Transcript The OSI Model Definition

THE OSI REFERENCE
MODEL
Open 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
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.
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Internationally recognized: Established by the International Organization of Standardization
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Non-proprietary: Public domain. Can be freely used by any vendor.
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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.
OSI Model Definition – 7 Layers
 Layer 1 – Physical
 Please
 Layer 2 – Data Link
 Do
 Layer 3 – Network
 Not
 Layer 4 – Transport
 Throw
 Layer 5 – Session
 Sausage
 Layer 6 – Presentation
 Pizza
 Layer 7 - Application
 Away
Why is the OSI model important?
 To the computer industry:
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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
Application
Presentation
Session
Transport
Network
Data Link
Physical
The 7 Layers of the OSI Model
Layer 7 - Application
The Application layer provides services to the software
through which the user requests network services.
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Examples:
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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:
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Microsoft Word or Excel
Adobe Photoshop
Physical
– Data Link – Network – Transport – Session – Presentation - Application
Layer 6 - Presentation
Manages data-format information for networked
communications (the network’s translator)
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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
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This layer is also responsible for certain protocol conversions, data encryption/decryption, or data
compression/decompression
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Examples:
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MIDI
JPG, GIF, TIF
MPEG
Physical
– Data Link – Network – Transport – Session – Presentation - Application
Layer 5 - Session
The Session layer establishes, maintains, and manages the
communication session between computers.
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Responsible for initiating, maintaining and terminating sessions
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Responsible for security and access control to session information (via session participant
identification)
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Responsible for synchronization services, and for checkpoint services
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Examples:
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NFS
SQL
RPC
Physical
– Data Link – Network – Transport – Session – Presentation - Application
Layer 4 - Transport
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.
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Manages the transmission of data across a network
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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)
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Provides acknowledgements of successful transmissions and requests retransmission for packets
which arrive with errors (error detection and recovery)
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Examples:
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TCP
UDP
Physical
– Data Link – Network – Transport – Session – Presentation - Application
Layer 3 - Network
The Network layer defines the processes used to route data
across the network and the structure and use of logical
addressing.
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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.)
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Responsible for deciding how to route transmissions between computers
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This layer also handles the decisions needed to get data from one point to the next point along a
network path
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This layer also handles packet switching and network congestion control
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Example:
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IP
Network routers
Physical
– Data Link – Network – Transport – Session – Presentation - Application
Layer 2 – Data Link
Concerned with the linkages and mechanisms used to move
data about the network and deals with the ways in which
data is reliably transmitted.
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Handles special data frames (packets) between the Network layer and the Physical layer
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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
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The data link layer is often conceptually divided into two sub-layers: logical link control (LLC) and
media access control (MAC).
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Examples:
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Network bridges
Ethernet
Wi-Fi
Physical
– Data Link – Network – Transport – Session – Presentation - Application
Layer 1 - Physical
This layer defines the electrical and physical specifications
for the networking media that carry the data bits across a
network.
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Converts bits into electronic signals for outgoing messages. Converts electronic signals into bits for
incoming messages. (Performs bit synchronization.)
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This layer manages the interface between the computer and the network medium (coax, twisted
pair, etc.)
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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
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Examples:
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Network hubs and repeaters
LAN and WAN topology
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?
Questions?
 Homework
 Review the definitions and
examples of each layer
 Take the test
 Repeat as necessary
Additional Resources
 http://georgech.com/osi-model/
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Presentation and class notes
Homework
Links for additional information and reference
Glossary of terms
Class forum
 George Chmielecki
 [email protected]
Class dismissed!