networkwolves 1:07 pm on February 24, 2015
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OSI Reference Model

All networks are use in today are based on concept of Open System Interconnection (OSI) Reference Model developed in 1984 by ISO (International Organization of Standards). Instead, ISO decided to established a common reference model for which other could then develop detailed interfaces, which in turn could become standards.

The OSI Model contains seven layers to perform communication. A layer serves the layer above it  and is served by the layer below it. Performing simultaneously from top layer to ground layer and connected horizontally to another instance.

OSI-1

OSI Model

The OSI model take the task and divide it into vertical stacks that is referred to different layers.

Application Layer

It is top on structure where end user themselves interact directly with network application. It is responsible for providing services to user.

Functionality:

  • It runs the authentication services to ensure the end user.
  • Determining the resources availability.
  • Synchronizing communication between applications layers of both ends.

Protocol supported: Firewalls ,Email Protocols (SMTP and POP3) ,Telnet ,HTTP ,FTP , Simple Network Management Protocol (SNMP).

Presentation Layer

It deals ” how should this data be presented “. It deals with syntax of data ensuring must be recognizable to the receiver providing properly data formatting and encryption. May be two application layers are using different syntax then presentation layer will do this translation part , providing freedom from compatibility problems.

Functionality:

  • Compatibility with operating system.
  • Proper encapsulation of data for network transmissions.
  • Data formating (ASCII , Binary)
  • Data encryption, compression and translation.

Protocol supported: ASCII, JPEG, GIF ,MPEG

Session Layer

It is the manager of two way communication between two remote hosts. It control the connection between local and remote application. Here overall communication called ” session “.

Functionality:

  • Handle the creation of session.
  • Maintenance of communication and exchanging the dialogues.
  • Termination of communication and restart procedure.

Protocol supported: RPC ,PAP ,NFS ,SQL.

Transport Layer

Fourth layer of OSI Model responsible for end to end communication between source to destination via one or more networks while maintain g the quality of service. Information is in the form of “Segments”.

Functionality:

  • Maintaing the communication between end to end.
  • Providing error free information to the destination.
  • Error correction and error recovery.
  • Retransmission on timeout if info are lose.
  • Transmitting the information in proper sequencing to destination.

Protocol supported: TCP and UDP.

Network Layer

Third Layer of OSI Model responsible to end to end communication between source to destination with in same network. Its using logical addressing called ” IP address “.Information is in the form of “Packets”.

Functionality:

  • Handles addressing and routing of data.
  • Finding valid path from source to destination called it routing.
  • Performing translation of logical address into physical address (Mac address).
  • Performing network congestion control and packet switching.

Protocol supported: IP ,IPX ,AppleTalk.

Datalink Layer

Second layer of OSI model performing host to host data transfer on directly connected link by error detecting and correction. Information is in the form of “frames”.

Functionality:

  • Receiving raw data from physical layer and packages to network layer and vice-versa.
  • Control how a computer on network gain access to data and permission to transmit it.
  • Performing frame synchronization and error checking not error recovery.
  • Use ” MAC addressing ” to identify the host in network.

Protocol supported: HDLC ,PPP ,frame-relay ,FDDI ,ATM.

Physical Layer

Bottom layer of OSI model defining the relationship of device to physical medium. All information is in the form of bits i.e 1s and 0s.

Functionality:

  • Receiving raw data from physical medium packages back to data link layer.
  • Transmitting all signal in the form of bits via physical medium.
  • Defines the protocol between two nodes over a communication medium.
  • Defines transmission mode i.e simplex, half duplex and full duplex.
  • Defines topology.
  • Defines modulation and demodulation of signals over devices.

Protocol supported: DSL ,USB ,Ethernet ,RJ-45 ,token-ring.

Flow of Information in OSI Model

Information is flow from top layer to bottom layer. Each layer adding their header to data received from upper layer process it and transfer it to lower layer. On other side of OSI model information flow from bottom layer to top layer while each layer remove their header to data received from lower process it and transfer to upper layer. Both OSI model Instance connected via communication medium.

Only layer 2 Datalink layer add both header and trailer to data and also strips both header and trailer on other side of OSI model. While other layers only adds and strips header.