ISO-OSI Reference Model

ISO-International Organization for Standardization

OSI-Open System International

            When networks first came into being, computers could typically communicate only with computers from the same manufacturer.

E.g., IBM PC can be Communicate only with same type of IBM PCs and it is not a flexible communication.

The OSI was model meant to help vendors create interoperable network devices and software in the form of protocols, so that different vendor’s networks could work with each other and it was introduced by ISO. By this help of OSI model can communicate.

  • different manufactures PCs
  • different hardware configuration
  • different software
  • Protocol support.

            OSI model is the primary architectural model for networks. It describes how data and network information are communicated from an application on one computer through the network.

A reference model is a conceptual blue print of how communication should take place between two computers. It addresses all the process required for effective communication and divides the process into logical grouping called layers.


  • It is an international level of standard.
  • Define what function occurred at each layer.
  • Simple component design
  • Trouble shooting become easy
  • Prevent changes to data travel from one layer to another
  • Simplifies teaching and learning
  • Reduce complexity
  • It encourages industry standardization by defining what function occurs at each layer of model.
  • Different devices, protocols can work with this process.

In OSI, concept total communication process has divided in to 7 layers. Based on hardware or software supports seven layers have divided in to two.

  1. Upper layer
  • Lower layer
  1. Upper layer

Upper layer works based on software or operating system and in each upper layer data known as protocol data unit (PDU) such as Application Protocol Data unit, Presentation  Protocol data unit etc.

  • Lower layer

Lower Layers works based on hardware components. The main function of upper layer is to send, formatted data from source to destination with some added information (headers). In each lower layer data identified with different names such as segment (in transport layer), packet (in network layer), frame (in data link layer) and bits (in physical layer).

Encapsulation:- The process of adding protocol headers with data is known as encapsulation.

De-encapsulation: – The process of removing protocol headers from data is known as de-encapsulation.

In communication system source devices takes place encapsulation and destination devices takes place de-encapsulation.

Functions of each layer

              Source                                                                              Destination

Application Layer:

When an application program that runs our application (eg: the request is handover to application layer. It is the layer, which interacts with user directly. It receives application from the user. Data communication starts from these layers. Its function is to check data availability (it checks is available or not) and if the data exists transfer data to presentation layer.

Protocol support in Application Layer

  • HTTP – Hyper text transfer protocol
  • FTP – File transfer protocol
  • SMTP – Simple Mail Transfer Protocol
  • SNMP – Simple Network Management Protocol
  • POP3 – Post Office Protocol
  • TFTP – Trivial File transfer Protocol
  • Telnet

Each layer passes certain information to next layer.

Presentation Layer

It translates data to common format such as

            * MPEG => moving picture Expert group

            * JPEG => Joint photographic Expert group

            * MP3

            * .TXT

            It also encrypts or decrypts Data Compression and Decompression takes place in this layer. The output of presentation layer is presentation PDU.

Session Layer

Session Layer establishes a logical connection from source to destination. It synchronizes dialog between the presentations Layer of two hosts and manage their data exchange. I.e. what type of communication to done (simplex, half-duplex, full duplex)? It is also responsible for keeping upper layer data separately.

A session is a particular time duration for which logical connection is present. This Layer establishes, manages and terminates session between two communicating hosts.

Session management protocols

NFS – Network File System (UNIX, Linux)

ASP – Apple talk session Management protocol

            [Protocol used MAC OS]

Transport Layer

The transport layer divides large amount of data in to smaller units. This is known as segmentation process. These small blocks are called segments. Transport layer add its own segment it is known as TCP Header. The header contains sequence number port information etc. It also establishes, maintains, and terminates virtual circuits with Destination.  It also ensures reliability Fault identification & corruption. Transport layer add port information with data.

  • Source port number

Source port numbers are system generated port address in the range of 1024 to 65535.

  • Destination port number

Destination port numbers are wellknown port numbers in the range of 0 to 1023. It is also known as protocol number because it is the predefined number for each protocol such as port 80 is HTTP, Port 23 is telnet etc.

Port number is used to create a virtual connection. It is a dedicated path used for communication between application Layer protocol and transport layer.

Protocol used in the transport layer

            TCP- Transmission Control Protocol

            UDP-User Datagram Protocol


  • Security ,
  • efficient transmission ,
  • connection oriented protocol,
  • data is send only after a connection is established,

Features of TCP

  1. Acknowledgement –available reply from destination.
  2. Windowing– To control transmission, acknowledgement after sending certain segment. When each data reaches destination, sending acknowledgement back to source consumes more time. So there fix an acknowledgement for a group of segment. It is called windowing.
  3. Error correction- when ack does not reaches after particular time the data will again transmit (retransmit)to destination.
  4. Flow control- it controls the flow of data which can accept by the destination.


  • Connectionless protocol.
  • No reliability available
  • Higher level programs run in UDP
  • Broadcasting-no need for acknowledgement
  • UDP is faster than TCP

Connection-Oriented Communication

In reliable transport operation, a device that wants to transmit sets up a connection-oriented communication with a remote device by creating a session. The transmitting device first establishes a connection-oriented session with its peer system, which is called a call setup or a three-way handshake. Data is then transferred; when the transfer is finished, a call termination takes place to tear down the virtual circuit. This is called four way hand shaking process.

Figure 1depicts a typical reliable session taking place between sending and receiving systems.

Steps in the connection-oriented session—the three-way handshake.

  • The first “connection agreement” segment is a request for synchronization.
  • The second and third segments acknowledge the request and establish connection parameters—the rules—between hosts. These segments request that the receiver’s sequencing is synchronized here as well so that a bidirectional connection will formed.
  • The final segment is also an acknowledgment. It notifies the destination host that the connection agreement has been accepted and that the actual connection has been established. Data transfer can now begin.

Network Layer

Network layer selects best path to deliver data. It does logical addressing (IP addressing) or logical communication starts from this layer. It adds its own header known as IP header IP header contains Source and Destination IP address. Network layer data is known as packets.  Router works in network layer of OSI model.

Source IP address

It is the IP address of source system, when we connect system to internet ISP provide a public IP which is assigned as source IP.

Destination IP address

It is the IP address of destination system. For destination IP address, source machine send a request to ISP’s DNS server (DNS-Domain Name System). DNS server resolve name ( We get the destination IP address.


  • IP– Internet protocol
  • IPX– integrated packet exchange
  • ICMP– internet control message protocol
  • DHCP-dynamic host configuration protocol

Service used to provide IP address automatically.

Data link layer

Data link layer also known as layer 2. It works using physical address (MAC). It adds header and footer with data. Header known as Ethernet header and footer is CRC footer. Ethernet header contains source MAC address and Destination MAC address. The data link layer data is known as frame. CRC footer is used for error checking.

Source MAC address- It is the MAC address of source system.

Destination MAC address- For destination MAC address, source system broadcast an ARP (Address resolution protocol) request.

ARP request

Source IP Destination IP
Source MAC ?

 Hence, we do not know the DMAC an ARP request will send. The request contains destination IP source IP and source MAC.

ARP reply

Source IP Destination IP
Source MAC Destination MAC


  • Ethernet
  • ARP – Address Resolution Protocol
  • RARP-Reverse ARP
  • Token ring
  • FDDI – Fiber Distributed Data Interface
  • ATM – Asynchronous Transfer Mode

By the concept of IEEE data link layer is divided in to two.

  1. LLC-Logical Link Control
    1. MAC-Media Access Control, sub layer


Used for identifying upper layer protocols (TCP/IP)

MAC sub layer

MAC defines physical media access information


  1. bridge
  2. switch
  3. WAP

Physical Layer

Physical layer provides Physical connectivity between devices. It converts data in to signals depending upon the medium. It also selects signaling standard and convert data to bit streams.


  •  Hub
  • Repeater

In Destination

  • Data transmitted through medium and reach physical layer of destination.
  • Destination physical layer store the signals together like a buffer.
  • Then data handover to data ink layer.
  • Data link breaks the header (Ethernet) and footer that are added in the data link layer of source. It checks destination MAC address as it is same as destination and also checks CRC footer. When header and footer removed, data transported to network layer.
  • In the network layer IP header breaks which added in the source. It checks destination IP, removes the header and handover the packet to transport layer.
  • Transport layer breaks TCP header which added in the source. Checks destination port address and the data reach to the application layer through the destination port number.
  • In the presentation layer data transferred to correct format. And application layer provides user interfaces.

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