lesson 2 osi reference model

8/14/2019 Lesson 2 OSI Reference Model

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Lesson 2Lesson 2

OSIOSI Reference ModelReference Model


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OverviewOverview

Upon completing this lesson, you will beable to perform tasks related to:

Layers in a general model of communicationThe OSI Model

How the OSI Model compares with the

TCP/IP Model


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OSI Reference ModelOSI Reference Model

Overview

During the past two decades tremendous

increase in the numbers of users and networksDifferent implementations of hardware and

software Incompatibility

Proprietary networks and different vendorproducts could not communicate with each

other


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OSI Reference ModelOSI Reference Model

Overview (continues)

In order to address this problem ISO (International

Organization for Standardization) researched manynetwork schemes (DecNET, SNA, TCP/IP, etc.) and

decided to build network model which would help

network vendors implement networks that could

communicate Interoperability

ISO released OSI (Open Systems Interconnection) in

1984.

OSI and Networking standards So What?


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Communication Model of two hostsCommunication Model of two hosts

Source Destination

Data Packets

1010101...

How can I connect two PCs to each other?

If source and destination are on different platforms?

What is the data packet consists of?

Application data + Control Information

Control Information : Source and destination addresses, FCS, etc


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Communication Model of two hostsCommunication Model of two hosts

WAN ? /

INTERNET ? /

CORPORATE

NETWORK ?

WAN ? /

INTERNET ? /CORPORATE

NETWORK ?


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Computer ProtocolsComputer Protocols

In order for data packets to travel from a source to adestination on a network, it is important that all thedevices on the network speak the same language orprotocol. A Protocol is a set of rules that make

communication on a network more efficient.


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Back to OSI Reference ModelBack to OSI Reference Model

Why a layered Network Model?Reduces complexity

Standardizes interfacesFacilitates modular engineering

Ensures interoperable technology

Accelerates evolution

Simplifies Teaching and Learning

Divide and Conquer


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Seven Layers of the OSI ModelSeven Layers of the OSI ModelApplication Layer (Layer 7)Application Layer (Layer 7)


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Presentation Layer (Layer 6)Presentation Layer (Layer 6)


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Session Layer (Layer 5)Session Layer (Layer 5)


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Transport Layer (Layer 4)Transport Layer (Layer 4)


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Network Layer (Layer 3)Network Layer (Layer 3)


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Data Link Layer (Layer 2)Data Link Layer (Layer 2)


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Physical Layer (Layer 1)Physical Layer (Layer 1)


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Data EncapsulationData EncapsulationApplication Data

Encapsulation Process SourceData Network Communication

Destination Data

Decapsulation Process

Application Data

Application Data is extended with

the control information needed in

each layer. In each layer these

control information is added as

headers and trailers.

Network Layer adds network addresses to the header.

Data Link Layer adds physical addresses to the header and FCS to the trailer.

Physical Layer convert to bits for transmission. (Last step)


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Data EncapsulationData EncapsulationEthereal.lnk


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Peer-to-peer communicationPeer-to-peer communication

Each layer must communicate with its peerlayer.

Each layer exchanges information called PDUs(Protocol Data Units)Data (Common) Layer 5,6,7 PDU

Segment, Datagram, TPDU Layer 4 PDU

Packet Layer 3 PDU

Frame Layer 2 PDU

Bits Layer 1


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TCP/IP Reference ModelTCP/IP Reference Model

OSI Reference Model is universally recognizedbut the historical and technical open standard ofthe Internet is Transmission Control Protocol /Internet Control Protocol (TCP/IP)

TCP/IP Reference Model and TCP/IP Stack

Related Link

http://www.ietf.org http://www4.ulpgc.es/tutoriales/tcpip/pru/3376fm.htm
http://www.ietf.org/http://www4.ulpgc.es/tutoriales/tcpip/pru/3376fm.htmhttp://www4.ulpgc.es/tutoriales/tcpip/pru/3376fm.htmhttp://www.ietf.org/


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TCP/IPTCP/IP ModelModel

DoD (U.S. Department of Defense)created this model.

A network that can survive any conditions.A protocol stack that can run over any

network infrastructure.


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OSI &OSI & TCP/IPTCP/IP ModelModel ComparisonComparison

Application

Presentation

Session

Transport

Network

Data Link

Physical

OSI Model

Application

Transport

Internet

Host to Network

TCP Model


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Layers in theLayers in the TCP/IPTCP/IP ModelModel

Application LayerConsiders that higher level protocols should

include session and presentation layer.

Therefore all three layer functionalitypackaged in this layer for TCP/IP Model.


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Layers in theLayers in the TCP/IPTCP/IP ModelModelTransport Layer

Quality of Service

Issues of Reliability

Flow Control

Error Correction

A connection oriented protocol TCP includes all of the conceptsabove.

Circuit switching vs. Packet Switching. Put a question mark

here but forget it now.


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Internet Layer Considers underlying physical network by creating a

virtual network view.

Internet Protocol (IP) designed for this layer doesnt

give so much importance to the Network Access layerbeneath.

Its only concern is to provide unreliable, best-effort

connectionless packet delivery.


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Network Access Layer (also called host-to-network layer)

It includes the LAN and WAN technology details, and

all the details in the OSI physical and data link layers


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TCP/IP Protocol GraphTCP/IP Protocol Graph

NFS : Network File System

SMTP : Simple Mail Transfer Protocol

TELNET : Terminal Emulation over network

FTP : File Transfer Protocol


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DNS : Domain Name

TFTP : Trivial File Transfer Protocol

RPC : Remote Procedure Call

SNMP : Simple Network Management Protocol


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TCP: Transmission Control

UDP : User Datagram Protocol

TFTP : Trivial File Transfer Protocol

IP : Internet Protocol ICMP : Internet Message Control Protocol


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ARP: Address Resolution Protocol

RARP : Reverse Address Resolution Protocol


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ComparisonComparison ofofTCP/IP with OSITCP/IP with OSI

Similarities

Both have layers

Both have application layers, though their

content includes different servicesBoth have compatible transport and network

layers

Packet-switched (not circuit-switched)technology is assumed

Networking professionals need to know both


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ComparisonComparison ofofTCP/IP with OSITCP/IP with OSI

Differences Session Layer + Presentation Layer + Application Layer (OSI) ~

Application Layer (TCP/IP)

Data Link + Physical Layer (OSI) ~ Network Access (TCP/IP)

TCP/IP has fewer layers but same functionality

OSI model is a guide and abstract model. Not an implementationguide. TCP/IP is a protocol suite consists of so many procotolsdeveloped for the Internet and they are widely used as industrystandards.

In TCP/IP a given protocol can be used by another protocol

within the same layer, whereas in the OSI model two layerswould be defined in such circumstances. For example ICMP andARP (Layer 3 Protocols) are carried in IP datagrams.


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Focus of the CCNA CurriculumFocus of the CCNA Curriculum

Application

FTP, HTTP, TFTP, SMTP, DNS, TELNET, SMTP

Presentation and Session Layer

Know definitions and named general functions Transport

TCP (the Internet)

Network

IP (the Internet) Data Link, Physical

Ethernet (common LAN Technology)


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Focus of the CCNA CurriculumFocus of the CCNA Curriculum


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Review Questions (Q1)Review Questions (Q1)


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Review Questions (A1)Review Questions (A1)


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R i Q ti (Q6)Re ie Q estions (Q6)


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Q (Q )


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R i Q i (A8)


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R i Q ti (Q9)


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Q ( )R i Q ti (A9)


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R i Q i (Q10)R i Q ti (Q10)


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R i Q ti (A10)R i Q ti (A10)


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lesson 2 osi reference model

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