OSI Model 7 Layers Explained PDF – Layers Functions-The Open Systems Interconnection model (OSI model) is a conceptual model that characterizes and standardizes the communication functions of a telecommunication or computing system without regard to their underlying internal structure and technology.

Its goal is the interoperability of diverse communication systems with standard protocols. The model partitions a communication system into abstraction layers. The original version of the model defined seven 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 comprise the contents of that path.

Two instances at the same layer are visualized as connected by a horizontal connection in that layer.

OSI Model 7 Layers Explained PDF - Layers Functions

OSI Model 7 Layers Explained PDF – Layers Functions

OSI Model 7 Layers Explained PDF – Layers Functions

What is OSI model?

The OSI model (Open System Interconnection) model defines a computer networking framework to implement protocols in seven layers. A protocol in the networking terms is a kind of negotiation and rule in between two networking entities.

Layers of OSI model:

Physical layer

The Physical layer is also called as the Layer 1. Here are the basic functionalities of the Physical layer:

  • Responsible for electrical signals, light signal, radio signals etc.
  • Hardware layer of the OSI layer
  • Devices like repeater, hub, cables, ethernet work on this layer
  • Protocols like RS232, ATM, FDDI, Ethernet work on this layer

The functions of the physical layer are :

  1. Bit synchronization: The physical layer provides the synchronization of the bits by providing a clock. This clock controls both sender and receiver thus providing synchronization at bit level.
  2. Bit rate control: The Physical layer also defines the transmission rate i.e. the number of bits sent per second.
  3. Physical topologies: Physical layer specifies the way in which the different, devices/nodes are arranged in a network i.e. bus, star or mesh topolgy.
  4. Transmission mode: Physical layer also defines the way in which the data flows between the two connected devices. The various transmission modes possible are: Simplex, half-duplex and full-duplex.

Data Link layer

The data link layer is also called as the Layer 2 of the OSI model. Here are the basic functionalities of the data link layer:

  • Responsible for encoding and decoding of the electrical signals into bits.
  • Manages data errors from the physical layer
  • Convers electrical signals into frames
  • The data link layer is divided into two sub-layers
    • The Media Access Control (MAC) layer
    • Logical Link Control (LLC) layer.
  • The MAC sublayer controls how a computer on the network gains access to the data and permission to transmit it.
  • The LLC layer controls frame synchronization, flow control and error checking.
  • MAC address is a part of the layer 2.
  • Devices like Switch work at this layer

The functions of the data Link layer are :

  1. Framing: Framing is a function of the data link layer. It provides a way for a sender to transmit a set of bits that are meaningful to the receiver. This can be accomplished by attaching special bit patterns to the beginning and end of the frame.
  2. Physical addressing: After creating frames, Data link layer adds physical addresses (MAC address) of sender and/or receiver in the header of each frame.
  3. Error control: Data link layer provides the mechanism of error control in which it detects and retransmits damaged or lost frames.
  4. Flow Control: The data rate must be constant on both sides else the data may get corrupted thus , flow control coordinates that amount of data that can be sent before receiving acknowledgement.
  5. Access control: When a single communication channel is shared by multiple devices, MAC sub-layer of data link layer helps to determine which device has control over the channel at a given time.

Network Layer

The Network layer is also called as the layer 3 of the OSI model. Here are the basic functionalities of the network layer:

  • Switching and routing technologies work here
  • Creates logical paths between two hosts across the world wide web called as virtual circuits
  • Routes the data packet to destination
  • Routing and forwarding of the data packets.
  • Internetworking, error handling, congestion control and packet sequencing work at this layer
  • Router works at layer three
  • Different network protocols like TCP/ IP, IPX, AppleTalk work at this layer

The functions of the Network layer are :

  1. Routing: The network layer protocols determine which route is suitable from source to destination. This function of network layer is known as routing.
  2. Logical Addressing: In order to identify each device on internetwork uniquely, network layer defines an addressing scheme. The sender & receiver’s IP address are placed in the header by network layer. Such an address distinguishes each device uniquely and universally.

Transport layer

The Transport  layer is also called as the layer 4 of the OSI model. Here are the basic functionalities of the Transport layer:

  • Responsible for the transparent transfer of data between end systems
  • Responsible for end-to-end error recovery and flow control
  • Responsible for complete data transfer.
  • Protocols like SPX, TCP, UDP work here

Session layer

The Session  layer is also called as the layer 5 of the OSI model. Here are the basic functionalities of the Session layer:

  • Responsible for establishment, management and termination of connections between applications.
  • The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end.
  • It deals with session and connection coordination.
  • Protocols like NFS, NetBios names, RPC, SQL work at this layer.

Presentation layer:

The Presentation layer is also called as the layer 6 of the OSI model. Here are the basic functionalities of the presentation layer:

  • Responsible for data representation on your screen
  • Encryption and decryption of the data
  • Data semantics and syntax
  • Layer 6 Presentation examples include encryption, ASCII, EBCDIC, TIFF, GIF, PICT, JPEG, MPEG, MIDI.

Application Layer:

The Application layer is also called as the layer 7 of the OSI model. Here are the basic functionalities of the Application layer:

  • Application layer supports application, apps, and end-user processes.
  • Quality of service
  • This layer is responsible for application services for file transfers, e-mail, and other network software services.
  • Protocols like Telnet, FTP, HTTP work on this layer.

The functions of the Application layer are :

  1. Network Virtual Terminal
  2. FTAM-File transfer access and management
  3. Mail Services
  4. Directory Services

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