OSI (Open System Interconnection) Model was developed to have a standard way of how two or more computers, servers can communicate with each other. It has 7 layers -

1. Physical layer - As name suggests, it does the physical connection between devices, information is present in the form of bits. In this layer Encoding, Decoding happens.

2. Data Link layer - It transmits the data from one node to another node. It adds source and Destination Mac address to the data packet to form it into a frame.

3. Network Layer - It finds best path for the packet to travel. It uses algorithms for finding these paths. Data is wrapped and source and destination Ports are added. Also source and destination IP address is added to the data packet. This is called Logical addressing. Routing, Transportation, Load balancing happens at Network layer.

4. Transport Layer - Here Data received from session layer is divided into small parts - segments known as segmentation. Sequencing is done. (Numbering of Segments). It helps in reassembling the segments at the destination. It does flow control (Speed at which data is transferred & speed at which it is received at destination is not same, so it needs to control the flow of data else data can be lost). It does error control, by adding checksum to every data segment. Protocols used - TCP, UDP.

5. Session layer - It creates and maintains the session. It does authentication and authorization before a session is established. (Authentication - Username, Password) (Authorization - Checking if you've the Permission to access the resource)

6. Presentation layer - It will convert the data got from application layer into machine readable binary format. The Data from the Application layer is extracted here, and manipulated here into required Format to transmit over the network. Here Compression and Decompression is done so that its easily transported. Encryption, Decryption also happens here. Protocol used - SSL

7. Application layer - Implemented in software which interact with one human to another. Protocols used - HTTP, HTTPS, SMTP, TELNET, POP. ( Here POP - For Receiving E-mails, SMTP - Sending E-mails, TELNET - For connecting to remote host / device using Telnet Client ).

Ping time -

It measures the Round-trip time, send from originating host to the destination of the computer, and they're echoed back.

Can we reduce this Ping time?

No, because we're getting best possible time. Signals are travelling at the speed of light through cables and satellites.

SSH - (Secure Shell)

It's used to login to a terminal to someone else's computer securely.

HTTP -

It is a client-server protocol, it tells how you requested the data from the server and it also tells us how server will send data back to the client. It uses TCP. (TCP - Transmission Control Protocol). It is stateless which means server doesn't save the session information.