Open system interconnection (OSI) model

Open System Interconnection Model (OSI model) is a reference model for communication between the end users in a network

    The Open system interconnection (OSI) Model, developed by the international standards Organization (ISO) describes the flow of information from one system to another.

         The ISO model is also called open system interconnection (OSI) reference model. It is a conceptual model that has seven layers

The open system interconnection reference model is defined as a stack of procedures or protocols consisting of seven logical layers each layer has a specific function and handles a unique data format.

    The OSI model groups and composes errands to transport ready information over the system. When two computers or devices communicate, data flows from the user-to-network interface the first layer starts at the bottom and rises upward to the seven layers of the stack.

Data flow from an upper layer to a lower layer results in conversion of data to a lower layer format with the addition of the lower header. This process is called Encapsulation

   When data flows from a lower layer to an upper layer, data is converted data is converted to the upper layer data format, and the lower layer header is discarded, this process is called Decapsulation.

Discussing networking concepts using the open system interconnection (OSI) Model has the following concepts:

  • Provides a common language or reference point between network professionals
  • Divides networking tasks into logical layers for easier comprehension
  • Allows specialization of features at different levels
  • Aids in Troubleshooting
  • Promotes standards interoperability between networks and devices.
  • Provides modularity in networking features (developers can change features without changing the entire approach)

Limitations of the Open System Interconnection (OSI) Model

  • OSI layers are theoretical and do not actually perform real functions
  • Industry implementations rarely have a layer-to-layer correspondence with reference to the OSI layers
  • Different protocol within the stack perform different functions that help send or receive the overall message
  • A particular protocol implementation may not represent every OSI layer (or may spread across multiple layers)

A graphical representation of the structure of the OSI Model

The OSI layer is Divided into 2 groups: Upper Layer and Lower Layer.

The Upper layer defines how applications communicate with each other.

The Lower layer defines the data transmission process that occurs between end-to-end devices.

(OSI) Open System Interconnection 7 Layer Structure

Upper Layer 
Application layer (data)Provides user interface (software, application works HTTP, HTTPS, FTP, SMTP protocol)
Presentation layer (data)Format of Data (image, jpeg, Audio Mp3, Video MP4, AVI, Encryption, Decryption, compression, Decompression)
Session layer (data)Create and Maintain the Session with Timeframe.  
Lower layer 
Transport layer (Segment)End to End delivery of data Two protocol works: TCP & UDP  
Network layer (packet)Add IP address source to destination
Datalink layer (Frame)Add MAC Address source to destination MAC and LLC
Physical layer (Bits)    Cables, Signals, connectors.

1) Physical Layer:

This layer is used to establish or terminate a connection to a communication medium.


It defines the electrical and mechanical specifications like cables, connectors and signaling options of the medium.

Converters from one media to another operate at this level.

This primary differentiator would be, cost effectiveness of installation, performance, convenience and maintenance it defines the cable or physical medium such as thin net, thick net, unshielded twisted pairs  

A Hub is a physical layer device which has very limited numbers of ports when a frame reaches from a source to the hub port it will broadcast to all ports including the source port which leads to a single broadcast.

    Hub Functions as a Half-duplex when a simultaneous data transfer (send & Receive) occurs on the same port. There is a greater possibility of collision occurring in the Hub environment.

Half-duplex: Data transmission is in a single direction at any given time, as a single wire pair enables only a one-way communication. This would lead to a collision. Full-duplex: Data transmission happens in two directions simultaneously as two wire pairs support a two-way communication  

2) Data Link Layer:

The data link layer organizes information on the system it handles data transfer between the network and the physical layers.

Switch: switch is a layer two device which connect multiple device in LAN segment and filter the frame by examining the MAC address. Frame: The data link layer where the mac address is used is called frame

Characteristics :

The data link layer receives data from the network layer

It adds the header and trailer to the data and passes data to the physical layer.

An information bundle comprises checksum, source & destination location, and also other information. The biggest bundle that can be sent through an information connection layer is called as the Maximum Transmission Unit (MTU)

Each data Unit is considered as a frame

A Data link layer protocol is used to move datagram frames over an individual link

Ex: MAC address flow in a datalink layer.

A host interface having a unique, 48-bit address is called as Ethernet address or MAC Address.

Hosts with multiple network interfaces should use the same MAC address on each interface. The data link layer protocol specific header denotes the MAC address of the packets source & destination when a packet is sent to all broadcast (host) a special MAC address is used. Switches work at the data link layer.

MAC address is usually represented as six colon separated pairs of hex digits, Ex: 8:0:15:12:ad:85 this no is unique and is associated with a particular Ethernet device (NIC)

MAC sub-layer:

This layer deals with the broadcast network and protocols. When many users require transmitting data over the media at the same time, MAC sub-layer helps determining multi-access based on protocol (CSMA/CD) MAC addressing and logical topologies are also defined.

Logical Link Control:

LLC is one of the sublayers in the data link layer which provides 1) flow control 2) Acknowledgement 3) Error notification

Network devices like bridges and switches operate at the data link layer.


Bridges are devices that are used to connect two LAN segments that use the same protocol.


Switches are intelligent devices with multiple ports and connects to multiple LAN segments.

Bridges and switches both have address learning mechanisms. Unlike hubs, these learn MAC addresses by broadcasting to all the connected ports except the source and generates a MAC table.

The MAC table is a list of learned addresses. Whenever a host transmits data the switch verifies with its MAC table if the address is found it will perform a unicast (one to one direction transfer) if the address is not available it does a broadcast until locating the address learn.

Bridges & switches support Full duplex (couple of wires send and receive data separately) which reduces chances of a collision.

Bridges use software for address learning while Switches use a hardware chip for address learning.

3) Network Layer:

The network layer determines the path and the logical IP Address.

A network layer is not require if two communicating devices lie on the same network however when the two devices connected on different networks a network layer is essential for providing source to destination delivery of packets.

Router: Router is a network layer device which connect different network to reach destination.
Packet: The network layer using the IP address is called packet.


When the packet intended for a device outside the network, other addressing structures required to identify the source and destination

The network layer adds a header that includes the logical address (IP) of the source and destination

Routers operate at this layer

The data unit at this layer known as packet

IP is responsible for routing. Directing datagrams from one network to another

EX: The typical process for a network layer is to break large datagrams, into smaller packets and the host receiving the packet will have to reassemble the fragmented datagram. At times the data may be larger than MTU IP identifies each host with a 32-bit IP address

IP addresses are written as decimal no, separated by four dots the no will be between 0 and 255

Even through IP packets are addressed using IP addresses must be used to actually transport data from one host to another.

The address Resolution Protocol (ARP) is used to map the IP address to its MAC address.

IP Routing Basics:

Routing is a process of identifying the best reachable path between source and destination.

Routers are used to connect and communicate between two or more different networks and between different geographical locations

Every router in a network will maintain a routing table to the destination either through static or dynamic routing.

OSI is a reference model for communication

Static Routing:

static routing is a form of routing that occurs when a router uses a manually configured route to forward a packet.

Dynamic Routing:

Dynamic routing provides optimal data routing unlike static routing; dynamic routing enables routers to select path according to real time logical network layout changes.

Routing Table:

A routing table is a list of known networks and destination networks learned through the routing process. The routing table provides detailed information about the interface through which it has to forward the metric to the destination. Every protocol has a different method of calculating metrics the best metric placed in the routing table every router maintains a routing table.

Metric: values used by routing protocol to reach destination called metrics.

4)Transport Layer:

This layer ensures that the entire message reaches its destination in order and handles the flow and error control at the source to destination level.


The data unit at this layer known as a segment

The transport layer uses two transport media

TCP (Transmission Control Protocol)UDP (User Datagram Protocol)
Connection orientedConnectionless
GuaranteeNo guarantee
AcknowledgementNo Acknowledgement
20 bytes add TCP information8 Bytes add UDP information

TCP Three-way Handshake

To establish a connection, TCP uses a three-way handshake.

Before a client makes attempt to connect with a server, the server should initially listen at a port to open it up for connections this referred to as a passive open.

Once the passive open established the client will initiate an active open.

Open system interconnection (OSI) model

The three-way handshake steps

1) SYN:

The active open performed by the client sending a SYN (synchronize) to the server the client sets the segments sequence number to a random value A.


Accordingly, the server answers with a Synchronize-Acknowledgement the acknowledged number placed at one more than the received sequence number A+1 the sequence number that the server chooses for the packet is another random no. B.


finally, the client sends an ACK back to the server the arrangement Number is set to the received acknowledgement value A+1 and the acknowledgment no. is set to one more than the received sequence number B+1

On completion of these actions, both client and server have received an affirmation of the connection

Connection oriented protocols require that a logical connection be established between two devices before transferring data.

Flow control:

TCP has a mechanism of managing and tracking the amount of data transferred this called flow control. It manages the traffic from the sender to the receiver such that there is no overload.


Windowing is the size or group of data segment which acknowledges the size of data that can received by the receiver in its buffer memory.


when data received from the sender to the receiver, it saves into the buffer memory. (buffer is a temporary memory)

The receiver must Acknowledge the window size to the sender.

The Sender must process the data based on the Acknowledgement.

5)Session layer:

The session Layer establishes, manages, synchronizes and terminates connection between the computers. Since the layer defines the start and stop point for any conversion between two hosts, it is termed as a session

The session layer allows users on different machines to establish a session between them before communication.

It controls the communication process.

It provides a dialog management facility.

It also helps the application for bidirectional communication and provides a messaging service during the communication process between the applications.


The session layer sets up, coordinates and terminates interactions and dialog control between communicating devices.

It provides either half duplex or full duplex service

In the establishment phase, rules and services for a session between the two devices negotiated. The participating devices must agree on the rules

Once rules are accepted the data transfer begins

Connection terminated once the session is complete and communication ends.

6) Presentation layer:

The presentation layer deals with the syntax and semantics of data exchanged between two devices

   This layer presents data to the application layer and is responsible for data translation and encoding every application sends/receives data in a specific format which is defined by the presentation layer

This layer defines the data format used in an application for communication

Various encryption standards are also defined at the presentation layer.


It transforms the data in the format that will accepted by the application layer.

External data representation (XDR) positioned at the presentation level.

Data received at the presentation layer is identify in the code format which includes data compression encryption and decryption

XDR- is a standard for data encoding and description it allows data transfer between different kinds of computer architectures and operating system.

7) Application layer:

The application layer of the OSI model is the starting point of communication and provides the means for the user to access information on the network using an application.


It provides a user interface to the user to communicate and the type of service to be use for the communication

This layer refers to the communication services to be used by the application for communicating between two devices – host and application

EX: internet explorer, outlook express, etc. are the applications that provide you an interface for communication whereas HTTP, HTTPS, SMTP, POP3, FTP etc. are the services use to communicate.

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