The OSI Model Cisco Networking Academy Program – Semester 1 J i m W h i t e – 2 0 0 2 Semester 1...

elCisco Networking Academy Program – Semester 1

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Semester 1

Chapter 2

The OSI Model

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The OSI Reference Model

• The OSI layer was introduced by the International Organization for Standardization (ISO) in 1984 in order to provide a reference model to make sure products of different vendors would interoperate in networks.

• OSI is short for Open Systems Interconnection.

• The OSI layer shows WHAT needs to be done to send data from an application on one computer, trough a network, to an application on another computer, not HOW it should be done.

• A layer in the OSI model communicates with three other layers: the layer above it, the layer below it, and the same layer at its communication partner.

• Data transmitted between software programs passes all 7 OSI layers.

• The Application, Presentation and Session layers are also known as the Upper Layers.

• The Data Link and Physical layers are often implemented together to define LAN and WAN specifications.

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What is a Protocol?

• A Protocol is a set of rules that make communication on a network more efficient.

• E.G. - When answering the telephone, someone says, "Hello," then the person calling says, "Hello. This is.... "; and so it goes back and forth

• One technical definition of a data communications protocol is: a set of rules, or an agreement, that determines the format and transmission of data. Layer n on one computer communicates with Layer n on another computer.

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What are the 7 layers of the OSI Model?

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Why use a Layered Model?

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Remembering the OSI Model

• All 7 - Application• People 6 - Presentation• Seem 5 - Session• To 4 - Transport• Need 3 - Network• Domino’s 2 - Data Link• Pizza 1 - Physical

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Remembering the OSI Model

• Away 7 - Application• Pizza 6 - Presentation• Sausage 5 - Session• Throw 4 - Transport• Not 3 - Network• Do 2 - Data Link• Please 1 - Physical

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Layer 1 – Physical Layer

• The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems.

• Physical layer specifications define characteristics such as:

• voltage levels

• timing of voltage changes

• physical data rates

• maximum transmission distances

• physical connectors

• Physical layer implementations can be categorized as either LAN or WAN specifications.

• more on next slide…

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Layer 1 – Physical Layer – Continued…

• Examples of LAN specifications are:

• Ethernet

• FastEthernet

• Token Ring

• FDDI

• Examples of WAN specifications are:

• HSSI

• V.24

• V.35

• BRI

• RS-232

• Transmits bits. (bitstream)

• Repeaters operate at this layer.

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Some Layer 1 Examples…

10 Base 2 – Co Ax

Fibre Optic Connectors

10 Base 5 – Thicknet

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Layer 2 – Data Link Layer

• Defines psychical addressing, network topology, and is also concerned with error notification, sequencing of frames and flow control.

• Examples of network topologies are:

• Star

• Bus

• Ring

• Physical addresses are also known as hardware and BIA's (Burned In Addressess) but most commonly as MAC addresses.

• Examples of Data Link LAN specifications are:

• Ethernet

• FastEthernet

• Token Ring

• FDDI

• More on next slide…

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Layer 2 – Data Link Layer – Continued…

• Examples of Data Link WAN specifications are:

• Frame Relay (operates also on the Physical layer)

• PPP (operates also on the Physical layer)

• X.25 (operates also on the Physical and Network layer)

• Transmits Frames.

• Bridges and Switches operate at this layer.

• The Data Link layer consists of two sublayers:

• LCC (Logical Link Control) sublayer

• Manages communication between devices over a single link of a network.

• Enables multiple higher-layer protocols to share a single physical data link.

• MAC sublayer

• Manages protocol access to the physical network medium.

• Determines hardware addresses.

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Some Layer 2 Examples…

Network Interface Card (NIC)

Workgroup Switch

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Layer 3 – Network Layer• Defines logical addressing for nodes and networks/segments.

• Enables internetworking, passing data from one network to another.

• Defines the logical network layout so routers can determine how to forward packets trough an internetwork.

• Routing occurs at this layer, hence Routed and Routing protocols reside on this layer.

• Routed protocols are used to encapsulate data into packets. The header added by the Network layer contains a network address so it can be routed trough an internetwork.

• Examples of Network layer Routed protocols are:

• IP, IPX, AppleTalk

• Routing protocols are used to create routing tables; routing tables are used to determine the best path / route. Routing protocols provide periodic communication between routers in an internetwork to maintain information on network links in a routing table.

• Examples of Network layer Routing protocols are:

• OSPF, IGRP/EIGRP, RIP, BGP.

• Transmits Packets.

• Routers operate at this layer.

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Layer 4 – Transport Layer

• The main purpose of this layers is making sure that the data is delivered error-free and in the correct sequence.

• Establishes, maintains and terminates virtual circuits.

• Provides error detection and recovery.

• Is concerned with reliable and unreliable transport. When using a connection-oriented, reliable transport protocol, such as TCP, acknowledgments are send back to the sender to confirm that the data has been received.

• Provides Flow Control and Windowing.

• Provides multiplexing; the support of different flows of data to different applications on the same host.

• Examples of Transport layer protocols are:

• TCP (connection-oriented, reliable, provides guaranteed delivery.)

• UDP (connectionless, unreliable, less overhead, reliability can be provided by the Application layer)

• SPX

•Transmits Segments.

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Layer 5 – Session Layer

• The session layer establishes, manages, maintains and terminates communication channels between software programs on network nodes. • Provides error reporting for the Application and Presentation layer. • Examples of Session layer protocols are:

• NFS • SQL • RPC

•Transmits Data.

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Layer 6 – Presentation Layer

• Defines coding and conversion functions. • Ensures that information sent from the application layer of one system is readable by the application layer of another system. • Includes common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes, common examples of these formats and schemes are:

• MPEG, QuickTime • ASCII, EBCDIC • GIF, TIFF, JPEG

• Transmits Data.

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Layer 7 – Application Layer

• Provides network services directly to applications. Software programs itself are not part of the OSI model. • Determines the identity and availability of communication partners, and determines if sufficient resources are available to start program-to-program communication. • This layer is closest to the user. • Examples of Application layer protocols are:

• Telnet • SMTP • FTP • SNMP

• Transmits Data.

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Data Encapsulation

• Data Encapsulation is the process of adding a header to wrap the data that flows down the OSI model.

• Each OSI layer may add it's own header to the data received from above. (from the layer above or from the software program 'above' the Application layer.)

• The 5 Steps of Data Encapsulation are:

1. The Application, Presentation and Session layers create DATA from users‘ input.

2. The Transport layer converts the DATA to SEGMENTS

3. The Network layer converts the SEGMENTS to PACKETS (or datagrams)

4. The Data Link layer converts the PACKETS to FRAMES

5. The Physical layer converts the FRAMES to BITS.

• At the sending computer the information goes from top to bottom while each layers divides the information received from upper layers in to smaller pieces and adds a header. At the receiving computer the information flows up the model discarding the corresponding header at each layer and putting the pieces back together.

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Data Encapsulation Diagram

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What are PDU’s?Each layer contains a Protocol Data Unit (PDU). PDU’s are used for peer-to-peer conversations.

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The TCP/IP Model

• The TCP/IP Model was develop by the Department of Defense in the late 60s’ to ensure data communications would not be interrupted even under the worse circumstances.

• Since then, TCP/IP has become the de facto method we use for data communications on the Internet.

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Exploring the 4 Layers of the TCP/IP Model

Application

Transport

Internet

Network Access

Includes all the functions of the OSI’s Application, Presentation, & Session layers including:

• Data representation

• Data encryption and

• Dialog control

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Application Layer Protocols

Application

Transport

Internet

Network Access

• FTP-File Transfer Protocol

• HTTP-Hypertext Transfer Protocol

• SMTP-Simple Mail Transfer Protocol

• DNS-Domain Name Service

• TFTP-Trivial File Transfer Protocol

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Application

Transport

Internet

Network Access

Uses the TCP protocol and is responsible for quality of service issues including:

• Reliability

• Flow Control and

• Error Correction

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Transport Layer Protocols

Application

Transport

Internet

Network Access

• TCP-Transmission Control Protocol

• UDP-User Datagram Protocol

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Application

Transport

Internet

Network Access

Uses the IP protocol and is responsible for:

• Path determination and

• Packet switching.

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Internet Layer Protocols

Application

Transport

Internet

Network Access

• IP – Internet Protocol

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Application

Transport

Internet

Network Access

Includes all the functions of the OSI’s Data Link & Physical layers including:

• Processes required by IP to ensure a packet reaches its destination.

• All the various LAN & WAN Technologies such as 100BaseTX & Frame Relay.

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Network Access Layer Protocols

Application

Transport

Internet

Network Access

• LAN and WAN Technologies

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Comparing the 2 Models

Application

Transport

Internet

Application

Presentation

Session

Transport

Network

Data Link

PhysicalNetwork Access

TCP/IP OSI

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Why are there 2 models?

• TCP/IP is the most popular “protocol specific” model used on the Internet.

• However, TCP/IP does not cover all the protocols and standards we will be exploring.

• The OSI Model is “protocol independent.” Therefore, all topics covered in the curriculum can fit into its framework.

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What should I know for the test?

• The OSI Model’s layers & their key characteristics and functions.

• The process of data encapsulation and the various PDUs.

• The TCP/IP layers and its corresponding protocols.

• Comparison of the two model’s layers.

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Sample CCNA Question #1

In the TCP/IP model which layer would deal with reliability, flow control, and error correction?

A – Transport Layer

B – Internet Layer

C – Network Layer

D – Application Layer

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Sample CCNA Answer #1

In the TCP/IP model which layer would deal with reliability, flow control, and error correction?

A – Transport Layer

B – Internet Layer

C – Network Layer

D – Application Layer

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Sample CCNA Question #2

What is the control information called that is placed before data when encapsulating that data for network transmission?

A – Frame

B – Header

C – Capsule

D – Routing Information

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Sample CCNA Answer #2

What is the control information called that is placed before data when encapsulating that data for network transmission?

A – Frame

B – Header

C – Capsule

D – Routing Information

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The OSI Model Cisco Networking Academy Program – Semester 1 J i m W h i t e – 2 0 0 2 Semester 1...

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