The OSI Model

The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

There are 7 Layers in the OSI Model, each describing a particular aspect of networking.

OSI Model
Data Link

It is an essential to memorize the layers of the OSI model and what they do.  There are two common pneumonics that are generally used, one starts at the top and the other at the bottom:

All People Seem To Need Data Processing

Please Do Not Throw Sausage Pizza Away

Either one can help you remember, but don’t forget which end you are starting on.  Now that you have them memorized, what does each layer do?

Application Layer

This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.

Presentation Layer

This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

Session Layer

This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.

Transport Layer

This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.

Network Layer

This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.

Data Link Layer

At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.

Physical Layer

This layer conveys the bit stream – electrical impulse, light or radio signal — through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

Note1:  The OSI Model was not just originally for Networking.  It was designed to help define communications within applications as well.

Note2:  Originally, the OSI Model was intended to help define the OSI Networking Suite, which was an alterntaive to TCP/IP.  It was not as widely accepted and ultimately TCP/IP became the dominant protocol for networking.

TCP/IP Internet Protocol Suite (aka Internet Protocol Suite)

Protocol/Internet Protocol, the suite of communications protocols used to connect hosts on the Internet. TCP/IP uses several protocols, the two main ones being TCP and IP. TCP/IP is built into the UNIX operating system and is used by the Internet, making it the de facto standard for transmitting data over networks.

OSI Model TCP Model Internet
Protocol Suite
Telnet, SSH
Data Link
Ethernet, Token Ring FDDI

TCP/IP takes its name from its two main protocols: transmission control protocol (TCP) and Internet protocol (IP). TCP operates at the transport layer, i.e., the middle layer in the seven layer OSI (open systems interconnection) reference model. This layer is responsible for maintaining reliable end-to-end communications across the network. IP, in contrast, is a network layer protocol, which is the layer just below the transport layer.

Whereas the IP protocol deals only with packets (i.e., the most fundamental unit of TCP/IP data transmission), TCP enables two hosts to establish a connection and exchange streams of data. TCP guarantees delivery of data and also guarantees that packets will be delivered in the same order in which they were sent.

The great success of TCP/IP is the result of the advantages that it offers over other network protocols and protocol suites, including the following:

It is a freely available protocol and not a secret protocol that is owned by a single company. This makes it possible for anyone with sufficient technical knowledge to improve it.

It is compatible with virtually all modern operating systems, and thus it enables almost any system to communicate with any other system.

It is also compatible with virtually all types of computer hardware and network configurations.

It is a routable protocol, which means that it can determine the most efficient path for every packet as it moves through the network. This makes TCP/IP highly scalable and thus the size of the network virtually unlimited (e.g., the Internet).

It provides reliable data delivery. Reliable means that it can guarantee that the data is delivered to its intended destination (e.g., through the use of error checking and the retransmission of corrupted or missing packets).

The use of a single (and relatively simple) addressing scheme, referred to as IP addressing, allows administrators to transfer their knowledge of TCP/IP to any TCP/IP network without the need to learn a new addressing scheme.

Credit to: Khurram Tanvir @

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