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8/12/2019 OSI Apptech

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The OSI Reference Model


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OSI Open System s Interconnection".

OSI model was first introduced in 1984 by the InternationalOrganization for Standardization (ISO). Outlines WHAT needs to be done to send data from one computer

to another.

Not HOW it should be done.

Protocols stacks handle how data is prepared for transmittal (to betransmitted)

In the OSI model, The specification needed are contained in 7 different layers that interact with each other.

The OSI Model


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What is THE MODEL?

Commonly referred to as the OSI reference model.

The OSI model is a theoretical blueprint that helps us understand how data gets

from one users computer to another.

It is also a model that helps develop standards so that all of ourhardware and software talks nicely to each other.

It aids standardization of networking technologies by providingan organized structure for hardware and software developers tofollow, to insure there products are compatible with current andfuture technologies.


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What Each Layer Does

2


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Gives end-user applications access

to network resources

Where is it on my computer? Workstation or Server Service in

MS Windows

3


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Presentation Layer

3


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Session Layer

Allows applications tomaintain an ongoing session

Where is it on mycomputer? Workstation and Server

Service (MS) Windows Client for

NetWare (NetWare)

3


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Figure 2.9 Transport l ayer

The tr anspor t layer is responsible forthe delivery of a message from one

process to another.


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Network Layer

Provides network-wide addressingand a mechanism to move packets

between networks (routing)

Responsibilities: Network addressing Routing

Example: IP from TCP/IP 3


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Network layer

The network layer is responsible forthe delivery of individual packets f rom

the source host to the destination host.


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Data Link Layer

Places data and retrieves itfrom the physical layer andprovides error detectioncapabilities

3


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Data l ink layer

The data link layer is responsible formoving frames f rom one hop (node) to

the next.


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Sub-layers of the Data Link Layer

MAC (Media Access Control) Gives data to the NIC Controls access to the media through:

CSMA/CD Carrier Sense Multiple Access/CollisionDetectionToken passing

LLC (Logical Link Layer) Manages the data link interface (or Service Access Points

(SAPs)) Can detect some transmission errors using a Cyclic

Redundancy Check (CRC). If the packet is bad the LLC willrequest the sender to resend that particular packet.


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Physical Layer

Determines the specs forall physical components Cabling Interconnect methods (topology

/ devices) Data encoding (bits to waves) Electrical properties

Examples: Ethernet (IEEE 802.3) Token Ring (IEEE 802.5) Wireless (IEEE 802.11b)

3


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Physical layer

The physical layer is responsiblefor the movement of individual bits

from one hop (node) to the next.


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Physical Layer (contd)

What are the Physical Layer components on mycomputer?NIC Network Interface Card Has a unique 12 character Hexadecimal number

permanently burned into it at the manufacturer. The number is the MAC Address/Physical address of a

computer

Cabling Twister Pair

Fiber Optic Coax Cable


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How Does It All Work Together

Each layer contains a Protocol Data Unit(PDU)

PDU s are used for peer-to-peer contact

between corresponding layers.

Data is handled by the top three layers, thenSegmented by the Transport layer.

The Network layer places it into packets andthe Data Link frames the packets fortransmission.

Physical layer converts it to bits and sends itout over the media.

The receiving computer reverses the process

using the information contained in the PDU .

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Figure 2.2 OSI layers


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Data Encapsulation In TCP/IP At each layer in the TCP/IP protocol stack

Outgoing data is packaged and identified for delivery to the layerunderneath

PDU Packet Data Unit the envelop information attached to apacket at a particular TCP/IP protocol

e.g. header and trailer

HeaderPDUs own particular opening componentIdentifies the protocol in use, the sender and intended recipient

Trailer (or packet trailer) Provides data integrity checks for the payload


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Encapsulation example: E-mail


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Encapsulation


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Figure 2.3 An exchange using the OSI model


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Figure 2.14 Summary of l ayers


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The Postal Analogy

A- Write a 20 page letter to a foreign country.

P- Translate the letter so the receiver can read it.

S- Insure the intended recipient can receive letter.

T- Separate and number pages. Like registered mail,tracks delivery and requests another package if one islost or damaged in the mail. N- Postal Center sorting letters by zip code to routethem closer to destination.D- Local Post Office determining which vehicles todeliver letters.

P- Physical Trucks, Planes, Rail, autos, etc whichcarry letter between stations.

P resentation

T ransport

Network

Physical

Data-Link

Session

A pplication

How would the OSI compare to the regular Post Office


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Remembering the 7 Layers

7 - Application All

6 - P resentation P eople

5 - S ession S eem

4 - Transport To

3 - Network Need

2 - Data Link Data

1 - P hysical P rocessing


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4 layers of the TCP/IP model

Layer 4: Application

Layer 3: TransportLayer 2: InternetLayer 1: Network access

It i s imp or tant to n ote that so m e of thelayers in th e TCP/IP m o d el have the s am e

nam e as layers in the OSI m od el.Do no t con fuse the layers of the two m od els .


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TCP/IP protocol stack


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TCP/IP Reference Model

Application

Transport

Internet

Network Access(Host-to-network)

LayerHTTP TELNET FTP SMTP SNMP

Protocols

TCP UDP

IP ICMP

ETHERNET PACKET RADIO


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Data Formats Application data

dataTCP

header dataTCP

header dataTCP

header

dataTCPheaderIP

header

dataTCPheaderIP

headerEthernetheader

Ethernettrailer

applicationlayer

transportlayer

networklayer

data linklayer

message

segment

packet

frame


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Packet Encapsulation (TCP/IP) The data is sent down the protocol stack

Each layer adds to the data by prepending headers

22Bytes 20Bytes 20Bytes 4Bytes

64 to 1500 Bytes

Comparing TCP/IP with OSI


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Comparing TCP/IP with OSI

OSI Model TCP/IP Hierarchy Protocols

7th

Application Layer

6 thPresentation Layer

5 thSession Layer

4 thTransport Layer

3 rdNetwork Layer

2 ndLink Layer

1 st

Physical Layer

Application Layer

Transport Layer

Network Layer

Link Layer

Link Layer : includes device driver and network interface cardNetwork Layer : handles the movement of packets, i.e. RoutingTransport Layer : provides a reliable flow of data between two hosts

Application Layer : handles the details of the particular application

How the OSI and TCP/IP Models Relate in a Networking Environment


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Layer 7 A pplication All Browsers Application

Layer 6 Presentation PeopleCommon DataFormat

Application

Layer 5 Session SeemNFS, SQL,

RPC, X-Win

Dialogues and

Conversations Application

Layer 4 T ransport To Computer Segment TCP and UDPQuality ofService, andReliability

Transport

Layer 3 N etwork N eed Router

Segment Networkinto SmallerBroadcast

Domains

Packet

RoutableProtocols.(IP, IPX,

AppleTalk)

Path Selection,Routing, and

AddressingInternet

Words toRemember

TCP/IPModel

OSIModelLayer

Pneumonic EquipmentEquipmentPurpose

OSI ModelName

FrameNDIS, ODI,MAC Address,Ether Talk

Data Protocols

NetworkAccess

Layer 1 Physical Processing

Repeater,Hub (Multi-port) B it Physical

Signals and

Media

NetworkAccess

Layer 2

Regular Computeror A Special

Gateway. Used tocombine networks

using different

communication protocols

D ata Link-MAC-LLC

One Collision AND

One Broadcast

Frames andMedia AccessControl (MAC)

Computer D ata

Redirector,FTP, Telnet,SMTP,SNMP,

NetwareCore

D ata

Bridge (2Ports) orSwitch and

NIC

Segment Networkinto SmallerColli sion Domains

How the OSI and TCP/IP Models Relate in a Networking Environment

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