OSI Model

The OSI Model (Open Systems Interconnection Model) is a conceptual framework used to describe the functions of a networking system.

What is The OSI Model Definition and Meaning?

This model defines a hierarchical architecture that logically partitions the functions required to support system-to-system communication. There are a total of seven layers that are responsible for specific tasks and functionalities. 

First developed in 1978 by French software engineer and pioneer, Hubert Zimmermann, The OSI Model has become widely adopted by all major computer and telecommunication companies since its inception in 1984. It belongs to the International Organization for Standards (ISO) and its identification is ISO/IEC 7498–1.  

The model was introduced to standardize networks so that multi-vendor systems could occur. Before this, it was only possible to have a one-vendor network, as all the devices from one vendor were not capable of communicating with others. The OSI Model is referenced many times when troubleshooting or describing network operations.  

Today, the OSI Model is the most utilized method when describing network communications. It is still widely used, as it offers the opportunity to identify threats across all tech stacks, and is essential for creating the mindset of “security-first” for cloud adaptation.

It also offers the ability to keep a data-centric posture and can be changed according to specific requirements to help secure cloud infrastructures. 

The Windows operating system utilizes an architectural network that is molded around the OSI model, and AppleTalk uses this model to give standards for the creation and development of networking software.

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Importance of OSI Model

The OSI Model is extremely important as it has literally defined the general jargon and terminology utilized in networking discussions, as well as documentation. It provides the ability to break down and pick apart highly complex communication processes, and evaluate all their important components. The OSI Model security architecture gives way to the understanding of the functions which support all communication that operates between different systems.

Characteristics of the OSI model

There are several key features that comprise the OSI Model: 

  • It is easy to see how hardware and software collaborate together.
  • We are able to understand new technologies as soon as they are developed.
  • Troubleshooting is easier by separate networks.
  • It gives the ability to compare basic functional relationships on different networks.

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OSI Reference Model

The OSI reference model refers to the communication layers between different computing systems. The purpose of the OSI reference model is to bridge the gap between developers and vendors so the digital communication products and software programs they create will interoperate, and to facilitate clear comparisons among communications tools.

OSI Model Explained

When asking, “what is an OSI reference model,” one needs to look at the larger picture of communication between systems, and realize it can be taken apart, in several different abstract layers, as there are 7 layers of the OSI Model. It is a reference model that shows how different applications talk to each other over a network and plays a key part in the deliverance of messaging between systems.

Advantages and Disadvantages of OSI Reference Model

The Advantages of OSI Reference Model

  • The OSI Reference Model in networking distinctly separates all services, interfaces, as well as protocols. Therefore, it is very flexible, and protocols within every layer are able to be replaced easily depending on the network’s nature.
  • All network models can be developed using this model, as it is so generic.
  • It is a model with layers which don’t affect other layers unless the interfaces between them change enormously.
  • It works with both connection-oriented and connectionless services.
  • Certain layers offer only partial functionality once it is practically deployed.
  • Its standards are based on theory and don’t offer adequate solutions when it comes to the practical implementation of networks.
  • When it was first launched, it was seen as “inferior quality,” as it didn’t meet the practical needs or the TCP/IP model.
  • The academia preferred using the TCP/IP model. Originally, there was huge resistance to its usage, as it was seen as a product of both the communities of Europe and the US government who were going to great efforts to force it upon both researchers, as well as programmers.

The Disadvantages of OSI Reference Model

  • It is a highly complex model, and when it was first implemented, it was slow and incurred a high cost.
  • Certain layers carry out the same services. Many of the 7 layers offer addressing, error control, as well as flow control.
  • It didn’t launch at the perfect time, as when it arrived, the TCP/IP protocols had already been implemented. Therefore, many businesses were, at first, reluctant to utilize it.

Principles of OSI Reference Model

  • It is a highly complex model, and when it was first implemented, it was slow and incurred a high cost.
  • Certain layers carry out the same services. Many of the 7 layers offer addressing, error control, as well as flow control.
  • It didn’t launch at the perfect time, as when it arrived, the TCP/IP protocols had already been implemented. Therefore, many businesses were, at first, reluctant to utilize it.

OSI Model simplified

So, what is the OSI Model? The OSI Model provides an eagle-eye view of how different networks operate. It does this with both hardware, as well as end-user applications. It’s a well-functioning model, as it gives programmers the opportunity to see exactly what is wrong in different networks, so they can pinpoint certain layers to troubleshoot.  

The OSI Model was created because back at the start of computer networking, certain products and networks could only communicate with networks of the same brand and product. For example, computers made by IBM could only communicate with other computers and networking devices made by IBM. This, of course, led to a great problem, as networking communication needed to function over many different systems and devices.

This wasn’t good for business, since people were forced to buy all their products from one computer manufacturer. What developed was certain top computing hardware manufacturers at the time agreed to utilize or support a networking model that was vendor-neutral together with their proprietary networking model.

There are two major components that make up the OSI Model – the basic reference model and protocols. Here, one of the layers in a network functions directly and communicates with the layers that are immediately below, as well as above it. This means that tools in Layer 2 will affect and communicate with tools in Layer 1 and 3.

Protocols give permission to each individual layer, which uses a specific host, to communicate with the layer that corresponds to it on a different host. Protocols offer the ability to send an email from a Layer 7 application such as Gmail, from Bangkok to a Microsoft Outlook account in Washington DC.

What Are The 7 Layers of The OSI Model?

Many ask “how many layers does the OSI Model contain?” The answer to this is seven different layers, each with its own function. 

As explained above, there is an OSI Model protocol stack which allows different layers to correspond between each other on different hosts. The OSI Model figure refers to the seven-layer model that is utilized in networking.

There are different devices in OSI Model layers, such as

  • Gateways (Session Layer)
  • Firewalls (Transport Layer)
  • Routers (Network Layer)
  • Switches, Bridges, Access Points (Data Link Layer)
  • Hubs, NICs, and Cables (Physical Layer).

SPECIAL TIP: A simple way to remember these layers is: All People Seem To Need Data Processing.The OSI Model’s seven layers defined and functions explained:

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SPECIAL TIP: A simple way to remember these layers is: All People Seem To Need Data Processing.The OSI Model’s seven layers defined and functions explained:

Layer 7 – Application Layer

The Application Layer in OSI Model provides the interface for the end-user who is using a device connected to a network. The user sees everything in this layer when it comes to loading an app like email. Through this layer, the following occurs – electronic messaging, support for file transfers, electronic mail, browsing the internet, and the ability to print on a network.   

This layer is the underlying service that supports programs and apps such as Microsoft Outlook. In other words, Microsoft protocols may include POP3, IMAP4, and SMTP, which are all protocols that are part of the Application Layer due to the fact that they all support the feature of email. This is how email works in the OSI Model. 

A terminal emulation program may be used here to connect to an OSI Model router. Should that occur, a secure shell would be used as the underlying service that would be part of the application layer and not the terminal emulation application itself.

Another important thing that occurs here is service advertisement, where one of the network’s devices wanting to offer a service, lets other network devices know about that service. An example is Apple’s AirPrint service, which is an app that discovers printers over the wifi OSI Model network that are compatible with it.  

Keep in mind, this is not the level where applications occur but rather the underlying service that supports it.

Layer 6 – Presentation Layer

In the Presentation Layer in OSI Model, data formatting will occur, such as a jpeg image. Very importantly, the question of “which layer of the OSI model addresses data encryption?” is answered here. 

For security purposes, if sensitive information is sent across the network, and it happens to be intercepted by a malicious user, it would appear jumbled and non-interpreted to them. Encryption prevents that from occurring. 

In essence, this layer translates and/or interprets all messages sent through the network.

Layer 5 – Session Layer

The Session Layer in OSI Model establishes all forms of request and response communication. It is responsible for setting up, maintaining, and tearing down a session. Once establishing a session, parameters may be exchanged which will be used during the session. 

For example, when setting up voice phone calls, negotiations are going to take place with regards to which UDP port numbers are going to be used for RTP (Real-time Transport Protocol), as well as what codec, and what method of encoding voice will be used. All this takes place at the Session Layer. 

Maintaining a session means making sure that nothing gets dropped, and if that occurs, that session will be re-established. Tearing down the session means that all participants of a conversation stop communication at the same time.

Layer 4 – Transport Layer

This Transport Layer is where traffic flow will be managed through the network layer to ensure there is as little congestion as possible, and it also checks for errors, making sure the service is of high quality by resending data when certain data has been corrupted. 

At this layer, popular methods of encryption and firewall security will occur. The Transport Layer in OSI Model concentrates on two protocols, TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). Industry professionals regard TCP as a reliable or connection-oriented protocol. 

A message that is sent to the receiver is referred to as SYN (Synchronization). Once that message is received, an acknowledgment referred to as ACK, is then sent back. This is called Synchronization and Acknowledgement (SYN-ACK), then acknowledgment (ACK) is sent by the original messenger.  UDP in OSI Model is regarded as an unreliable protocol or connectionless and is primarily used when there are issues with overhead. Windowing will also occur at layer 4, as well as buffering.

Layer 3 – Network Layer

The Network Layer in OSI Model is a routing layer that coordinates data conversation’s related parts to make sure that files are transferred. While the second layer handles the way that the physical layer transfers data, this layer organizes that data for purposes of transfer and reassembly. It handles all Routing Protocols and finds the best path to deliver data from a specific network to another. 

The Network Layer is also responsible for logical addressing, for example, the IPv4 OSI Model and IPv6 OSI Model. This is where routers which have ports would connect to switches which then go out to end devices. The router makes forwarding decisions based on the information of IP addresses. 

Switching will occur at this layer, such as packet switching, where a forwarding decision or switching the packet from the incoming interface over to the outgoing or egress interface takes place. Connection services such as flow control will also occur at this layer.

The Data Link Layer in OSI Model is broken up into two sublayers, the MAC (Media Access Control) sublayer, and the LLC (Logical Link Control) sublayer. The MAC layer covers the physical addressing of the network device, such as the MAC address on interface cards. It’s a 48-bit address which makes all cards unique from all other cards on all other devices.

Logical topology, which looks like a ring, takes place at the data link layer. The method of data transmission occurs, and it is easy to tell if there is any troubleshooting needed here. 

At the LLC sublayer lie connection services and synchronization of transmission which agrees when the bits start and stop. The types of synchronization that may occur are:

  • Isochronous Synchronization – Where devices search for a common device that is external for clocking
  • Asynchronous Synchronization – Where devices utilize their own internal clocks and at the same time using one or more start/stop bits
  • Synchronous Synchronization – Where both sender and receiver share clocking over a separate channel

At this layer, you will find ethernet switches to connect printers, PCs, and regular networking devices.

Here lies the method in which the network’s information is taken apart into frames and then transmitted over the Physical Layer. At this layer, error detection and correction occur, as well as certain addressing in order for different devices to differentiate themselves from one another in larger systems.

Layer 1 – Physical Layer

The Physical Layer in OSI Model is concerned with how bits are represented, and how we get them across the network. Layer 1 refers to all the physical and electrical features of devices. 

Devices that may be related to this layer are ethernet cables, as they are physical layer components, as bits travel over them, as well as fiber optic cables to send data, and network interface cards inside computers which encode data, so that it can be sent out on the wire and receive data. 

Other examples of devices are phone cords which are utilized for the purpose of DSL and dial-up services, as well as radio signals utilized in all forms of wireless communication. 

The Physical Layer functions also include signals’ conversion into something that another layer of the OSI Model can use, referred to above as a bit, and changing the signal for multiple usages of many users across the same connections.  

This also refers to our wiring standards and the jacks we use to plug into walls, such as the RJ45 jack, which allows for 8 different wires to be used as conductors. The way the wiring is put in is based on a standard called the T568B, which indicates the certain colors that go into the corresponding slots. 

Another aspect of this layer is the physical topology of the network, such as bus topology. Other aspects include synchronization, multiplexing strategy and bandwidth usage.

Here, at Layer 1, it converts electrical signals, as well as physical media without having any effect on information received through the link. However, you will also find more advanced media converters at the Data Link Layer, Layer 2.

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OSI Model vs TCP IP Model

The TCP/IP Model stands for the Internet Protocol Suite  It’s important to note that the OSI Model is not currently used, although it is widely referenced; the TCP/IP Model is used instead. The concepts are identical, however, the layers are slightly different.   The TCP/IP Model was first tested in 1975 and in March 1982, it was used in all the military computer networking of the US Department of Defense. DEC, IBM, and AT&T were the first giant organizations to use TCP/IP. It was used as the protocol standard for ARPANET (the Internet’s predecessor) in 1983.

Difference Between OSI Model and TCP IP Model

Just like the OSI Model, the TCP/IP Model is a model that standardizes computer networking. However, where the OSI Model contains 7 layers, the original TCP/IP Model contains only 4.

Similarities Between OSI Model and TCP IP Model

Both Models contain the Application, Transport, Internet, and Link layers. They are both numbered in ascending order, however, the direction depends on whether the traffic is being received or sent.

Eventually, a fifth layer was added to the TCP/IP Model, as well as a renamed layer. The Link Layer was split into Data Link and Physical, and the Internet layer is now known as the Network Layer. 

When comparing the layers of the OSI Model and the TCP/IP model, it is evident that the Application (Level 7), Presentation (Level 6), and Session (Level 5) Layers of the OSI/Model are compressed all into the Application (Level 5) Layer of the TCP/IP Model. All the other layers match identically. 
The data encapsulation process in OSI Model or TCP/IP Model occurs when certain extra information is added to the data item in order to place additional features on it.

What is the Bluetooth OSI Model?

Within the technology of Bluetooth, mobile devices are connected wirelessly over a short distance and form a PAN (Personal Area Network). The Bluetooth architecture has its own model which doesn’t follow the OSI or TCP/IP Model. 
It has the physical (radio and baseband), data link (baseband, LMP, L2CAP), middleware (RFComm protocol, AT & SDP commands, adopted protocols), and application (all the application profiles that allow for Bluetooth app interactions). 

What is the DoD OSI Model?

DoD stands for Department of Defense. As stated above, in March 1982, the TCP/IP Model was used in all military computer networking. The DoD Model is a smaller version of the OSI Model. It’s condensed into four layers. These layers are as follows:

DoD Model

-Process/Application Layer Application, Presentation and Session
-Host-to-Host Layer Transport
-Internet Network
-Network Access

OSI Model

-Application, Presentation and Session
-Data Link, Physical Layer

The OSI Model for Dummies

The OSI Model has seven layers. It consists of 2 acronyms, PDNTSPA (Bottom/Up) and APSTNDP (Up/Bottom). The best way on how to remember the 7 layers of the OSI Model is through the first acronym: Please Do Not Throw Salami Pizza Away; the second acronym can be remembered as All People Seem To Need Data Processing.

The OSI Model flow chart shows how data flows in the OSI Model. This is an example of the flow of data:

  1. An application, like Gmail, creates data that an end-user sends. The Application Layer puts on a header/encapsulation field which has information like screen size, as well as fonts, and the data is then passed on to Layer 6.
  1. Layer 6 places the Presentation Layer header information, which will then be converted to ASCII for example, and this new data will flow onto Layer 5.
  1. The same process as the one above applies to the Session Layer and all the information managed here will be passed on to Layer 4.
  1. In the Transport Layer, the data that the segment was received will be placed in the header, and this will be passed on to the next layer.
  1. Layer 3 places all the Network Layer information, for example, the address of the source and destination, to aid this layer in determining the most successful delivery path for the packets, and all this data will go on to Layer 2. 
  1. This layer places the Data Link Layer header and trailer information, making sure that the data is not corrupt, and this information will then flow on to the next layer for transmission over the media.
  1. On this Physical Layer (Layer 1), all the ones and zeros of the bit stream get transmitted. Bit synchronization will take place here which makes sure that the end-user information is put together in the exact order it was sent.

All these steps will take place in reverse order on the device that it is received.  

Highlighting The Benefits of Perimeter 81 for The OSI Model

Adaptive Layered Model: Changes in one layer do not affect other layers, provided that the interfaces between the layers do not change drastically. Layers in the OSI Model are distinguished based on services, interfaces, and protocols. 

Supports Multiple Protocols: The OSI Model supports connection-oriented as well as connectionless services.

Independent Generic Model: The OSI Model  is a generic model and acts as a guidance tool to develop any independent network model, differing itself from the TCP/IP protocol which is based on more specific and standard protocols.

OSI Model FAQs

What is The OSI Model? 
The OSI Model (Open Systems Interconnection Model) is a conceptual framework used to describe the functions of a networking system. The OSI model defines a hierarchical architecture that logically partitions the functions required to support system-to-system communication. There are a total of seven layers which are responsible for specific tasks and functionalities.
What Are The 7 Layers of The OSI Model?
Layer 7 – Application
Layer 6 – Presentation
Layer 5 – Session
Layer 4 – Transport
Layer 3 – Network
Layer 2 – Data Link
Layer 1 – Physical
OSI Model vs TCP IP Model
The main difference between The OSI Model and the TCP/IP Model is that The OSI Model has seven layers whereas the TCP/IP Model consists of four layers (Application, Transport, Internet, and Network Access).
How Do Real World Protocols Map to The OSI Model?
The OSI Model Protocol includes 7 layers:

Layer 7 – Application (SSH, FTP, Telnet)
Layer 6 – Presentation (HTTP, SNMP, SMTP)
Layer 5 – Session (RPC, Named Pipes, NETBIOS)
Layer 4 – Transport (TCP, UDP)
Layer 3 – Network (IP)
Layer 2 – Data Link (Ethernet)
Layer 1 – Physical (Cat-5)
What Are the OSI Model Layer Functions?
Layer 7 – Application is an abstraction layer and what the end user sees. 
Layer 6 – Presentation
Layer 5 – Session
Layer 4 – Transport
Layer 3 – Network
Layer 2 – Data Link
Layer 1 – Physical

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