THE OSI REFERENCE MODELOpen Systems Interconnection Reference Model

Agenda

Define OSI Model

Explain why it is important

Review each of 7 levels Definition Examples

OSI Packaging

Review

The OSI Model Definition

Internationally recognized: Established by the International Organization of Standardization

Non-proprietary: Public domain. Can be freely used by any vendor.

The OSI model is a layered model that describes how information moves from an application program running on one networked computer to an application program running on another networked computer.

Open Systems Interconnection (OSI) Reference Model is a set of internationally recognized non-proprietary standards for

networking and for operating systems involved in networking functions.

OSI Model Definition – 7 Layers

Layer 1 – Physical

Layer 2 – Data Link

Layer 3 – Network

Layer 4 – Transport

Layer 5 – Session

Layer 6 – Presentation

Layer 7 - Application

Please

Do

Not

Throw

Sausage

Pizza

Away

Why is the OSI model important?

To the computer industry: Change Design Troubleshooting Training and education Interoperability between vendors

To you: Help with troubleshooting network problems Will be on certification exams

MCP CCNA

Will be asked in an interview

An Analogy

ApplicationPresentationSession

TransportNetwork

Data LinkPhysical

The 7 Layers of the OSI Model

Layer 7 - Application

Examples: Internet Explorer, Safari, and other browsers FTP Mail

Many applications that run on your computer are NOT part of the Application layer. This means that the following are not part of layer 7 because they do not request network services: Microsoft Word or Excel Adobe Photoshop

The Application layer provides services to the software through which the user requests network services.

Physical – Data Link – Network – Transport – Session – Presentation - Application

Layer 6 - Presentation

For outgoing messages, it converts data into a generic format for network transmission; for incoming messages, it converts data from the generic network format to a format that the receiving application can understand

This layer is also responsible for certain protocol conversions, data encryption/decryption, or data compression/decompression

Examples: MIDI JPG, GIF, TIF MPEG

Manages data-format information for networked communications (the network’s translator)

Physical – Data Link – Network – Transport – Session – Presentation - Application

Layer 5 - Session

Responsible for initiating, maintaining and terminating sessions

Responsible for security and access control to session information (via session participant identification)

Responsible for synchronization services, and for checkpoint services

Examples: NFS SQL RPC

The Session layer establishes, maintains, and manages the communication session between computers.

Physical – Data Link – Network – Transport – Session – Presentation - Application

Layer 4 - Transport

Manages the transmission of data across a network

Manages the flow (flow control) of data between parties by segmenting long data streams into smaller data chunks (based on allowed “packet” size for a given transmission medium) (packet sequencing)

Provides acknowledgements of successful transmissions and requests retransmission for packets which arrive with errors (error detection and recovery)

Examples: TCP UDP

The functions defined in this layer provide for the reliable transmission of data segments, as well as the disassembly and assembly of the data before and after transmission.

Physical – Data Link – Network – Transport – Session – Presentation - Application

Layer 3 - Network

Handles addressing messages for delivery, as well as translating logical network addresses and names into their physical counterparts (Logical Addresses are managed by local network admins.)

Responsible for deciding how to route transmissions between computers

This layer also handles the decisions needed to get data from one point to the next point along a network path

This layer also handles packet switching and network congestion control

Example: IP Network routers

The Network layer defines the processes used to route data across the network and the structure and use of logical

addressing.

Physical – Data Link – Network – Transport – Session – Presentation - Application

Layer 2 – Data Link

Handles special data frames (packets) between the Network layer and the Physical layer

At the sending end this layer handles conversion of data into raw formats that can be handled by the Physical Layer. At the receiving end, this layer packages raw data from the physical layer into data frames for delivery to the Network layer

The data link layer is often conceptually divided into two sub-layers: logical link control (LLC) and media access control (MAC).

Examples: Network bridges Ethernet Wi-Fi

Concerned with the linkages and mechanisms used to move data about the network and deals with the ways in which

data is reliably transmitted.

Physical – Data Link – Network – Transport – Session – Presentation - Application

Layer 1 - Physical

Converts bits into electronic signals for outgoing messages. Converts electronic signals into bits for incoming messages. (Performs bit synchronization.)

This layer manages the interface between the computer and the network medium (coax, twisted pair, etc.)

This layer tells the driver software for the MAU (media attachment unit) (eg. network interface cards (NICs), modems) what needs to be sent across the medium

Examples: Network hubs and repeaters LAN and WAN topology

This layer defines the electrical and physical specifications for the networking media that carry the data bits across a

network.

Physical – Data Link – Network – Transport – Session – Presentation - Application

The 7 Layers of the OSI Model

Summing up

What is the ISO Model?

Why is it important?

What are the 7 layers?

Homework Review the definitions and

examples of each layer Take the test Repeat as necessary

Questions?

Additional Resources

http://georgech.com/osi-model/ Presentation and class notes Homework Links for additional information and reference Glossary of terms Class forum

George Chmielecki george.chmielecki@verizon.net

Class dismissed!