An Overview of the OSI Model
The Open System Interconnection model is a conceptual/ reference model, created by the International Organization for Standardization. The purpose of OSI is to standardize the communication between computing systems using the standard protocols. In simple words, it provides standard protocols for 2 systems to be able to be able to communicate successfully.
The model follows a layered architecture, segregating the data flow into 7 abstraction layers which shall be discussed in detail in the upcoming sections. The lower layers, from network to physical layers are known as media layers or network support layers, the other layers including the transport layer are the host layers and the top 3 layers are the user support layers.
Application layer
Starting from the top, at layer 7 we have the application layer which is the human-computer interaction layer, supporting the access of network services. The communication is initiated at this layer which directly interacts with the data received from users via the user interface web browsers and email clients, the programs running on the end systems. This layer deals with the protocols related to message syntax and semantics and similar data manipulation, which the software relies on. Some of the protocols are Hyper Text Transfer Protocol(HTTP), File Transfer Protocol(FTP), Simple Mail Transfer Protocol(SMTP), Domain Name System(DNS), Simple Network Management Protocol(SNMP) and Dynamic Host Configuration Protocol(DHCP).
Presentation layer
Following the application layer is the presentation layer at layer 6. The key functions are encryption, compression and representation of data. The presentation layer encrypts data at the sender’s end during the communication process to ensure safety and integrity of the data and decrypts at the receiver end, end to end encrypted data. Data compression is also performed by this layer for fast message delivery and improve efficiency. Layer 6 is also responsible for the translation, necessary when the 2 systems are using different standards. Implemented if required.
Session layer
The session layer is at layer 5 of the model which handles session synchronization, checkpointing and recovery of data exchange. A session is simply the time between opening and closing the connection. So this layer takes care that session is alive for the right amount of time and ends once the connection closes. Implemented if required.
Transport layer
The transport protocols are implemented at layer 4, the transport layer. As the name suggests, it takes care of the process-process data transfer which is, end to end communication between two processes. The sender breaks the messages received from the previous layers inti segments and passes to the network layer. At the receiver end, the segments are reassembled to form the original message and passes to the application layer. The additional functions the transport layer is responsible are flow control, congestion control, error control and reliable data transfer. The 2 protocols used in this layer are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
Network layer
Layer 3 handles the 2 main functionalities at the network layer routing and forwarding the packets from source to destination based on the IP address. Packets are the small chunks of segments. Routing is finding the best physical route for the overall communication while forwarding is determining the best path at a point. The protocols are Internet Protocol (IPv4 / IPv6), Internet Control Message protocol (ICMP), Internet Group Management Protocol (IGMP) and so on.
Data Link layer
At layer 2 we have the data link layer which transfers the data frames between neighboring network elements in the same network. Frames are the small chunks generated from packets. It is also responsible for error and flow control. Then how is it different from transport layer? Transport layer handles inter network communication with error and flow control while link layer takes care of the intra network communication with error and flow control. This layer uses MAC and LLC physical addressing. Some of the protocols include Point to Point Protocol (PPP), Link Access Procedure (LAP), Synchronous Data Link Protocol (SDLC).
Physical layer
The bottom most layer of the model, layer 1 is the physical layer for binary bit transmission. This layer typically deals with the physical media used for data transfer like cables and switches using signal convention in traditional media.
Communication in the protocol stack
If Alice has to send a message to Bella, Alice’s web browser passes the message to the application layer. The application layer chooses a protocol and sends to the presentation layer. The encrypted and compressed message is sent to the session layer which will begin the session. Now, the data reaches the transport layer and broken down into segments and which are passed on using either TCP or UDP. The segments are further broken down into packets at the network layer where the packets are routed based on requirements and the destination IP address. The packets are now split into frames at the data link layer. Finally, it is passed onto the physical layer where it is simplified into bits and transferred via the physical media to the next network element. This whole process is known as encapsulation which occurs at the source.
Once the binary data reaches Bella’s system’s physical layer reassembled into frames, they flow upwards to the link layer where they are reassembled as packets. These packets flow up to the network layer and combined to get the segments which are then sent to the transport layer and combined to get back the original message. The original message goes up the session layer where the session is closed and then the presentation layer which decrypts the message. This message is finally sent to Bella’s browser via the application layer. This is decapsulation which occurs at the destination. Now Bella will be able to read Alice’s message.
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