OSI stands for Open System Interconnection is a reference model that describes how information from a software application in one computer moves through a physical medium to the software application in another computer.

OSI consists of seven layers, and each layer performs a particular network function.

OSI model divides the whole task into seven smaller and manageable tasks. Each layer is assigned a particular task.

Each layer is self-contained, so that task assigned to each layer can be performed independently.

Below there is a table with all layers given with their individual task.

Layer 7	Application	Network services provided to end-user applications such as a web browser.
Layer 6	Presentation	The presentation layer formats or translates data for the application layer based on the syntax or semantics that the application accepts.
Layer 5	Session	The session layer controls synchronization of communication between different computers.
Layer 4	Transport	Breaks the data in chunk and Transfer the data between systems and hosts.
Layer 3	Network	Delivering frames to their intended destinations among based on the addresses contained inside the frame.
Layer 2	Data Link	Directly connected nodes are used to perform node-to-node data transfer where data is packaged into frames. Also corrects Error.
Layer 1	Physical	Transmitting electrically or optically raw data from sender to receiver

Below I am going to give some idea about important terminologies regarding OSI model –

• MAC Address (Media Access Control) – It is built into all the network devices.
  • It is a 48 bit address represented in hexadecimal form.
  • Each hex character contains 4 bit.
  • There are 12 hex characters
  • E.G. – 00–40–05–60–7D–49
  • The first six digit represents the NIC manufactures. Last six digit refers to the unique ID of each NIC.
• Frames – The data transfer in network happens in frames.
  • Different network have different frame size.
  • All NIC in a same network must have same frame size.
  • Generally the frame size is 1500 bytes

Data Transmission in Hub – When a specific NIC sends the data to a specific host in a network it just sends the data to the hub and the hub sends the data to all the connected host and the host with the matching MAC address will receive and others will discard it.

Data Transmission in Switch – Later we use switch which filter the MAC address and sends the data to only the destination address.

FCS – It is 4 byte long.

• Data Link Layer parts –
  • LLC(Logical Link Control) – NIC that talks to the system’s operating system (usually via device drivers). The LLC handles multiple network protocols and provides flow control.
  • MAC Address – which is discussed above.
• Network Layer –

Here it gives a logical address called IP address. It is a 32 bit address and represented as dotted decimal. There is a device called router which sends data though IP address.

In real life there are many ways the hosts in a network is connected. Upon which the efficiency and other parameters depends and discussed topologies are been developed through the decades.

Bus & Ring Topology

• A bus topology use a connection line that connects all computers.
• Ring topology connects all the computer on a network with a ring of cable.
  In ring topology the data gets transferred in uni direction (clockwise or anti clockwise )

Problems – Both suffered from same problem with cable break down.

Star Topology

Here all machines are connected to a central node.

Pros – It is robust and if one connection goes down then entire network don’t get affected.

Cons – If the central node goes down then the entire network goes down.

Mesh Topology

In wireless network every machine is connected to every network then it is a mesh topology. There are mainly two type of topology is there-

• Fully Mesh – All machines are connected to all other machines.
• Total number of Links – n * (n – 1) / 2
• Partial Mesh – At least two machines are not connected.

Hybrid Topology

Here in the central box all the machines are connected to a internal bus. It is often known as star-bus topology.

All the hosts are connected to a central node but inside the box every connection is actually bus topology.

Internet

It is a distributed vast network that connect all type of devices together.

Data Communication

There are total 5 fundamental parts of any data communication –

• Message – For a data communication one of the most important thing is message or the data.
• Sender – The second thing is sender there must be a person or application which is sending the data.
• Receiver – The third part obviously is the receiver there also must be a receiver in order to receive the data.
• Transmission Medium – It is the way by which the data is going to be transferred. It can be many types for example – Wire, wireless etc.
• Protocol – It is similar to human conversation some device send some message/request to another machine and it sends back reply as another message/request. In general protocol is a set of rule upon which devices communicate with each other.