa) PDU conception each

    protocol on the diff. layerhas its own format.b) Headers are addedwhile a packet is goingdown the stack at eachlayer.c) Trailers are usually

    added on the secondlayer.

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    Pretty similar to OSITCP/IP has less layers(four)Main difference in layers is after layer 4

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    Applications and Services run on itEnables human network to interface the underlying data networkApplications on that layer (E-mail clients, web browsers, Chats,etc.) top-stack applications (As people are on the top of the

    stack)Applications provide people with a way to create messageApplication layer services establish an interface to the networkProtocols provide the rules and formats that govern how data istreated

    Protocols on the destination and the host must match

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    III The OSI Layers7. Application Layer

    Provides user interfaces andsupport for services

    Resource sharing and deviceredirection

    Remote file access Remote printer access Inter-process communication Network management Directory services Electronic messaging such as

    mail) Network virtual terminals

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    Coding and conversion of Application layer data to ensure thatdata from the source device can be interpreted by theappropriate application on the destination device.Compression of the data in a manner that can be decompressedby the destination device.Encryption of the data for transmission and the decryption of

    data upon receipt by the destination.This is the layer at which application programmers consider datastructure and presentationExamples: GIF, JPEG, TIFF, etc.Sometimes n distinction is made between the presentation and

    application layers. For example http/https. HTTP is generallyregarded as an application layer protocol although it hasPresentation layer aspects such as the ability t identifycharacter encoding for roper conversion

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    The OSI Layers6. Presentation Layer

    Translation (connects differentcomputer systems)

    Compression (transmissionefficiency)

    Encryption (SSL security)

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    Functions at this layer create and maintaindialogs between source and destinationapplicationsAuthenticationPermissionsSession Restoration (Checkpoint or recovery)

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    The OSI Layers5. Session Layer

    Session establishment,maintenance and termination(Deciding who sends, andwhen.)

    Session support (security,name recognition, logging )

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    Tracking the individual communication between applications onthe source and destination hostsSegmenting data and managing each pieceReassembling the segments into streams of application dataIdentifying the different applicationsConversation MultiplexingSegmentsConnection-oriented conversationsReliable deliveryOrdered data reconstruction

    Flow controlTCP Web BrowserUDP Video Streaming Applications

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    The OSI Layers4. Transport Layer

    Connectionless and connection-oriented services

    Process-Level Addressing Multiplexing and Demultiplexing Segmentation, Packaging and

    Reassembly Connection Establishment,

    Management and Termination Acknowledgments and

    Retransmissions Flow Control

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    Addressing (IPV4)Encapsulation (Inserts a header with sourceand destination IPs)Routing (Move a packet over the Internet)Decapsulation (Open the packet and checkthe destination host)IP is connectionless

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    The OSI Layers3. Network Layer

    Logical Addressing Routing (where the packet is

    destinated to) Datagram Encapsulation Fragmentation and Reassembly

    (handling too big packets ) Error Handling and Diagnostics (

    using status messages forexample )

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    End to end packet delivery

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    It is the role of the OSI Data Link layer to prepare Network layer

    packets for transmission and to control access to the physical media.Allows the upper layers to access the media using techniques such asframingControls how data is placed onto the media and is received from themedia using techniques such as media access control and error

    detectionFrame - The Data Link layer PDUNode - The Layer 2 notation for network devices connected to acommon mediumMedia/medium - The physical means for the transfer of information

    between two nodesNetwork - Two or more nodes connected to a common mediumThe Data Link layer is responsible for the exchange of framesbetween nodes over the media of a physical network.

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    The OSI Layers2. Data Link Layer2.1. Logical Link Control LLC )

    Establishment and control oflogical links between localdevices on a network.

    2.2. Media Access Control MAC) The procedures used by devices

    to control access to the networkmedium.

    Frame sequencing Frame acknowledgment Addressing Frame delimiting Frame error checking PD U : frame

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    The role of the Physical layer is to encode thebinary digits that represent Data Link layerframes into signals and to transmit andreceive these signals across the physicalmedia that connect network devices.Copper cableFiber


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    The OSI Layers1. Physical Layer

    Definition of HardwareSpecifications (of cables,connectors, wireless radiotransceivers, network interfacecards )

    Encoding and Signaling (bitrepresentation)

    Data Transmission and Reception(half duplex, full duplex )

    Topology and Physical NetworkDesign (mesh, ring, bus)

    PD U : bit

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    OSI Summary

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    The way people learn NetworkingA standard for softwareA standard for hardware

    Seven layers architectureEach layer independent on the othersSimilar to TCP/IP(TCP/IP explained)OSI is used as a model for developingnetwork aware applications(Here I mean thatpeople use its structure to model software)

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