1

OSI Reference Model

2

Agenda

The Layered Model Layers 1 & 2: Physical &

Data Link Layers Layer 3: Network Layer Layers 4–7: Transport,

Session, Presentation, and Application Layers

3

The Layered Model

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Layered Communication

Source: Tanenbaum, 1996

I like rabbits

L: DutchIk hou vankonijnen

Fax #:---L: DutchIk hou vankonijnen

Message

Information for the Remote Translator

Informationfor the RemoteSecretary

Location A

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Layered Communication

I like rabbits

L: DutchIk hou vankonijnen

L: DutchIk hou vankonijnen

Fax #:---L: DutchIk hou vankonijnen

Fax #:---L: DutchIk hou vankonijnen

J’aimeles lapins

Information for the Remote Translator

Informationfor the RemoteSecretary

Location A Location B

Message

6

Layered Communication

I like rabbits

L: DutchIk hou vankonijnen

L: DutchIk hou vankonijnen

Fax #:---L: DutchIk hou vankonijnen

Fax #:---L: DutchIk hou vankonijnen

J’aimeles lapins

Information for the remote translator

Informationfor the remotesecretary

Location A Location B

1

2

3Layers

Message

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Why a Layered Network Model?

7 Application

6 Presentation

5 Session

4 Transport

3 Network

2 Data Link

1 Physical

• Reduces complexity (one big problem to seven smaller ones)

• Standardizes interfaces

• Facilitates modular engineering

• Assures interoperable technology

• Accelerates evolution

• Simplifies teaching and learning

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Devices Function at Layers

7 Application

6 Presentation

5 Session

4 Transport

3 Network

2 Data Link

1 Physical

NIC Card

Hub

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Host & Media Layers

7 Application

6 Presentation

5 Session

4 Transport

3 Network

2 Data Link

1 Physical

Host layers:Host layers: Provide accurate data delivery between computers

Media layers:Media layers: Controlphysical delivery of messages over the network}}

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Layer Functions

Provides network services to application processes (such as electronic mail, file transfer, and terminal emulation)

7 Application

11

Layer Functions

Network services to applicationsNetwork services to applications

• Ensures data is readable by receiving system

• Format of data• Data structures• Negotiates data transfer

syntax for application layer

7 Application

6 Presentation Data representation

12

Layer Functions

Inter-host communication• Establishes, manages, and

terminates sessions between applications

7 Application

6 Presentation

5 Session

Network services to applicationsNetwork services to applications

Data representationData representation

13

Layer Functions

7 Application

6 Presentation

5 Session

Transport4

Inter-host communicationInter-host communication

Network services to applicationsNetwork services to applications

Data representationData representation

End-to-end connection reliability

• Concerned with data transport issues between hosts

• Data transport reliability• Establishes, maintains, and

terminates virtual circuits• Fault detection and recovery• Information flow control

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Layer Functions

7 Application

6 Presentation

5 Session

Transport4

Network3

Inter-host communicationInter-host communication

Network services to applicationsNetwork services to applications

Data representationData representation

End-to-end connection End-to-end connection reliabilityreliability

Addresses and best path• Provides connectivity and path

selection between two end systems

• Domain of routing

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Layer Functions

7 Application

6 Presentation

5 Session

Transport4

Network3

Data Link2

Inter-host communicationInter-host communication

Network services to applicationsNetwork services to applications

Data representationData representation

End-to-end connection End-to-end connection reliabilityreliability

Addresses and best pathAddresses and best path

Access to media• Provides reliable transfer of data

across media• Physical addressing, network

topology, error notification, flow control

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Layer Functions

7 Application

6 Presentation

5 Session

Transport4

Network3

Data Link2

Physical1

Inter-host communicationInter-host communication

Network services to applicationsNetwork services to applications

Data representationData representation

End-to-end connection End-to-end connection reliabilityreliability

Addresses and best pathAddresses and best path

Access to mediaAccess to media

Binary transmission• Wires, connectors, voltages,

data rates

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7 Application

6 Presentation

5 Session

4 Transport

3 Network

2 Data Link

1 Physical

Host A

Peer-to-Peer Communications

Application

Presentation

Session

Transport

Network

Data Link

PhysicalBits

Frames

Packets

Segments

Host B

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Data Encapsulation

Application

Presentation

Session

Transport

Network

Physical

Data Link

Application

Presentation

Session

Transport

Network

Physical

Data Link

Data} {DataNetwork

Header

Frame

HeaderFrame

TrailerDataNetwork

Header

0101101010110001

Host A Host B

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Layers 1 & 2: Physical & Data Link Layers

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Physical and Logical Addressing

0000.0c12.3456

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MAC Address

24 bits

ROMRAM

24 bits

0000.0c12. 3456Serial NumberVendor Code

• MAC address is burned into ROM on a network interface card

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Layer 3: Network Layer

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Network Layer: Path Determination

Which Path?Which Path?

• Layer 3 functions to find the best path through the internetwork

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Network Layer: Communicate Path

1

2

3

4

5

6

7

8

9

10 11

Addresses represent the path of media connections

Routing helps contain broadcasts

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Addressing—Network and Node

Network Node

1

2

123

1

3 1

1.1

2.1

3.1

1.2

1.3

• Network address—Path part used by the router

• Node address—Specific port or device on the network

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Protocol Addressing Variations

Network Node

1 1

GeneralExample

Network Host

10. 8.2.48

TCP/IPExample (Mask 255.0.0.0)

Network Node

1aceb0b. 0000.0c00.6e25

Novell IPXExample

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Network Layer Protocol Operations

Each router provides its services to support upper layer functions

X Y

AA

CC

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Network Layer Protocol Operations

Each router provides its services to support upper layer functions

BB

X Y

AA

CC

Presentation

Data LinkPhysical

Data LinkPhysical

Router A Router B Router C

Data LinkPhysical

Data LinkNetworkTransportSessionPresentationApplication

Physical

Host X Host Y

Data LinkNetworkTransportSession

Application

Physical

Network Network Network

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Routed Versus Routing Protocol

• Routed protocol used between routers to direct user traffic

1.02.03.0

1.12.13.1

DestinationNetwork

NetworkProtocol

Protocol Name

Examples: IP, IPX, AppleTalk, DECnet Exit Port

to Use

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Routed Versus Routing Protocol

• Routed protocolused betweenrouters to directuser traffic

Examples: IP, IPX, AppleTalk, DECnet

• Routing protocolused only betweenrouters to maintainrouting tables

Examples: RIP, IGRP, OSPF

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Static Versus Dynamic Routes

Uses a protocol route that a networkadministrator enters into the router

Static Route

Uses a route that a network protocoladjusts automatically for topology ortraffic changes

Dynamic Route

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Static Route Example

Point-to-point orcircuit-switched

connection

“Stub” network

Only a single networkconnection with no need

for routing updates

AA

BB

Fixed route to address reflects administrator’s knowledge

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Adapting to Topology Change

AA BB

CCDD

Can an alternate route substitute for a failed route?

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Adapting to Topology Change

AA BB

CCDD

XX

Can an alternate route substitute for a failed route?

YesYes—With dynamic routing enabled

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LAN-to-LAN Routing Example

Network 1

Network 3

E0E1 To0

Host 5Host 4 Network 2

TokenRing

802.3 Net 2, Host 5

Routing Table

DestinationNetwork

Outgoing Interface

123

E0To0E1

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LAN-to-LAN Routing

Network 1

Network 3

E0E1 To0

Host 5Host 4 Network 2

TokenRing

802.3 Net 2, Host 5

Routing Table

Net 2, Host 5802.5

From LAN to LAN

DestinationNetwork

Outgoing Interface

123

E0To0E1

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LAN-to-WAN Routing

Data

1.3 2.4 Data

AA

TokenRing

2.4

Token Ring 1.3 2.4 Data

BB

1.3From LAN

Frame Relay 1.3 2.4 DataFrame Relay

1.3 2.4 Data

To WAN

Ethernet

Data

1.3 2.4 Data

1.3 2.4 Data

To LAN

1.3 2.4 Data

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Layers 4–7: Transport, Session, Presentation, and Application Layers

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Transport Layer

Segments upper-layer applications Establishes an end-to-end connection Sends segments from one end host to another Optionally, ensures data reliability

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Transport Layer— Segments Upper-Layer Applications

Electronic

Mail

File

Transfer

Application

Presentation

Session

Segments

DataApplication

PortTransport Data

Application

Port

Terminal

Session

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Transport Layer— Establishes Connection

Synchronize

Synchronize

Acknowledge

Negotiate Connection

Receiver

Data Transfer

Connection Established

(Send Segments)

Sender

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Transport Layer— Sends Segments with Flow Control

Transmit

Buffer FullNot Ready

StopProcessSegments

Buffer OKReadyGo

Resume Transmission

ReceiverSender

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Transport Layer— Reliability with Windowing

•Window Size = 1

Sender

Send 1 Receive 1

Receiver

Ack 2

Send 2 Receive 2Ack 3

Sender

Send 1Send 2

Receive 1Receive 2

Receiver

•Window Size = 3

Send 3 Receive 3Ack 4

Send 4

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Transport Layer— An Acknowledgement Technique

Sender Receiver

Send 2Send 1

Send 3Ack 4

Send 5Send 4

Send 6Ack 5

Send 5Ack 7

1 2 3 4 5 6 7 1 2 3 4 5 6 7

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Transport to Network Layer

End-to-End SegmentsEnd-to-End Segments

Routed PacketsRouted Packets

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Session Layer

• Network File System (NFS)

• Structured Query Language (SQL)

• Remote-Procedure Call (RPC)

• X Window System

• AppleTalk Session Protocol (ASP)

• DEC Session Control Protocol (SCP)

Service Request

Service Reply

Coordinates applications as they interact on different hosts

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Presentation Layer

ASCIIEBCDICEncrypted

• Text• Data

login:

PICTTIFFJPEGGIF

• Graphics• Visual images

• Sound

• VideoMPEGQuickTime

MIDI

Provides code formatting and conversion for applications

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Application Layer

Word Processor

Presentation Graphics

Spreadsheet

Database

Design/Manufacturing

Project Planning

Others

COMPUTERAPPLICATIONS

Electronic Mail

File Transfer

Remote Access

Client-Server Process

Information Location

Network Management

Others

NETWORKAPPLICATIONS INTERNETWORK

APPLICATIONS

Electronic Data Interchange

World Wide Web

E-Mail Gateways

Special-Interest Bulletin Boards

Financial Transaction Services

Internet Navigation Utilities

Conferencing (Voice, Video, Data)

Others

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