# The Open Systems Interconnection model (OSI Model) is a conceptual model that characterizes and standardizes the internal functions of a communication system by partitioning it into abstraction layers. The model is a product of the Open Systems Interconnection project at the International Organization for Standardization (ISO), maintained by the identification ISO/IEC 7498-1.
An open system is a set of protocols that allow any two different systems to communicate regardless of their underlying structure. The purpose of OSI model is to show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software.The OSI model isn’t a protocol; it is a model for understanding and designing a network architecture that is flexible, robust and interoperable.
The model groups communication functions into seven logical layers. A layer serves the layer above it and is served by the layer below it. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of that path. Two instances at one layer are connected by a horizontal connection on that layer.
this picture is from: http://en.wikipedia.org/wiki/OSI_model
0SI MODEL LAYERS
| Layer | Protocol Data Unit (PDU) |
| Application | Data |
| Presentation | Data |
| Session | Data |
| Transport | Segment |
| Network | Packet |
| Data Link | Frame |
| Physical | Bits |
| Layer | Protocol Data Unit (PDU) |
| Application | Data |
| Presentation | Data |
| Session | Data |
| Transport | Segment |
| Network | Packet |
| Data Link | Frame |
| Physical | Bits |
between them there is a wire to send data
Layer 1: Physical layer
which
Defines the physical and electrical medium for data transfer
*Physical layer components: cables, jacks, patch panels, punch blocks, hubs, and MAUs
*Physical layer concepts: topologies, analog versus digital/encoding, bit synchronization, baseband versus broadband, multiplexing, and serial data transfer
*Unit of measurement: Bits
Layer 2 :Data link layer
which
Establishes, maintains, and decides how transfer is accomplished over the physical layer and ensures error-free transmission over the physical layer
*Physical addresses (the hexadecimal address that is burned into the ROM of the NIC), known as the MAC address uniquely identify each hardware device work at the Data Link Layer
*Data Link Layer components: network interface cards and bridges
*Unit of measurement: frames
Layer 3: Transport layer
**This layer ensures messages are delivered error-free, in sequence and with no losses or duplications
*Protocols that work at this layer segment messages, ensure correct reassembly at the receiving end, perform message acknowledgement and message traffic control
*The Transport Layer contains both connection-oriented and connectionless protocols
*Unit of measurement used: segments or messages
Layer 4 :Network layer
which is:
Controls the operations of routing and switching information to different networks
Translates logical addresses or names to physical addresses
*Internet Protocol (IP) is a Network Layer protocol
*Devices that work at the network layer are routers and IP switches
*Network Layer components: IP addresses, subnets
*Unit of measurement: packets
Layer 5: Session layer
The Session Layer manages session establishment, maintenance and termination between network devices
Example: when you log on and log off
*This layer controls the name and address database for the OS
*NetBIOS (Network Basic Input Output System) is a protocol that works at this layer
Layer 6 : Presentation layer
This layer translates the data format from sender to receiver in the various OSes that may be used
*Presentation Layer concepts include: character code conversion, data compression, and data encryption
*Redirectors work on this layer, such as mapped network drives that enable a computer to access file shares on a remote computer
Layer 7: Application layer
Serves as a the window for users and application processes to access network services
*This layer is where message creation begins
End-user protocols such as FTP, SMTP, Telnet, and RAS work at this layer
*This layer is not the application itself, but the protocols that are initiated by this layer
Comparison between OSI Model & TCP Model
| OSI Model | TCP Model |
| Application Layer | Application Layer |
| Presentation Layer | |
| Session Layer | |
| Transport Layer | Transport Layer |
| Network Layer | Internet Layer |
| Data Link Layer | Network Access Layer |
| Physical Layer |
** The OSI Physical layer id skipped altogether on the TCP Model
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