© 1999, Cisco Systems, Inc. OSI Reference Model.

© 1999, Cisco Systems, Inc. www.cisco.com

OSI Reference ModelOSI Reference Model

2-2CSE: Networking Fundamentals—OSI Model © 1999, Cisco Systems, Inc. www.cisco.com

Agenda

• The Layered Model

• Layers 1 & 2: Physical & Data Link Layers

• Layer 3: Network Layer

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


The Layered ModelThe Layered Model



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



I like rabbits





J’aimeles lapins

Information for the Remote Translator

Informationfor the RemoteSecretary

Location A Location B

Message



I like rabbits





J’aimeles lapins

Information for the remote translator

Informationfor the remotesecretary

Location A Location B

1

2

3Layers

Message


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


Devices Function at Layers

7 Application

6 Presentation

5 Session

4 Transport

3 Network

2 Data Link

1 Physical

NIC Card

Hub


Host 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

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}

}


Layer Functions

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

7 Application


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


Layer Functions

Inter-host communication• Establishes, manages, and

terminates sessions between applications

7 Application

6 Presentation

5 Session


Data representationData representation


Layer Functions

7 Application

6 Presentation

5 Session

Transport4

Inter-host communicationInter-host communication



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


Layer Functions

7 Application

6 Presentation

5 Session

Transport4

Network3




End-to-end connection reliabilityEnd-to-end connection reliability

Addresses and best path• Provides connectivity and path

selection between two end systems

• Domain of routing


Layer Functions

7 Application

6 Presentation

5 Session

Transport4

Network3

Data Link2





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


Layer Functions

7 Application

6 Presentation

5 Session

Transport4

Network3

Data Link2

Physical1





Addresses and best pathAddresses and best path

Access to mediaAccess to media

Binary transmission• Wires, connectors, voltages,

data rates


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


Data Encapsulation

Application

Presentation

Session

Transport

Network

Physical

Data Link

Application

Presentation

Session

Transport

Network

Physical

Data Link

Host A Host B

Data} {


Data Encapsulation

Application

Presentation

Session

Transport

Network

Physical

Data Link

Application

Presentation

Session

Transport

Network

Physical

Data Link

Data} {DataNetwork

Header

Host A Host B


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

Host A Host B


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


Layers 1 & 2: Physical & Data Link

Layers

Layers 1 & 2: Physical & Data Link

Layers



Physical and Logical Addressing

0000.0c12.3456


MAC Address

24 bits

ROMRAM

24 bits

0000.0c12. 3456Serial NumberVendor Code

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


Layer 3: Network Layer

Layer 3: Network Layer



Network Layer: Path Determination

Which Path?Which Path?

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


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


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


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


Network Layer Protocol Operations

• Each router provides its services to support upper layer functions

X Y

AA

CC


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


Multiprotocol Routing

IP 15.17.132.6

IP 15.16.42.8

IP 15.16.50.3

Routing Tables

IP

TokenRing

TokenRing

AppleTalk 200.167

AppleTalk 100.110

Apple

IPX 4b.0800.0121.ab13

IPX 3a.0800.5678.12abNovell

DECnet 5.8

DECnet 10.1

DEC

VAX

VAX

• Routers pass traffic from all routed protocols over the internetworkRouters pass traffic from all routed protocols over the internetwork


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


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


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


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


Adapting to Topology Change

AA BB

CCDD

• Can an alternate route substitute for a failed route?



AA BB

CCDD

XX



AA BB

CCDD

XX

• Can an alternate route substitute for a failed route?

YesYes—With dynamic routing enabled


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


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


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

To WAN

To LAN


LAN-to-WAN Routing

Data

1.3 2.4 Data

AA

TokenRing

2.4


BB

1.3From LAN

Frame Relay 1.3 2.4 DataFrame Relay

1.3 2.4 Data

To WAN

To LAN


LAN-to-WAN Routing

Data

1.3 2.4 Data

AA

TokenRing

2.4


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


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

Application Layers

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

Application Layers



Transport Layer

• Segments upper-layer applications

• Establishes an end-to-end connection

• Sends segments from one end host to another

• Optionally, ensures data reliability


Transport Layer— Segments Upper-Layer Applications

Electronic

Mail

File

Transfer

Application

Presentation

Session

Segments

DataApplication

PortTransport Data

Application

Port

Terminal

Session


Transport Layer— Establishes Connection

Synchronize

Synchronize

Acknowledge

Negotiate Connection

Receiver

Data Transfer

Connection Established

(Send Segments)

Sender


Transport Layer— Sends Segments with Flow Control

Transmit

Buffer FullNot Ready

StopProcessSegments

Buffer OKReadyGo

Resume Transmission

ReceiverSender


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


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


Transport to Network Layer

End-to-End SegmentsEnd-to-End Segments

Routed PacketsRouted Packets


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


Presentation Layer

ASCIIEBCDICEncrypted

• Text• Data

login:

PICTTIFFJPEGGIF

• Graphics• Visual images

• Sound

• VideoMPEGQuickTime

MIDI

• Provides code formatting and conversion for applications


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• Internetwork applications

can extend beyond the enterprise (i.e., to suppliers, etc.)


Summary

• OSI reference model describes building blocks of functions for program-to-program communications between similar or dissimilar hosts

• Layers 4–7 (host layers) provide accurate data delivery between computers

• Layers 1–3 (media layers) control physical delivery of data over the network

58Presentation_ID © 1999, Cisco Systems, Inc. www.cisco.com

Date post:	14-Jan-2016
Category:	Documents
Upload:	albert-gilbert
View:	226 times
Download:	1 times

© 1999, Cisco Systems, Inc. OSI Reference Model.

Documents