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 [email protected]
Class dismissed!