Open system interconnection (OSI)  developed by ISO in 1974. It is not a physical model, its a set of guide lines for how to data transfer.OSI consist 7 layers. each layers have different functions.  OSI communicate with various hardware and software networks.Each layer functions are given.

OSI Model

   Layer 7 :Application Layer
      It is responsible for providing networking services to user. eg. HTTP,FTP,DNS,DHCP...etc.It is hardware software interface.its also known as desktop layer.Identification of services is done using port numbers.Ports are entry and exit unit of layer.
  Layer 6 :Presentation Layer
      It is Responsible for converting data into standard format.eg MPEG , JPEG , MP3 ,ASCII...etc.Presentation layer is responsible for Encoding and Decoding , Encryption and Decryption , Compression and Decompression.
 Layer 5 :Session Layer
      It is responsible for establishing ,maintaining and terminating session.it also provide dialog control between devices.it coordinate communication by offering 3 different modes simlex , half duplex , full duplex.
 Layer 4 :Transport Layer
     It is responsible for an end to end connection.Data from upper layer is cut from this layer called segments. .Following tasks are performed at transport layer

• Identifying Services
• Multiplexing and Demultiplexing
• Segmentation
• Sequencing and Reassembling
• Error Correction
• Flow control

Layer 3 :Network Layer
    This layer create packets from segments by adding header and recreate segments. Its responsible to find the best path for packets to destination.This layer mainly use TCP/IP protocol .
Layer 2 :Data link Layer
     Datalink layer is divided into 2,
         Media Access Control(MAC)
         Logical Link Control(LLC)
 Media Access Control(MAC) : It define how packets are placed on the media. Physical addressing
 (MAC address) and logical topologies are defined here.
 Logical Link Control(LLC) : It is responsible for identifying network layer protocol and encapsulating
  packets.
Layer 1 :Physical Layer
  It is used to perform data transmission in  the help of media. Hub,Cables,Modems are working at physical layer.