OSI Reference model

In 1984, the International Organization for Standardization (ISO) developed the OSI Reference Model . OSI model is used to describe how information is transferred from one networking component  to another network component.

    OSI (Open System Interconnect) model separates the network communication process into seven layers, each layer has a different but specific processing  function. application Layer (layer 7), Presentation Layer (layer 6), Session Layer (layer 5), Transport Layer (layer 4), Network Layer (layer 3), Data Link Layer (layer 2), Physical Layer (layer 1).

* Application, Presentation and Session layer  call as "Upper Layer OR software Layer"
* Transport layer  call as "heart of OSI model"
* Network, Data link and Physical layer  call as "Lower Layer OR  Hardware Layer"

Devices and Protocols

Layer7:  Application Layer
    The seventh layer , or topmost layer, of the OSI Reference Model is the application layer, it is a responsible for providing network services to the users, it is also called as Desktop layer, Identification of all services done by using Port Numbers.
* Total No Of Port (0  to 65535)
* Reserved Port  (0 to 1023)
* Open Client Port (1024 to 65535)

Layer6: Presentation Layer
    The six layer , of the OSI Reference Model is the presentation layer, it is a responsible for converting data into standard format. the presentation layer determines how data is transmitted and represented to the user.
Example: ASCII, JPEG, MPEG, MP3 etc.
 function of presentation layer:
    * Encoding and Decoding
    * Encryption and Decryption
    * Compression and Decompression

Layer5: Session Layer
    The fifth layer, of the OSI Reference Model is the session layer, the session layer is responsible for initiating the  setup and teardown of connections. the session layer is responsible for setting up, maintaining and tearing down network connections.
Example: RPCs ( Remote Procedure Calls), NFS ( Network File System) and SQL( Standard Query Language ).

Layer4: Transport Layer
    The fourth layer, of the OSI Reference Model is the transport layer,it is a heart of OSI layer, it is responsible  for end to end connectivity. Transport Layer provides a variety of services between two host computers, including connection establishment and termination, flow control, error recovery, and segmentation of large data blocks into smaller parts for transmission.
Following task are performed in this layer:
* Identifying Services.
*  Multiplexing and De-multiplexing
* Segmentation
* Sequencing and Reassembling
*  Error Correction
* Flow Controls
* Windowing

Layer3: Network Layer
      The third layer, of the OSI Reference Model is the network layer, it is responsible for providing best path for data to reach the destination. logical addressing works on this layer, router and layer 3 switch are  network layer devices.
Function of Network layer:
* Defines logical addresses used at layer3.
* Find best path, based on the network numbers of logical addresses, to reach destination components
* Connects different data link layer types together, such as Ethernet, fiber distributed data interface (FDDI), Serial, and Token Ring.
In this layer two type of protocol used:
>> Routed Protocol
    ex. IP, IPx, Apple Talk
>> Routing Protocol
    ex. RIP, IGRP, OSPF, EIGRP

Layer2: Data Link Layer
      The second layer, of the OSI Reference Model is the data link layer, it provides  for logical addresses for components, the data link layer provides for physical, or hardware addresses (MAC). Formats data into frames appropriate for transmission onto some physical medium. Defines rules for when the medium can be used. Defines means by which to recognize transmission errors.
Data Link Layer divided into two sub layer:
>> LLC ( Logical Link Control ) it is a WAN Protocol
    ex. PPP, HDLC, Frame Relay
>> MAC ( Media Access Control ) it is a physical address, it is a 48 bits address.
Data Link Address Type:
* Unicast     --> Represents a single device on a segment
* Broadcast --> Represents every device on a segment
* Multicast  --> Represents a group of devices on a segment

Layer1: Physical Layer
      The first layer, of the OSI Reference Model is the physical layer, it is a responsible for the physical mechanics of a network connection, data will be connected into Binary 0`s and 1`s, Data will be in the form of electrical pulse ,if it is coaxial or twisted pair cable. Defines the electrical, optical, cabling, connectors, and procedural details required for transmitting bits, represented as some form of energy passing over a physical medium.
* The type of interface (NIC) used on the networking device
* The type of cable (fiber, coaxial, twisted pair ) used for connecting devices
*The connection used on each end of the cable.

PDU Terms in OSI Reference Model