OSI also known as Open System Interconnection was the first standard model for networking used by telecommunication companies in the early 1980s. It explains the seven layers that computer systems use to communicate over a network.
The layers assist network professionals in visualizing what happens inside their network and can assist network managers and computer programmers with network problems.
OSI was introduced in 1983 by representatives from major computer and telecommunications corporations, and it was recognized as a worldwide standard by ISO in 1984.
The modern Internet is based on TCP/IP model, not the OSI model. The OSI 7-layer model is generally used because it helps to visualize and understand how network work, as well as isolate and diagnose network problems.
The 7 Layers of the OSI Model:
Easy way to remember the layers:
From Application to Physical (Layer 7 to Layer 1):
All People Seem To Need Data Processing
All Pros Search Top Notch Donut Places
A Penguin Said That Nobody Drinks Pepsi
A Priest Saw Two Nuns Doing Pushups
From Physical to Application (Layer 1 to Layer 7):
Please Do Not Throw Sausage Pizza Away
Pew! Dead Ninja Turtles Smell Particularly Awful
People Don’t Need To See Paula Abdul
Pete Doesn’t Need To Sell Pickles Anymore
Table of Contents:
Layer 7 - Application Layer
The application layer gets information from users directly and displays incoming data to the user. So this layer is used by the end user-softwares like web browsers and email clients. Protocols are provided that permit software send and receive information and display them to the users. Some protocols used are;
HTTP - Hypertext Transfer Protocol
FTP - File Transfer Protocol
POP - Post Office Protocol
SMTP - Simple Mail Transfer Protocol
DNS - Domain Name System
Layer 6 - Presentation Layer
The presentation layer denotes the preparation translation of an application format to a network format or vice versa. That means how two devices should encode, encrypt, and compress data so it is received securely on the other end. Any data transmitted by the application layer is prepared for transmission across the session layer by the presentation layer.
Layer 5 - Session Layer
In the Session layer, communication channels are created when two computers or other network devices need to communicate with each other. These channels are called sessions. It is responsible for setting up sessions, ensuring coordination between devices, and terminating them after communication finishes. During a data transfer, the session layer can also define checkpoints; if the session is stopped, devices can continue data transmission from the latest checkpoint.
Layer 4 - Transport Layer
The transport layer coordinates data transmission between end systems and hosts. It takes data sent at the session layer and divides the data into segments on the transmitting end and reassembles the segments on the receiving end converting them back into data that the session layer may use. Flow control is handled by the transport layer. It provides data at a pace that matches the receiving device's connection speed, and error control, determining if the data was received with errors and thus requesting again. The Transmission Control Protocol(TCP) is the best example of a Transport Layer.
Services of Transport Layer:
Connection-Oriented Communication
Same Order Delivery
Data Integrity
Flow Control
Traffic Control
Multiplexing
Byte orientation
Layer 3 - Network Layer
The network layer is responsible for two primary functions. One is to divide segments into network packets and then reassemble the packets on the receiving end. The second function is to route packets by identifying the best path over a physical network. To route packets to a target node, the network layer employs network addresses, usually Internet Protocol(IP) addresses.
Features of Network Layer:
Transport data packets from source to destination without changing or using them.
Fragment the packets if the packed are too large for delivery.
Decide the route for transmission of packets from available multiple routes.
Source and destination addresses are added to the data packets inside the network layer.
Services of Network Layer:
Packetizing
Routing and Forwarding
Error Control
Flow Control
Congestion Control
Layer 2 - Data Link Layer
In the Data Link layer, the packets are again divided into frames and sent from source to destination. The Data Link layer provides node-to-node data transmission which is two directly connected nodes and handles error correction from the physical layer. It is considered the most complex layer in the OSI model. It also encodes, decodes, and organizes outgoing and incoming data.
This layer is divided into two parts:
Logical Link Control(LLC) - It deals with multiplexing which is the flow of data among applications and other services. It also does error checking and synchronized frames.
Media Access Control(MAC) - connects devices and defines permissions to transmit and receive data using MAC addresses.
Functions performed at Data Link Layer
Framing
Addressing
Error Control
Flow Control
Access Control
Protocols of Data Link Layer:
Synchronous Data Link Protocol (SDLC)
High-Level Data Link Protocol (HDLC)
Serial Line Interface Protocol (SLIP)
Point to Point Protocol (PPP)
Link Access Procedure (LAP)
Link Control Protocol (LCP)
Network Control Protocol (NCP)
Layer 1 - Physical Layer
The physical layer is at the bottom of the OSI model which is the electrical and physical representation of the system. It has the responsibility of the physical cables and wireless connectivity between network nodes. It describes the connection, the electrical cable, or wireless technology that connects the devices and is responsible for transmitting raw data, which is just a series of 0s and 1s, as well as bit rate control.
Advantages of OSI Model
Can determine the hardware and software necessary to establish a network.
Can understand and convey the procedure that components use while communicating over a network.
Can troubleshoot by determining which network layer is having an issue and putting efforts into that layer.
OSI Model vs TCP/IP Model
TCP/IP also known as Transfer Control Protocol/Internet Protocol is older than the OSI model and was created by the US Department of Defense(DoD).
OSI Model | TCP/IP Model |
Have separated 3 layers Application, Presentation, and Session. | Have combined the 5, 6, and 7 layers of the OSI model as the application layer. |
Have separated 2 layers Data Link and Physical. | Have combined the 1 and 2 layers of the OSI model as the Network Access layer. |
Take over responsibility for sequencing and acknowledgment tasks. | Does not take over responsibility for sequencing and acknowledgment tasks, allowing the underlying transport layer to do so. |
A generic, protocol-independent model designed to represent all types of network communication. | A functional model that is based on particular, standard protocols and is meant to handle specific communication problems. |
Simple applications do not use all seven layers. | All layers are used by most applications. |
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