OSI Model

Introduction

The OSI Model when it was first created came out in two parts :

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* The First part was the **Abstract Model**.
* The actual model with its seven layers.
* The Second part was actually a **Set of specially created protocols**, that were designed to illustrate how the protocols would fit inside this **abstract model**.
* Protocols not actually used in any network system.
* Devices to teach how protocols work with Networks to carry data.
* Model for organizations creating new protocols.
* Stands for Open Systems Interconnection Reference Model.
* Created as a Reference Model and Teaching aid.
* Not intended to reflect any actual network architecture.

Mnemonics for memorizing these Layers in the proper order :

  • All People Seem To Need Data Processing.
  • Please Do Not Throw Sausage Pizza Away

Layer 1 - Physical Layer

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* Bottom Layer
* Concerned with Physical transmission of raw Data.
* Transmits data in form 1s and 0s.
* Defines encoding methods to transmit Data.
* Defines how bits are placed on media.
* Defines how to know when bits Start & Stop.
* Defines specifications for media usage.
* Defines kinds of media permitted.
* Defines how physical connections are made
* Defines pin usage in physical connections.
* Specifies the Standards that apply to specific types of media.

Layer 2 - Data Layer

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* Provides error-free transmission from one node to the next over physical media.
* Establishes and terminates links between nodes.
* Responsible for traffic control.
* Transmits and receives frames sequentially.
* Responsible for frame acknowledge.
* Provides and expects frame acknowledge.
* Detects and recovers from errors on Physical Layer.
* Retransmits no acknowledged.
* Handles duplicate frame receipt.
* Responsible for frame delimiting.
* Creates and recognizes frame boundaries.
* Responsible for error checking
* Checks received frames for Data integrity.
* Provides media access management.
* Determines when node is allowed to use physical media.

Layer 3 - Network Layer

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* Controls the operations of the subnetwork it is on.
* Determines best physical path for data.
* Uses network conditions to choose best path.
* Uses priority of service to determine best path.
* Uses other factors to determine best path.

Layer 4 - Transport Layer

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* Ensures messages delivered error-free.
* Ensures messages delivered in sequence.
* Ensures messages delivered with no loss or duplication.
* Relieves higher protocols of concern for transfer of data.
* Size and complexity of transport protocols dependent on service provided by network layer.

Message Segmentation Function :
* Accepts messages from session layer
* Splits message into smaller units
* Imposes message size limits on network layer protocols
* Prepares header for each smaller unit created
* Passes smaller units to network layers
* Reassembles message at destination
* Header for smaller units contain certain elements
* Header contains start and end flag
* Header contains sequence information

Other Transport Layer Functions :
* Message acknowledgment
* Provides reliable end-to-delivery of messages.
* End-to-end delivery accompanied by acknowledgments.
* Message traffic control
* Controls rate of traffic send when no buffers available
* Session multiplexing
* Breaks all the data coming in on one link into separate data streams
* Those data streams called sessions.
* Tracks which message belongs to which session.

End-to-End Layers :
* Transport layer above layers not responsible for transmission between nodes.
* Transport and above layers responsible for source to destination transmission
* Source-to-destination transmissions also called end-to-end transmissions.
* Upper layers not concerned with underlying communications facility

Layer 5 - Session Layer

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* Responsible for establishing sessions between processes running on different computers.
* Provides several function to accomplish this.
* Session establishment, maintenance, and termination.
* Session support

Establishment Maintenance Termination :

  • Allows application processes on different machines to do several things between the machines.
  • Allows processes to establish a connection.
  • Allows processes to use a connection.
  • Allows processes to terminate a connection.
  • Each connection called a session.

Session Support :

  • Performs the function of allowing processes to communicate over network.
  • Performs security
  • Performs name recognition.
  • Performs logging on.
  • Performs other functions that are less common.

Layer 6 - Presentation Layer

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* Formats data to be presented to the application layer.
* Translator for the network.
* At sending station translates data from application layer format to common format.
* At receiving station translates data from common format to format used by application layer.

Presentation Layer Functions :
* Character code translation.
* ASCII to EBCDIC
* Data conversation.
* Bit order
* CR-CR/LF
* Integer-floating point
* Data compression
* Reduces number of bits needed to transmit data.
* Data encryption
* Encryption of data for security purposes.
* Encryption of passwords.

Layer 7 - Application Layer

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* Serves as window for uses and applications to access network service.
* Provides a variety of commonly used functions.

Application Layer Common Function :

  • Resource sharing.
  • Device redirection
  • Remote file access.
  • Remote printer access.
  • Network management.
  • Directory services.
  • Email
  • Instant messaging