presentation layer three primary functions

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Presentation Layer in OSI model

Prerequisite : OSI Model

Introduction : Presentation Layer is the 6th layer in the Open System Interconnection (OSI) model. This layer is also known as Translation layer, as this layer serves as a data translator for the network. The data which this layer receives from the Application Layer is extracted and manipulated here as per the required format to transmit over the network. The main responsibility of this layer is to provide or define the data format and encryption. The presentation layer is also called as Syntax layer since it is responsible for maintaining the proper syntax of the data which it either receives or transmits to other layer(s).

Functions of Presentation Layer :

The presentation layer, being the 6th layer in the OSI model, performs several types of functions, which are described below-

• Presentation layer format and encrypts data to be sent across the network.
• This layer takes care that the data is sent in such a way that the receiver will understand the information (data) and will be able to use the data efficiently and effectively.
• This layer manages the abstract data structures and allows high-level data structures (example- banking records), which are to be defined or exchanged.
• This layer carries out the encryption at the transmitter and decryption at the receiver.
• This layer carries out data compression to reduce the bandwidth of the data to be transmitted (the primary goal of data compression is to reduce the number of bits which is to be transmitted).
• This layer is responsible for interoperability (ability of computers to exchange and make use of information) between encoding methods as different computers use different encoding methods.
• This layer basically deals with the presentation part of the data.
• Presentation layer, carries out the data compression (number of bits reduction while transmission), which in return improves the data throughput.
• This layer also deals with the issues of string representation.
• The presentation layer is also responsible for integrating all the formats into a standardized format for efficient and effective communication.
• This layer encodes the message from the user-dependent format to the common format and vice-versa for communication between dissimilar systems.
• This layer deals with the syntax and semantics of the messages.
• This layer also ensures that the messages which are to be presented to the upper as well as the lower layer should be standardized as well as in an accurate format too.
• Presentation layer is also responsible for translation, formatting, and delivery of information for processing or display.
• This layer also performs serialization (process of translating a data structure or an object into a format that can be stored or transmitted easily).

Features of Presentation Layer in the OSI model: Presentation layer, being the 6th layer in the OSI model, plays a vital role while communication is taking place between two devices in a network.

List of features which are provided by the presentation layer are:

• Presentation layer could apply certain sophisticated compression techniques, so fewer bytes of data are required to represent the information when it is sent over the network.
• If two or more devices are communicating over an encrypted connection, then this presentation layer is responsible for adding encryption on the sender’s end as well as the decoding the encryption on the receiver’s end so that it can represent the application layer with unencrypted, readable data.
• This layer formats and encrypts data to be sent over a network, providing freedom from compatibility problems.
• This presentation layer also negotiates the Transfer Syntax.
• This presentation layer is also responsible for compressing data it receives from the application layer before delivering it to the session layer (which is the 5th layer in the OSI model) and thus improves the speed as well as the efficiency of communication by minimizing the amount of the data to be transferred.

Working of Presentation Layer in the OSI model : Presentation layer in the OSI model, as a translator, converts the data sent by the application layer of the transmitting node into an acceptable and compatible data format based on the applicable network protocol and architecture.  Upon arrival at the receiving computer, the presentation layer translates data into an acceptable format usable by the application layer. Basically, in other words, this layer takes care of any issues occurring when transmitted data must be viewed in a format different from the original format. Being the functional part of the OSI mode, the presentation layer performs a multitude (large number of) data conversion algorithms and character translation functions. Mainly, this layer is responsible for managing two network characteristics: protocol (set of rules) and architecture.

Presentation Layer Protocols : Presentation layer being the 6th layer, but the most important layer in the OSI model performs several types of functionalities, which makes sure that data which is being transferred or received should be accurate or clear to all the devices which are there in a closed network. Presentation Layer, for performing translations or other specified functions, needs to use certain protocols which are defined below –

• Apple Filing Protocol (AFP): Apple Filing Protocol is the proprietary network protocol (communications protocol) that offers services to macOS or the classic macOS. This is basically the network file control protocol specifically designed for Mac-based platforms.
• Lightweight Presentation Protocol (LPP): Lightweight Presentation Protocol is that protocol which is used to provide ISO presentation services on the top of TCP/IP based protocol stacks.
• NetWare Core Protocol (NCP): NetWare Core Protocol is the network protocol which is used to access file, print, directory, clock synchronization, messaging, remote command execution and other network service functions.
• Network Data Representation (NDR): Network Data Representation is basically the implementation of the presentation layer in the OSI model, which provides or defines various primitive data types, constructed data types and also several types of data representations.
• External Data Representation (XDR): External Data Representation (XDR) is the standard for the description and encoding of data. It is useful for transferring data between computer architectures and has been used to communicate data between very diverse machines. Converting from local representation to XDR is called encoding, whereas converting XDR into local representation is called decoding.
• Secure Socket Layer (SSL): The Secure Socket Layer protocol provides security to the data that is being transferred between the web browser and the server. SSL encrypts the link between a web server and a browser, which ensures that all data passed between them remains private and free from attacks.

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Presentation Layer: What It Is, Design Issues, Functionalities

Description and Functions of Presentation Layer in the OSI model: In this tutorial, we are going to learn what the Presentation layer is and the Functions of the Presentation Layer in the OSI model in Computer Networking. We will also discuss the Design issues with the Presentation Layer and the working of the Presentation Layer with the help of its diagram. By Monika Jha Last updated : May 05, 2023

What is Presentation Layer?

The Presentation Layer is concerned with the syntax and semantics of the information exchanged between two communicating devices.

• The presentation layer takes care that the data is sent in that way the receiver of the data will understand the information (data) and will be able to use the data.
• Languages that are syntax can be different from the two communicating machines. In this condition, the presentation layer plays the role of translator between them.
• It is possible for two machines to communicate with different data representations, data structures to be exchanged can be defined in an abstract way.
• These abstract data structures will be managed by the presentation layer and this layer allows higher-level data structures (For example banking records), to be defined and exchanged.

This figure shows the relationship of the presentation layer to the session layer and application layer.

Design Issues with Presentation Layer

The following are the design issues with presentation layer:

• To manage and maintain the Syntax and Semantics of the information transmitted.
• Encoding data in a standard agreed-upon way just like a string, double, date, etc.
• It Performs Standard Encoding scheme on the wire.

Functionalities of the Presentation Layer

Specific functionalities of the presentation layer are as follows:

1. Translation

• The processes or running programs in two machines are usually exchanging the information in the form of numbers, character strings and so on before being transmitted. The information should be changed to bitstreams because different computers use different encoding schemes.
• The Presentation layer is responsible for compatibility between these encoding methods.
• The Presentation layer at the sender's side changes the information from its sender dependent format.
• The Presentation layer at the receiving machine changes the common format into its receivers dependent format.

Example: Convert ASCII code to EBCDIC code.

2. Encryption

• The system must be able to assure privacy regarding the message or information as it also carries sensitive information.
• Encryption means that the sender transforms the original information or message to another form, this data after encryption is known as the ciphertext and this ciphertext sends the resulting message out over the network.
• Decryption concerned with the transform of the message back to its original form. This decrypted data is known as plain text.

3. Compression

• Data Compression means reduces the number of bits to be transmitted by this reduce the bandwidth of the data.
• Data Compression becomes particularly important in the transmission of multimedia such as audio, video, text, etc.

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Presentation Layer: Protocols, Examples, Services | Functions of Presentation Layer

Presentation Layer is the 6th layer in the Open System Interconnection (OSI) model where all application programmer consider data structure and presentation, beyond of simply sending the data into form of datagram otherwise packets in between the hosts. Now, we will explain about what is presentation layer with its protocols, example, service ; involving with major functions of presentation Layer with ease. At the end of this article, you will completely educate about What is Presentation Layer in OSI Model without any hassle.

What is Presentation Layer?

Presentation layer is capable to handle abstract data structures, and further it helps to defined and exchange of higher-level data structures.

Presentation Layer Tutorial Headlines:

Let’s get started,   functions of presentation layer.

Presentation layer performs various functions in the OSI model ; below explain each one – 

Protocols of Presentation Layer

Example of presentation layer protocols:.

Here, we will discuss all examples of presentation layer protocols; below explain each one –  

Multipurpose Internet Mail Extensions (MIME) : MIME protocol was introduced by Bell Communications in 1991, and it is an internet standard that provides scalable capable of email for attaching of images, sounds and text in a message.

Network News Transfer Protocol (NNTP) : This protocol is used to make connection with Usenet server and transmit all newsgroup articles in between system over internet.

Apple Filing Protocol (AFP ) : AFP protocol is designed by Apple company for sharing all files over the entire network .

NetWare Core Protocol (NCP) : NCP is a Novell client server model protocol that is designed especially for Local Area Network (LAN). It is capable to perform several functions like as file/print-sharing, clock synchronization, remote processing and messaging.

Network Data Representation (NDR) : NDR is an data encoding standard, and it is implement in the Distributed Computing Environment (DCE).

Tox : The Tox protocol is sometimes regarded as part of both the presentation and application layer , and it is used for sending peer-to-peer instant-messaging as well as video calling.

eXternal Data Representation (XDR) : This protocol provides the description and encoding of entire data, and  it’s main goal is to transfer data in between dissimilar computer architecture.

Presentation Layer Services

Design issues with presentation layer, faqs (frequently asked questions), what is meant by presentation layer in osi model.

Presentation Layer is the 6th layer in the Open System Interconnection (OSI) model that is the lowest layer, where all application programmer consider data structure and presentation, beyond of simply sending the data into form of datagram otherwise packets in between the hosts.

What protocols are used in the presentation layer?

Can you explain some presentation layer examples, what are the main functions of the presentation layer, what are services of presentation layer in osi.

Presentation layer has a responsibility for formatting, translation, and delivery of the information for getting to process otherwise display .

Now, i hope that you have completely learnt about what is presentation layer with its protocols, example, service ; involving with major functions of presentation Layer with ease. If this post is useful for you, then please share it along with your friends, family members or relatives over social media platforms like as Facebook, Instagram, Linked In, Twitter, and more.

Also Read: Data Link Layer: Protocols, Examples | Functions of Data Link Layer

Related posts.

  Layer 6 Presentation Layer

De/Encryption, Encoding, String representation

The presentation layer (data presentation layer, data provision level) sets the system-dependent representation of the data (for example, ASCII, EBCDIC) into an independent form, enabling the syntactically correct data exchange between different systems. Also, functions such as data compression and encryption are guaranteed that data to be sent by the application layer of a system that can be read by the application layer of another system to the layer 6. The presentation layer. If necessary, the presentation layer acts as a translator between different data formats, by making an understandable for both systems data format, the ASN.1 (Abstract Syntax Notation One) used.

OSI Layer 6 - Presentation Layer

The presentation layer is responsible for the delivery and formatting of information to the application layer for further processing or display. It relieves the application layer of concern regarding syntactical differences in data representation within the end-user systems. An example of a presentation service would be the conversion of an EBCDIC-coded text computer file to an ASCII-coded file. The presentation layer is the lowest layer at which application programmers consider data structure and presentation, instead of simply sending data in the form of datagrams or packets between hosts. This layer deals with issues of string representation - whether they use the Pascal method (an integer length field followed by the specified amount of bytes) or the C/C++ method (null-terminated strings, e.g. "thisisastring\0"). The idea is that the application layer should be able to point at the data to be moved, and the presentation layer will deal with the rest. Serialization of complex data structures into flat byte-strings (using mechanisms such as TLV or XML) can be thought of as the key functionality of the presentation layer. Encryption is typically done at this level too, although it can be done on the application, session, transport, or network layers, each having its own advantages and disadvantages. Decryption is also handled at the presentation layer. For example, when logging on to bank account sites the presentation layer will decrypt the data as it is received.[1] Another example is representing structure, which is normally standardized at this level, often by using XML. As well as simple pieces of data, like strings, more complicated things are standardized in this layer. Two common examples are 'objects' in object-oriented programming, and the exact way that streaming video is transmitted. In many widely used applications and protocols, no distinction is made between the presentation and application layers. For example, HyperText Transfer Protocol (HTTP), generally regarded as an application-layer protocol, has presentation-layer aspects such as the ability to identify character encoding for proper conversion, which is then done in the application layer. Within the service layering semantics of the OSI network architecture, the presentation layer responds to service requests from the application layer and issues service requests to the session layer. In the OSI model: the presentation layer ensures the information that the application layer of one system sends out is readable by the application layer of another system. For example, a PC program communicates with another computer, one using extended binary coded decimal interchange code (EBCDIC) and the other using ASCII to represent the same characters. If necessary, the presentation layer might be able to translate between multiple data formats by using a common format. Wikipedia

• Data conversion
• Character code translation
• Compression
• Encryption and Decryption

The Presentation OSI Layer is usually composed of 2 sublayers that are:

CASE common application service element

SASE specific application service element

Layer 7   Application Layer

Layer 6   presentation layer, layer 5   session layer, layer 4   transport layer, layer 3   network layer, layer 2   data link layer, layer 1   physical layer.

Presentation layer and Session layer of the OSI model

There are two popular networking models: the OSI layers model and the TCP/IP layers model. The presentation layer and session layer exist only in the OSI layers models. The TCP/IP layers model merges them into the application layer.

The Presentation Layer

The presentation layer is the sixth layer of the OSI Reference model. It defines how data and information is transmitted and presented to the user. It translates data and format code in such a way that it is correctly used by the application layer.

It identifies the syntaxes that different applications use and formats data using those syntaxes. For example, a web browser receives a web page from a web server in the HTML language. HTML language includes many tags and markup that have no meaning for the end user but they have special meaning for the web browser. the web browser uses the presentation layer's logic to read those syntaxes and format data in such a way the web server wants it to be present to the user.

On the sender device, it encapsulates and compresses data before sending it to the network to increase the speed and security of the network. On the receiver device, it de-encapsulates and decompresses data before presenting it to the user.

Examples of the presentation layer

Example standards for representing graphical information: JPEG, GIF, JPEG, and TIFF.

Example standards for representing audio information: WAV, MIDI, MP3.

Example standards for representing video information: WMV, MOV, MP4, MPEG.

Example standards for representing text information: doc, xls, txt, pdf.

Functions of the presentation layer

• It formats and presents data and information.
• It encrypts and compresses data before giving it to the session layer.
• It de-encrypts and decompresses the encrypted and compressed data it receives from the session layer.

Session layer

The session layer is the fifth layer of the OSI layers model. It is responsible for initiating, establishing, managing, and terminating sessions between the local application and the remote applications.

It defines standards for three modes of communication: full duplex, half-duplex, and simplex.

In the full duplex mode, both devices can send and receive data simultaneously. The internet connection is an example of the full duplex mode.

In the half duplex mode, only one device can send data at a time. A telephone conversation is an example of the half-duplex mode.

In the simplex mode, only one device can send data. A radio broadcast is an example of the simplex mode.

Functions of the session layer

• It is responsible for terminating sessions, creating checkpoints, and recovering data when sessions are interrupted.
• It opens and maintains logical communication channels between network applications running on the local host and network applications running on the remote host.
• If a network application uses an authentication mechanism before it opens a logical communication channel (session) with the remote host, it handles the authentication process.

Examples of the session layer

Structure Query Language (SQL), Remote Procedure Call (RPC), and Network File System (NFS) are examples of the session layer.

By ComputerNetworkingNotes Updated on 2023-04-25

ComputerNetworkingNotes CCNA Study Guide Presentation layer and Session layer of the OSI model

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Application Layer Protocols

Application, session and presentation.

The Presentation Layer

The presentation layer has three primary functions:

• Formats, or presents, data from the source device into a compatible form for receipt by the destination device.
• Compression of the data in a way that can be decompressed by the destination device.
• Encryption of the data for transmission and the decryption of data upon receipt by the destination.

As shown in the figure, the presentation layer formats data for the application layer and it sets standards for file formats. Some well-known standards for video include QuickTime and Motion Picture Experts Group (MPEG). QuickTime is an Apple computer specification for video and audio, and MPEG is a standard for video and audio compression and coding.

Among the well-known graphic image formats that are used on networks are Graphics Interchange Format (GIF), Joint Photographic Experts Group (JPEG), and Portable Network Graphics (PNG) format. GIF and JPEG are compression and coding standards for graphic images. PNG was designed to address some of the limitations of the GIF format and to eventually replace it.

The Session Layer

As the name implies, functions at the session layer create and maintain dialogs between source and destination applications. The session layer handles the exchange of information to initiate dialogs, keep them active, and to restart sessions that are disrupted or idle for a long period of time.

Presentation Layer

Last Edited

What is the Presentation Layer?

Presentation Layer is the Layer 6 of the seven-layer Open Systems Interconnection (OSI) reference model . The presentation layer structures data that is passed down from the application layer into a format suitable for network transmission. This layer is responsible for data encryption, data compression, character set conversion, interpretation of graphics commands, and so on. The network redirector also functions at this layer.

Presentation Layer functions

• Translation:  Before being transmitted, information in the form of characters and numbers should be changed to bit streams. Layer 6 is responsible for interoperability between encoding methods as different computers use different encoding methods. It translates data between the formats the network requires and the format the computer.
• Encryption:  Encryption at the transmitter and decryption at the receiver
• Compression:  Data compression to reduce the bandwidth of the data to be transmitted. The primary role of  data compression  is to reduce the number of bits to be transmitted. Multimedia files, such as audio and video, are bigger than text files and compression is more important.

Role of Presentation Layer in the OSI Model

This layer is not always used in network communications because its functions are not always necessary. Translation is only needed if different types of machines need to talk with each other. Encryption is optional in communication. If the information is public there is no need to encrypt and decrypt info. Compression is also optional. If files are small there is no need for compression.

Explaining Layer 6 in video

Most real-world protocol suites, such as TCP/IP , do not use separate presentation layer protocols. This layer is mostly an abstraction in real-world networking.

An example of a program that loosely adheres to layer 6 of OSI is the tool that manages the Hypertext Transfer Protocol (HTTP) — although it’s technically considered an application-layer protocol per the TCP/IP model.

However, HTTP includes presentation layer services within it. HTTP works when the requesting device forwards user requests passed to the web browser onto a web server elsewhere in the network.

It receives a return message from the web server that includes a multipurpose internet mail extensions (MIME) header. The MIME header indicates the type of file – text, video, or audio – that has been received so that an appropriate player utility can be used to present the file to the user.

In short, the presentation layer

Makes sure that data which is being transferred or received should be accurate or clear to all the devices which are there, in a closed network.

• ensures proper formatting and delivery to and from the application layer;
• performs data encryption; and
• manages serialization of data objects.

Presentation Layer Of OSI Model For Beginners

Presentation Layer:

Presentation Layer is the sixth layer in the OSI model and here are some of the functionalities of the presentation layer:

Translation

Before being transmitted, the data remains in the form of characters and numbers. This data has to be changed to bitstreams before transmission. The presentation layer is responsible for interoperability between encoding methods as different computers use different encoding methods. It translates data between the formats that a network requires and the format a computer needs.

It carries out the process of encryption at the transmitter end and the process of decryption at the receiver end.

Encryption and decryption are ways to protect the confidentiality of the data stored on computer systems or wired over the internet or other computer networks.

Also Read:   Network Layer Of OSI Mode: Functionalities and Protocols

In terms of security, modern-day encryption methods play a vital role in the security assurance of IT systems and communications as they can provide not only confidentiality but also,  authentication and integrity.

Data compression

Data compression is also known by different names like source coding or bit-rate reduction. As evident from these names, data compression involves encoding information using fewer bits than the original representation. So in this way, the data compression can be either lossy or lossless.

Even though lossless compression reduces bits by identifying and eliminating statistical redundancy, no data information is lost in the lossless compression.

On the other hand, the lossy compression reduces bits by identifying unnecessary information and removing it.

Data compression is useful in computer networks because it helps in the following ways:

• Reducing resource usages such as data storage space or transmission capacity. B
• Reducing the need for an expensive hardware for the data representation. For example, if a video is highly compressed before transmission, an expensive hardware might be required to decompress the video data before playing it.

Thus, data compression is also very helpful in real-time applications over the internet like real-time video or audio streaming.

Data conversion

There are different types of operating systems such as Windows, Linux, Mac OS etc. are being used all around the world. Data conversion is, thus, responsible for the conversion of computer data from one format to another.

Different computers encode data in different ways on the basis of certain standards. On top of that, each computer program handles data in a different manner. Data conversion comes in handy in those situations when the representation of data is needed on different platforms.

The presentation layer can be composed of two sublayers: common application service element (CASE) and specific application service element (SASE).

Also Read:   LLC Layer (Logical Link Control): Data Link Layer Of OSI Model

The common application service element sublayer provides services for the application layer and request services from the session layer. It provides support for common application services whereas the specific application service element sublayer provides application specific services (protocols) like remote database access, file transfer, virtual terminal.

If you have any comments or thoughts related to it, feel free to ask and correct us. Also, don’t miss our complete coverage on the Computer networks .

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The Presentation Layer of OSI Model

The presentation layer (Layer 6) ensures that the message is presented to the upper layer in a standardized format. It deals with the syntax and the semantics of the messages.

The main functions of the presentation layer are as follows −

• It encodes the messages from the user dependent format to the common format and vice versa, for communication among dissimilar systems.
• It is responsible for data encryption and decryption of sensitive data before they are transmitted over common channels.
• It is also responsible for data compression. Data compression is done at the source to reduce the number of bits to be transmitted. It reduces the storage space and increases the file transfer rate. It is particularly useful for transmission of large multimedia files.

• Related Articles
• The Transport Layer of OSI Model
• The Session Layer of OSI Model
• The Data Link Layer of OSI Model
• Explain the functions of Presentation Layer.
• What is Presentation Layer?
• The OSI Reference Model
• What is a presentation layer?
• Advantages and Disadvantages of the OSI Model
• Computer Networks – Layers of OSI Model
• What is the OSI Reference Model?
• OSI Model in Computer Networking
• Why Does the OSI Reference Model Matter?
• What are the critiques of OSI Model and Protocols?
• Explain the advantages and disadvantages of the OSI reference Model
• OSI vs. TCP/IP Reference Model

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The OSI Model – The 7 Layers of Networking Explained in Plain English

By Chloe Tucker

This article explains the Open Systems Interconnection (OSI) model and the 7 layers of networking, in plain English.

The OSI model is a conceptual framework that is used to describe how a network functions. In plain English, the OSI model helped standardize the way computer systems send information to each other.

Learning networking is a bit like learning a language - there are lots of standards and then some exceptions. Therefore, it’s important to really understand that the OSI model is not a set of rules. It is a tool for understanding how networks function.

Once you learn the OSI model, you will be able to further understand and appreciate this glorious entity we call the Internet, as well as be able to troubleshoot networking issues with greater fluency and ease.

All hail the Internet!

Prerequisites

You don’t need any prior programming or networking experience to understand this article. However, you will need:

• Basic familiarity with common networking terms (explained below)
• A curiosity about how things work :)

Learning Objectives

Over the course of this article, you will learn:

• What the OSI model is
• The purpose of each of the 7 layers
• The problems that can happen at each of the 7 layers
• The difference between TCP/IP model and the OSI model

Common Networking Terms

Here are some common networking terms that you should be familiar with to get the most out of this article. I’ll use these terms when I talk about OSI layers next.

A node is a physical electronic device hooked up to a network, for example a computer, printer, router, and so on. If set up properly, a node is capable of sending and/or receiving information over a network.

Nodes may be set up adjacent to one other, wherein Node A can connect directly to Node B, or there may be an intermediate node, like a switch or a router, set up between Node A and Node B.

Typically, routers connect networks to the Internet and switches operate within a network to facilitate intra-network communication. Learn more about hub vs. switch vs. router.

Here's an example:

For the nitpicky among us (yep, I see you), host is another term that you will encounter in networking. I will define a host as a type of node that requires an IP address. All hosts are nodes, but not all nodes are hosts. Please Tweet angrily at me if you disagree.

Links connect nodes on a network. Links can be wired, like Ethernet, or cable-free, like WiFi.

Links to can either be point-to-point, where Node A is connected to Node B, or multipoint, where Node A is connected to Node B and Node C.

When we’re talking about information being transmitted, this may also be described as a one-to-one vs. a one-to-many relationship.

A protocol is a mutually agreed upon set of rules that allows two nodes on a network to exchange data.

“A protocol defines the rules governing the syntax (what can be communicated), semantics (how it can be communicated), and synchronization (when and at what speed it can be communicated) of the communications procedure. Protocols can be implemented on hardware, software, or a combination of both. Protocols can be created by anyone, but the most widely adopted protocols are based on standards.” - The Illustrated Network.

Both wired and cable-free links can have protocols.

While anyone can create a protocol, the most widely adopted protocols are often based on standards published by Internet organizations such as the Internet Engineering Task Force (IETF).

A network is a general term for a group of computers, printers, or any other device that wants to share data.

Network types include LAN, HAN, CAN, MAN, WAN, BAN, or VPN. Think I’m just randomly rhyming things with the word can ? I can ’t say I am - these are all real network types. Learn more here .

Topology describes how nodes and links fit together in a network configuration, often depicted in a diagram. Here are some common network topology types:

A network consists of nodes, links between nodes, and protocols that govern data transmission between nodes.

At whatever scale and complexity networks get to, you will understand what’s happening in all computer networks by learning the OSI model and 7 layers of networking.

What is the OSI Model?

The OSI model consists of 7 layers of networking.

First, what’s a layer?

No, a layer - not a lair . Here there are no dragons.

A layer is a way of categorizing and grouping functionality and behavior on and of a network.

In the OSI model, layers are organized from the most tangible and most physical, to less tangible and less physical but closer to the end user.

Each layer abstracts lower level functionality away until by the time you get to the highest layer. All the details and inner workings of all the other layers are hidden from the end user.

How to remember all the names of the layers? Easy.

• Please | Physical Layer
• Do | Data Link Layer
• Not | Network Layer
• Tell (the) | Transport Layer
• Secret | Session Layer
• Password (to) | Presentation Layer
• Anyone | Application Layer

Keep in mind that while certain technologies, like protocols, may logically “belong to” one layer more than another, not all technologies fit neatly into a single layer in the OSI model. For example, Ethernet, 802.11 (Wifi) and the Address Resolution Protocol (ARP) procedure operate on >1 layer.

The OSI is a model and a tool, not a set of rules.

OSI Layer 1

Layer 1 is the physical layer . There’s a lot of technology in Layer 1 - everything from physical network devices, cabling, to how the cables hook up to the devices. Plus if we don’t need cables, what the signal type and transmission methods are (for example, wireless broadband).

Instead of listing every type of technology in Layer 1, I’ve created broader categories for these technologies. I encourage readers to learn more about each of these categories:

• Nodes (devices) and networking hardware components. Devices include hubs, repeaters, routers, computers, printers, and so on. Hardware components that live inside of these devices include antennas, amplifiers, Network Interface Cards (NICs), and more.
• Device interface mechanics. How and where does a cable connect to a device (cable connector and device socket)? What is the size and shape of the connector, and how many pins does it have? What dictates when a pin is active or inactive?
• Functional and procedural logic. What is the function of each pin in the connector - send or receive? What procedural logic dictates the sequence of events so a node can start to communicate with another node on Layer 2?
• Cabling protocols and specifications. Ethernet (CAT), USB, Digital Subscriber Line (DSL) , and more. Specifications include maximum cable length, modulation techniques, radio specifications, line coding, and bits synchronization (more on that below).
• Cable types. Options include shielded or unshielded twisted pair, untwisted pair, coaxial and so on. Learn more about cable types here .
• Signal type. Baseband is a single bit stream at a time, like a railway track - one-way only. Broadband consists of multiple bit streams at the same time, like a bi-directional highway.
• Signal transmission method (may be wired or cable-free). Options include electrical (Ethernet), light (optical networks, fiber optics), radio waves (802.11 WiFi, a/b/g/n/ac/ax variants or Bluetooth). If cable-free, then also consider frequency: 2.5 GHz vs. 5 GHz. If it’s cabled, consider voltage. If cabled and Ethernet, also consider networking standards like 100BASE-T and related standards.

The data unit on Layer 1 is the bit.

A bit the smallest unit of transmittable digital information. Bits are binary, so either a 0 or a 1. Bytes, consisting of 8 bits, are used to represent single characters, like a letter, numeral, or symbol.

Bits are sent to and from hardware devices in accordance with the supported data rate (transmission rate, in number of bits per second or millisecond) and are synchronized so the number of bits sent and received per unit of time remains consistent (this is called bit synchronization). The way bits are transmitted depends on the signal transmission method.

Nodes can send, receive, or send and receive bits. If they can only do one, then the node uses a simplex mode. If they can do both, then the node uses a duplex mode. If a node can send and receive at the same time, it’s full-duplex – if not, it’s just half-duplex.

The original Ethernet was half-duplex. Full-duplex Ethernet is an option now, given the right equipment.

How to Troubleshoot OSI Layer 1 Problems

Here are some Layer 1 problems to watch out for:

• Defunct cables, for example damaged wires or broken connectors
• Broken hardware network devices, for example damaged circuits
• Stuff being unplugged (...we’ve all been there)

If there are issues in Layer 1, anything beyond Layer 1 will not function properly.

Layer 1 contains the infrastructure that makes communication on networks possible.

It defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating physical links between network devices. - Source

Fun fact: deep-sea communications cables transmit data around the world. This map will blow your mind: https://www.submarinecablemap.com/

And because you made it this far, here’s a koala:

OSI Layer 2

Layer 2 is the data link layer . Layer 2 defines how data is formatted for transmission, how much data can flow between nodes, for how long, and what to do when errors are detected in this flow.

In more official tech terms:

• Line discipline. Who should talk for how long? How long should nodes be able to transit information for?
• Flow control. How much data should be transmitted?
• Error control - detection and correction . All data transmission methods have potential for errors, from electrical spikes to dirty connectors. Once Layer 2 technologies tell network administrators about an issue on Layer 2 or Layer 1, the system administrator can correct for those errors on subsequent layers. Layer 2 is mostly concerned with error detection, not error correction. ( Source )

There are two distinct sublayers within Layer 2:

• Media Access Control (MAC): the MAC sublayer handles the assignment of a hardware identification number, called a MAC address, that uniquely identifies each device on a network. No two devices should have the same MAC address. The MAC address is assigned at the point of manufacturing. It is automatically recognized by most networks. MAC addresses live on Network Interface Cards (NICs). Switches keep track of all MAC addresses on a network. Learn more about MAC addresses on PC Mag and in this article . Learn more about network switches here .
• Logical Link Control (LLC): the LLC sublayer handles framing addressing and flow control. The speed depends on the link between nodes, for example Ethernet or Wifi.

The data unit on Layer 2 is a frame .

Each frame contains a frame header, body, and a frame trailer:

• Header: typically includes MAC addresses for the source and destination nodes.
• Body: consists of the bits being transmitted.
• Trailer: includes error detection information. When errors are detected, and depending on the implementation or configuration of a network or protocol, frames may be discarded or the error may be reported up to higher layers for further error correction. Examples of error detection mechanisms: Cyclic Redundancy Check (CRC) and Frame Check Sequence (FCS). Learn more about error detection techniques here .

Typically there is a maximum frame size limit, called an Maximum Transmission Unit, MTU. Jumbo frames exceed the standard MTU, learn more about jumbo frames here .

How to Troubleshoot OSI Layer 2 Problems

Here are some Layer 2 problems to watch out for:

• All the problems that can occur on Layer 1
• Unsuccessful connections (sessions) between two nodes
• Sessions that are successfully established but intermittently fail
• Frame collisions

The Data Link Layer allows nodes to communicate with each other within a local area network. The foundations of line discipline, flow control, and error control are established in this layer.

OSI Layer 3

Layer 3 is the network layer . This is where we send information between and across networks through the use of routers. Instead of just node-to-node communication, we can now do network-to-network communication.

Routers are the workhorse of Layer 3 - we couldn’t have Layer 3 without them. They move data packets across multiple networks.

Not only do they connect to Internet Service Providers (ISPs) to provide access to the Internet, they also keep track of what’s on its network (remember that switches keep track of all MAC addresses on a network), what other networks it’s connected to, and the different paths for routing data packets across these networks.

Routers store all of this addressing and routing information in routing tables.

Here’s a simple example of a routing table:

The data unit on Layer 3 is the data packet . Typically, each data packet contains a frame plus an IP address information wrapper. In other words, frames are encapsulated by Layer 3 addressing information.

The data being transmitted in a packet is also sometimes called the payload . While each packet has everything it needs to get to its destination, whether or not it makes it there is another story.

Layer 3 transmissions are connectionless, or best effort - they don't do anything but send the traffic where it’s supposed to go. More on data transport protocols on Layer 4.

Once a node is connected to the Internet, it is assigned an Internet Protocol (IP) address, which looks either like 172.16. 254.1 (IPv4 address convention) or like 2001:0db8:85a3:0000:0000:8a2e:0370:7334 (IPv6 address convention). Routers use IP addresses in their routing tables.

IP addresses are associated with the physical node’s MAC address via the Address Resolution Protocol (ARP), which resolves MAC addresses with the node’s corresponding IP address.

ARP is conventionally considered part of Layer 2, but since IP addresses don’t exist until Layer 3, it’s also part of Layer 3.

How to Troubleshoot OSI Layer 3 Problems

Here are some Layer 3 problems to watch out for:

• All the problems that can crop up on previous layers :)
• Faulty or non-functional router or other node
• IP address is incorrectly configured

Many answers to Layer 3 questions will require the use of command-line tools like ping , trace , show ip route , or show ip protocols . Learn more about troubleshooting on layer 1-3 here .

The Network Layer allows nodes to connect to the Internet and send information across different networks.

OSI Layer 4

Layer 4 is the transport layer . This where we dive into the nitty gritty specifics of the connection between two nodes and how information is transmitted between them. It builds on the functions of Layer 2 - line discipline, flow control, and error control.

This layer is also responsible for data packet segmentation, or how data packets are broken up and sent over the network.

Unlike the previous layer, Layer 4 also has an understanding of the whole message, not just the contents of each individual data packet. With this understanding, Layer 4 is able to manage network congestion by not sending all the packets at once.

The data units of Layer 4 go by a few names. For TCP, the data unit is a packet. For UDP, a packet is referred to as a datagram. I’ll just use the term data packet here for the sake of simplicity.

Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are two of the most well-known protocols in Layer 4.

TCP, a connection-oriented protocol, prioritizes data quality over speed.

TCP explicitly establishes a connection with the destination node and requires a handshake between the source and destination nodes when data is transmitted. The handshake confirms that data was received. If the destination node does not receive all of the data, TCP will ask for a retry.

TCP also ensures that packets are delivered or reassembled in the correct order. Learn more about TCP here .

UDP, a connectionless protocol, prioritizes speed over data quality. UDP does not require a handshake, which is why it’s called connectionless.

Because UDP doesn’t have to wait for this acknowledgement, it can send data at a faster rate, but not all of the data may be successfully transmitted and we’d never know.

If information is split up into multiple datagrams, unless those datagrams contain a sequence number, UDP does not ensure that packets are reassembled in the correct order. Learn more about UDP here .

TCP and UDP both send data to specific ports on a network device, which has an IP address. The combination of the IP address and the port number is called a socket.

Learn more about sockets here .

Learn more about the differences and similarities between these two protocols here .

How to Troubleshoot OSI Layer 4 Problems

Here are some Layer 4 problems to watch out for:

• Blocked ports - check your Access Control Lists (ACL) & firewalls
• Quality of Service (QoS) settings. QoS is a feature of routers/switches that can prioritize traffic, and they can really muck things up. Learn more about QoS here .

The Transport Layer provides end-to-end transmission of a message by segmenting a message into multiple data packets; the layer supports connection-oriented and connectionless communication.

OSI Layer 5

Layer 5 is the session layer . This layer establishes, maintains, and terminates sessions.

A session is a mutually agreed upon connection that is established between two network applications. Not two nodes! Nope, we’ve moved on from nodes. They were so Layer 4.

Just kidding, we still have nodes, but Layer 5 doesn’t need to retain the concept of a node because that’s been abstracted out (taken care of) by previous layers.

So a session is a connection that is established between two specific end-user applications. There are two important concepts to consider here:

• Client and server model: the application requesting the information is called the client, and the application that has the requested information is called the server.
• Request and response model: while a session is being established and during a session, there is a constant back-and-forth of requests for information and responses containing that information or “hey, I don’t have what you’re requesting.”

Sessions may be open for a very short amount of time or a long amount of time. They may fail sometimes, too.

Depending on the protocol in question, various failure resolution processes may kick in. Depending on the applications/protocols/hardware in use, sessions may support simplex, half-duplex, or full-duplex modes.

Examples of protocols on Layer 5 include Network Basic Input Output System (NetBIOS) and Remote Procedure Call Protocol (RPC), and many others.

From here on out (layer 5 and up), networks are focused on ways of making connections to end-user applications and displaying data to the user.

How to Troubleshoot OSI Layer 5 Problems

Here are some Layer 5 problems to watch out for:

• Servers are unavailable
• Servers are incorrectly configured, for example Apache or PHP configs
• Session failure - disconnect, timeout, and so on.

The Session Layer initiates, maintains, and terminates connections between two end-user applications. It responds to requests from the presentation layer and issues requests to the transport layer.

OSI Layer 6

Layer 6 is the presentation layer . This layer is responsible for data formatting, such as character encoding and conversions, and data encryption.

The operating system that hosts the end-user application is typically involved in Layer 6 processes. This functionality is not always implemented in a network protocol.

Layer 6 makes sure that end-user applications operating on Layer 7 can successfully consume data and, of course, eventually display it.

There are three data formatting methods to be aware of:

• American Standard Code for Information Interchange (ASCII): this 7-bit encoding technique is the most widely used standard for character encoding. One superset is ISO-8859-1, which provides most of the characters necessary for languages spoken in Western Europe.
• Extended Binary-Coded Decimal Interchange Code (EBDCIC): designed by IBM for mainframe usage. This encoding is incompatible with other character encoding methods.
• Unicode: character encodings can be done with 32-, 16-, or 8-bit characters and attempts to accommodate every known, written alphabet.

Learn more about character encoding methods in this article , and also here .

Encryption: SSL or TLS encryption protocols live on Layer 6. These encryption protocols help ensure that transmitted data is less vulnerable to malicious actors by providing authentication and data encryption for nodes operating on a network. TLS is the successor to SSL.

How to Troubleshoot OSI Layer 6 Problems

Here are some Layer 6 problems to watch out for:

• Non-existent or corrupted drivers
• Incorrect OS user access level

The Presentation Layer formats and encrypts data.

OSI Layer 7

Layer 7 is the application layer .

True to its name, this is the layer that is ultimately responsible for supporting services used by end-user applications. Applications include software programs that are installed on the operating system, like Internet browsers (for example, Firefox) or word processing programs (for example, Microsoft Word).

Applications can perform specialized network functions under the hood and require specialized services that fall under the umbrella of Layer 7.

Electronic mail programs, for example, are specifically created to run over a network and utilize networking functionality, such as email protocols, which fall under Layer 7.

Applications will also control end-user interaction, such as security checks (for example, MFA), identification of two participants, initiation of an exchange of information, and so on.

Protocols that operate on this level include File Transfer Protocol (FTP), Secure Shell (SSH), Simple Mail Transfer Protocol (SMTP), Internet Message Access Protocol (IMAP), Domain Name Service (DNS), and Hypertext Transfer Protocol (HTTP).

While each of these protocols serve different functions and operate differently, on a high level they all facilitate the communication of information. ( Source )

How to Troubleshoot OSI Layer 7 Problems

Here are some Layer 7 problems to watch out for:

• All issues on previous layers
• Incorrectly configured software applications
• User error (... we’ve all been there)

The Application Layer owns the services and functions that end-user applications need to work. It does not include the applications themselves.

Our Layer 1 koala is all grown up.

Learning check - can you apply makeup to a koala?

Don’t have a koala?

Well - answer these questions instead. It’s the next best thing, I promise.

• What is the OSI model?
• What are each of the layers?
• How could I use this information to troubleshoot networking issues?

Congratulations - you’ve taken one step farther to understanding the glorious entity we call the Internet.

Learning Resources

Many, very smart people have written entire books about the OSI model or entire books about specific layers. I encourage readers to check out any O’Reilly-published books about the subject or about network engineering in general.

Here are some resources I used when writing this article:

• The Illustrated Network, 2nd Edition
• Protocol Data Unit (PDU): https://www.geeksforgeeks.org/difference-between-segments-packets-and-frames/
• Troubleshooting Along the OSI Model: https://www.pearsonitcertification.com/articles/article.aspx?p=1730891
• The OSI Model Demystified: https://www.youtube.com/watch?v=HEEnLZV2wGI
• OSI Model for Dummies: https://www.dummies.com/programming/networking/layers-in-the-osi-model-of-a-computer-network/

Chloe Tucker is an artist and computer science enthusiast based in Portland, Oregon. As a former educator, she's continuously searching for the intersection of learning and teaching, or technology and art. Reach out to her on Twitter @_chloetucker and check out her website at chloe.dev .

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COMPUTER NETWORK BASICS

• Introduction To Computer Networks
• Uses of Computer Networks
• Line Configuration
• Types of Network Topology
• Transmission Modes
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Data link layer

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Transport layer

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ISO/OSI REFERENCE MODEL

• Introduction to Reference Models
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• OSI Model: Transport Layer
• OSI Model: Session Layer
• OSI Model: Presentation Layer
• OSI Model: Application Layer

TCP/IP REFERENCE MODELCOMPUTER NETWORKS

• The TCP/IP Reference Model
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Session layer

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Computer Networks

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• Protocols and Standards
• Connection Oriented and Connectionless
• OSI Vs TCP/IP

Presentation layer

• Presentation Layer

Application layer

• HTTP Protocol
• FTP Protocol
• SMTP Protocol
• POP Protocol
• SNMP Protocol
• Electronic Mail
• MIME Protocol
• World Wide Web
• DNS Protocol

Presentation Layer - OSI Model

The primary goal of this layer is to take care of the syntax and semantics of the information exchanged between two communicating systems. Presentation layer takes care that the data is sent in such a way that the receiver will understand the information(data) and will be able to use the data. Languages(syntax) can be different of the two communicating systems. Under this condition presentation layer plays a role translator.

In order to make it possible for computers with different data representations to communicate, the data structures to be exchanged can be defined in an abstract way. The presentation layer manages these abstract data structures and allows higher-level data structures(eg: banking records), to be defined and exchanged.

Functions of Presentation Layer

• Translation: Before being transmitted, information in the form of characters and numbers should be changed to bit streams. The presentation layer is responsible for interoperability between encoding methods as different computers use different encoding methods. It translates data between the formats the network requires and the format the computer.
• Encryption: It carries out encryption at the transmitter and decryption at the receiver.
• Compression: It carries out data compression to reduce the bandwidth of the data to be transmitted. The primary role of Data compression is to reduce the number of bits to be 0transmitted. It is important in transmitting multimedia such as audio, video, text etc.

Design Issues with Presentation Layer

• To manage and maintain the Syntax and Semantics of the information transmitted.
• Encoding data in a standard agreed upon way. Eg: String, double, date, etc.
• Perform Standard Encoding on wire.
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1. Overview

The OSI model stands for Open Systems Interconnection model and is a standardized framework that describes a networking system’s communication functions. It consists of 7 layers, and each layer has specific functionality to perform.

In this tutorial, we’ll discuss the OSI model and its layers in detail . We’ll present the description and functionality of each layer.

2. Introduction to the OSI Model

In the early days of networking systems, the networking devices were limited to communicate to the devices manufactured by the same vendors. For example, a networking device manufactured by a company X could only communicate with other devices manufactured by the same company, not with any other companies’ devices.

To ensure that the networking devices manufactured nationally and internationally can communicate with each other, the devices should be developed through a standard procedure. To bring standardization in the computing and communication system, the International Organization for Standardization (ISO) published the OSI model in 1984.

The OSI model consists of 7 layers: Application Layer, Presentation Layer, Session Layer, Transport Layer, Network Layer, Datalink Layer, Physical Layer. Each of these layers has a different role to play, and they work collaboratively to transmit the data from one networking device to another.

Let’s see how the OSI model:

Now, we’ll discuss the primary function of each of the layer:

3. Physical Layer

The lowest layer in the OSI model, the physical layer, is responsible for the transmission and reception of raw data between the network devices and the transmission medium . It stores the information or data in the form of bits and transit bits from one node to another:

The physical layer plays a vital role in controlling the transmission rate . Whenever some device sends data to the physical layer, it receives the data, converts the data to bits, and sends the data link layer. While transferring the bits, it defines the number of bits transmitted per second.

It facilitates the synchronization at the bit level by setting a clock that controls both the receivers and senders.

It defines the transmission mode in which the data flows between two networking devices.

The physical layer also defines the physical topology . A physical topology is an actual layout of the computer cables and other network devices.

The network protocols which are used in the physical layer are RS-232 , UTP cables , DSL .

4. Data Link Layer

The data link layer is responsible for the data transfer between two directly connected nodes. When the physical layer receives data, it converts it to bits and sends it to the data link layer. The data link layer ensures the data or packet received from the physical layer is error-free and then transfers the data from one node to another. It sends the packet to another node using its MAC address.

The data link layer is further divided into two sub-layers: Logical Link Control (LLC) and Media Access Control (MAC) . The LLC layer is responsible for checking the errors in the received packet, and it synchronizes the frames. The MAC layer controls the access of networking devices to the transmission medium. It also gives permission to transfer the data from one node to another.

As soon as the raw data from the physical layer reaches the data link layer, it converts the raw data into packets, called frames. With a frame, it also adds a header and trailer. A header and trailer include information for hardware destination and source address. In this way, it also helps the packet to reach the destination:

The primary responsibility of the data link layer is to control the flow of the data. Suppose some data is transferred from one high processing server to another low processing server. The data link layer makes sure the data rate matches between two servers and no data get corrupted.

The data link layer is also responsible for error control and access control .

The most popular computer networking technology used in the data link layer is Ethernet .

5. Network Layer

The network layer transmits data from one transmitting station to another. It also helps select the shortest path from one host to another to deliver the data at the best possible time. This process is called routing. It also puts the IP addresses of the sender and receiver into the header.

The network layer provides a logical connection between different network devices. It also plays an important role in addressing. When the incoming data arrives at the network layer, it adds the destination and source address with the header:

The network layers use various protocols like IPv4/IPv6 , ARP , ICMP to carry out its functionalities.

6. Transport Layer

The transport layer works between the network layer and the application layer. It ensures the end-to-end and complete transfer of the data. It also provides acknowledgment to the sender node when the data transmission is complete. If there is an error occurred during transmission, the transport layer re-transmit the data.

The transport layer receives the packets from the network layer and divides the packets into smaller packets, known as segments. This process is called segmentation . It also controls the flow and error of the data and helps data to reach the receiver end successfully:

At the receiver’s end, the transport layer receives the data in a segment form. It reassembles the segment and gives acknowledgment for the successful transformation of data.

As the transportation layer deals with packet transfer between sender and receiver station, it mainly uses TCP , UDP protocols to transfer packets.

7. Session Layer

The session layer is used to establish a connection, maintain and synchronizes the sessions between communicating devices. It also provides authentication and ensures security.

When transmitting data in the form of segments, the session layer adds some synchronization points. If some error occurs, the transmission restarts from the last synchronization point. This process is called synchronization and recovery:

The session layer also plays an important role as a dialog controller. A dialog controller identifies the mode of communication for a session. It allows communication between two systems in half-duplex or full-duplex .

In order to allow applications to communicate with different computers, the session layer uses network protocols like NetBIOS  or  PPTP .

8. Presentation Layer

The presentation layer gives syntax and semantics of the data exchanged between the two networking systems. It provides translations and is also known as a translation layer.

It also ensures data privacy by providing encryption and decryption to the data. It helps in the reduction of the number of bits needed to represent data. This process is formally known as data compression:

In order to ensure the privacy and security of the data being transferred, the presentation layer uses SSL or TSL protocol.

9. Application Layer

The application layer is the last layer in the OSI model, and it is very close to the software application. It acts as a window between a software application and an end-user. It helps to access the data from the network and display the information to the user.

The application layer performs several functions like it allows a user to access a file remotely, manage the files, retrieve specific data from a computer:

Popular protocols that are used in the application layer are e.g.  HTTP , FTP , DNS , SNMP .

10. Conclusion

In this tutorial, we’ve discussed the OSI model in detail. We presented a concise discussion on each layer of the OSI model with its functionalities.

The OSI Model

What is the osi model.

How a single bit travels from one computer to the next is a complex concept. In 1984, the open systems interconnection (OSI) model was published as a framework for network communication. The model breaks down computer network communication into seven layers. All of the layers work together to create a digital message. The message is built as it moves down the protocol stack. However, it is not sent to another network until it reaches the physical layer.

The model helps IT, computer science, and cybersecurity professionals understand how a single bit travels from one computer to the next by breaking the system into these layers.

From physical devices to user interfaces (UI), this model explains the communication role of each layer in overall computer networking. This article will start by introducing the Physical Layer (Layer 1).

Layer 1: the physical layer

The physical layer is where data moves across network interfaces as digital signals. Additionally, this is where the transmitting and receiving of network communication occurs. Starting with the Application Layer the message moves down the OSI model, and it eventually reaches the Physical Layer for transmission. When the message is received by the physical layer, the message will then move up the OSI layers until it reaches the final application layer.

Layer 2: data-link layer

Electrical signals received (or transmitted) to the physical layer are linked and translated to digital logic in the data-Link layer . Computer devices may be networked at the Data-Link layer, but only as a Local Area Network (LAN). Connecting a LAN to another LAN occurs at Layer 3.

Within Layer 2, the Protocol Data Unit (PDU) known as a frame consists of a header, footer, and data. Understanding how a frame is structured is important for network traffic analysis.

Additionally, within Layer 2, physical addresses are assigned and are also known as MAC addresses and/or hardware addresses in networking. MAC addresses are unique to each device on a local network. They are 48-bits in length and are assigned in hexadecimal characters.

Some other things to note about Layer 2 is that there are a few protocols that reside in it that we should know about:

• Ethernet : The most common type of LAN, Ethernet is the standard used to connect computing devices, routers, and switches in a wired network.
• IEEE 802.11 : “Wi-Fi” or “Wireless LAN.”
• Fiber Distributed Data Interface (FDDI) : Network standard for fiber optic LAN connections.
• Link Layer Discovery Protocol (LLDP) : A Link Layer protocol used for advertising neighbors, identity, and capabilities on a LAN.
• Address Resolution Protocol (ARP) : Converts and links Internet Protocol (IP) addresses to MAC addresses on a LAN.
• Cisco Discovery Protocol (CDP) : Similar to LLDP, but Cisco proprietary. The protocol collects neighbor information of directly connected LAN devices.

Additionally, Layer 2 is split into two sublayers:

• Logical Link Control (LLC) : Responsible for establishing the logical link between devices on a local network.
• Media Access Control (MAC) : Responsible for the procedures used by devices across a network medium.

Layer 3: network layer

When we think of the internet, we are thinking of interconnected networks. Interconnecting networks refer to a Local Area Network (LAN) connection to neighboring or remote networks. Layer 3 of the OSI model, the network layer , is where internetworking takes place and is where logical addresses are assigned to networked devices. A primary function of this layer is to route network packets from one LAN to another. Routing requires IP addresses and logical mapping of other networks across the internet to properly deliver messages. Another important function of Layer 3 is its ability to fragment and reassemble large communication. When Layer 3 passes a message down to Layer 2 for transmission, message length limits may be encountered in some cases.

Additionally, Layer 3 is the layer where the protocols used to route communication between networks reside. A few common network protocols are:

• Internet Protocol (IP) : IPv4 and IPv6 are two versions of IP, and IPv4 is the most common protocol of the Internet .
• Internet Protocol Secure (IPSec) : A more secure version of IP which leverages cryptography.
• Routing Information Protocol (RIP) : Distance-vector routing protocol that uses hop count as a metric of routing.
• Enhanced Interior Gateway Routing Protocol (EiGRP) : Cisco proprietary. A distance-vectoring protocol used for automating network configurations and routing decisions.
• Internet Control Message Protocol (ICMP) : Network protocol used for error reporting of network issues.
• Border Gateway Protocol (BGP) : A routing protocol designed to exchange routing information automatically on the internet.

Within Layer 3, the Protocol Data Unit (PDU) is the packet . Packets encapsulate data intended for transmission with header and footer data.

The IPv4 protocol encapsulates data with IPv4 header information necessary for delivery. For example, the 32-bit packet format contains the source address, the destination address, protocol, time-to-live (TTL), etc. in the IPv4 header data.

Layer 4: transport layer

The transport layer , Layer 4, is responsible for being the go-between the abstract layers of the OSI model (Layers 7-5) and the concrete communication layers (Layers 3-1).

Depending on the type of application, the transportation of that application’s communication will need to be handled in a specific way. For example, basic web browsing communication uses Hypertext Transfer Protocol (HTTP) . HTTP communicates via a specific connection service type and port. The transport layer is responsible for delivering/receiving the HTTP communication and maintaining the connection throughout the HTTP communication.

The Protocol Data Unit (PDU) at Layer 4 is known as a data segment . Segmentation is the process of dividing raw data into smaller pieces. Once the raw data is packaged from the higher application layers it is segmented at the transport layer before being passed to the Network Layer.

The transport layer protocols are divided into two categories depending on their connection service type:

Connection-oriented services

This connection type establishes a logical connection between two devices prior to beginning communication across a network. Connection-oriented protocols typically maintain service connection by following a set of rules that initiate, negotiate, manage, and terminate the communication. The Transport Layer protocols will also retransmit any data that is received without acknowledgment. The most common Connection-Oriented protocol is the Transmission Control Protocol (TCP) and its process to manage a connection between two devices is called the Three-Way Handshake . In TCP communication, the communicating devices typically share a client/server relationship where a client initiates communication with a service. The handshake involves the process of sending special TCP messages to synchronize a state of negotiated connection in communication.

Connectionless services

In connectionless communication, the protocol does not establish a connection between client and server. Instead, once a request is made to the server, the server sends all data without initiation, negotiation, or management of connection. Connectionless protocols also do not attempt to correct any interruptions in data transmission. Once the server sends the data, the server is not concerned if the client receives it.

When TCP or UDP are used to establish communication, the communication is assigned a port as the Layer 4 address. A port is a logical assignment given to processes and their respective application protocols on a computing system. A few important facts to memorize about ports are:

• There are 65,535 valid port numbers available to assign to a communication process.
• Ports 0 - 1023 are Well-Known Ports : Assigned to universal TCP/IP application protocols. These protocols are the most common such as HTTPS, SSH, FTP, DNS, and the list goes on. They are registered to these protocols by a global
• Ports 1024 - 49,151 are Registered Ports : Reserved for application protocols that are not specified as universal TCP/IP application protocols.
• Ports 49,152 - 65,535 are Private/Dynamic Ports : These ports may be used for any process without the need to register the port with the global assigning authority.
• When TCP and IP are used together, a Layer 4 port and a Layer 3 IP address are assigned to the connection. This is called a socket. For example, 8.8.8.8:443 is a socket indicating that communication to IP address 8.8.8.8 is to connect to port 443 on the server.

Layer 5: session layer

The session layer starts, manages, and terminates sessions between end-user application processes. Sessions are considered the persistent connection between devices. A session is application-focused; sessions are not concerned with layers 1-4. Instead, the session layer controls dialog between two networked devices. It is considered to facilitate host-to-host communication. Sessions dialog may be controlled through synchronization checkpoints, and through management of communication modes. There are two modes of communication permitted at Layer 5:

• Half-Duplex : Communication travels in both directions between sender and receiver, but only one device may transmit a message at a time.
• Full-Duplex : Communication travels in both directions between sender and receiver, and messages may be sent simultaneously in either direction.

The session layer resembles a phone conversation. For example, when a person picks up a phone and calls someone else a session is created. Once the communication on the call is completed, the session is terminated by hanging up the phone. In computing, software applications are making the phone call and establishing a session.

Two common Layer 5 protocols still used today are:

• Remote Procedure Call (RPC)

Layer 6: presentation layer

The presentation layer is primarily responsible for presenting data so that the recipient will understand the data. Data formatting and encoding protocols apply at Layer 6 to ensure data is legible and presented properly in the application receiving it. Data compression is also a function of Layer 6. If necessary, data may be compressed to improve data throughput over network communication.

Some common Layer 6 protocols are ASCII , JPEG , GIF , MPEG , and PNG .

Another main function of the presentation layer is the encryption and decryption of data sent across a network. Most encryption communication protocols straddle multiple layers of the OSI model, but the actual encryption function is Layer 6.

Two of the most common secure communication protocols are:

Transport Layer Security (TLS)

• Secure Socket Layer (SSL)

Layer 7: application layer

The topmost layer of the OSI model is the application layer . On computer systems, applications display information to the user via the UI.

Note : Software applications running on a computer are NOT considered to reside in the application layer. Instead, they leverage application layer services and protocols that enable network communication.

For example, the user can craft messages and access the network from the application layer. A web browser application allows a user to access a web page. The user may input information and receive information through the web browser. However, the application layer protocol HTTP performs the network communication function. The web browser and HTTP work closely together, and the distinction between the two may be subtle. Yet, HTTP is the web browsing protocol for all web browser applications. In contrast, no single web browser software exclusively utilizes HTTP.

HTTP is one of many common application layer protocols. Below are a few additional protocols to know. It is also good practice to memorize the associated port assigned to the protocols:

The OSI model breaks down computer network communication into seven layers. All of the layers work together to create a digital message. Understanding the OSI model will help you communicate with other network technologists. Computer networking may seem complex, but, with a bit of study, you can gain this knowledge to become an effective Cybersecurity Analyst.

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