The OSI & TCPIP Model Explained

The OSI & TCP/IP Model – Networking 101

By Published Network Fundamentals

Fundamental Networking – CCNA – The OSI & TCP/IP Model

In this post, we’ll go over fundamental networking. As an overview, we begin to unravel the world of networking. Over the years, because the concept and technology have evolved to be layered and complex the brilliant engineers at the International Organization for Standardization & Defense Advance Research Project Agency (DARPA) came up with a model to better understand, visualize, and troubleshoot networks.

 

What is the TCP & OSI Reference Model?

The OSI reference model (Open Systems Interconnection) was created by the International Organization for Standardization (ISO), an international standard-setting body. It was designed to be a reference model for describing the functions of a communication system. The OSI reference model is a framework for creating and establishing networking standards and networks devices and describes how applications on different devices can communicate through the network.

The OSI Reference Model has a total of seven layers, with each respective layer describing different functions of data traveling through a network. Here is the graphical representation of these layers:


The OSI reference model layers are numbered from the the Physical layer, which is where the network data transmission begins. The OSI reference model is important to remember, and you must remember the model! This is often asked in various ways not just in your certification exams but also on interviews and daily troubleshooting in the networking world! As you prepare for your exam, you must remember this. One easy way that people memorize this is through the mnemonic:  Please Do Not Throw Sausage Pizza Away

 

So, what is the purpose of these layers?’

They are most commonly used by vendors. They enable them to implement some functionality into a networking device, which then enables easier interoperability with devices from other vendors.

Here is a brief description of each of the seven layers of the OSI reference model.

  • Physical – (Layer 1) The Physical layer defines how data bits transmit from one device to another. This involves physical aspect of the network such as cables, network connectors, and network cards receive and transmit bits.
  • Data – (Layer 2) The Data Link Layer is the Data Encapsulation and packets in a frame. A frame contains a header and a trailer that enable devices to communicate. A header (most commonly) contains a source and destination MAC address. A trailer contains the Frame Check Sequence field, which is used to detect transmission errors.  The trailer portion is used to determine if the frame arrived without error. The data link layer has two sublayers:

1. Logical Link Control – The Purpose is for flow control and error detection.
2. Media Access Control – MAC Address is used for hardware addressing and for controlling the access method.

  • Network – (Layer 3) Network Layer defines device addressing, routing, and path determination. Device (logical) network addressing is used to identify a host on a network (it is also known as the device IP address).
  • Transport – (Layer 4) Network Transport segments large chunks of data received from the upper layer protocols. Transport protocol at layer 4 establishes as well as terminates connections between endpoints It is often used for flow control and data recovery.
  • Session – (Layer 5) Session layer defines how to establish and terminate a session between the two systems.
  • Presentation – (Layer 6) Presentation Layer defines data formats. Compression and encryption are found  this layer.
  • Application – (Layer 7) The application layer closest to the user, hence the name application. It allows network applications to communicate with other network applications which is visible the end user.

It is a common practice to reference a protocol by the layer number or layer name. For example, HTTPS is referred to as an application (or Layer 7) protocol. Network devices are also sometimes described according to the OSI layer on which they operate – e.g. a Layer 2 switch or a Layer 7 firewall.

The following table shows which protocols reside on which layer of the OSI model:

TCP/IP model

The TCP/IP model was created in the 1970s by the Defense Advance Research Project Agency (DARPA) as an open, vendor-neutral, public networking model. Just like the OSI model, it describes general guidelines for designing and implementing computer protocols. It consists of four layers: Network Access, Internet, Transport, and Application:

TCP IP model

The following picture show the comparison between the TCP/IP model and OSI model:

TCP IP and OSI model comparison

As you can see from the picture above, the TCP/IP model has fewer layers than the OSI model. The Application, Presentation, and Session layers of the OSI model are merged into a single layer in the TCP/IP model. Physical and Data Link layers found in OSI Model are called Network Access layer in the TCP/IP model. See below for a brief description of each layer:

  • Network Access– The Network Access Layer defines the protocols and hardware required to deliver data across a physical network.
  • Internet –  The Internet Layer defines the protocols for the logical transmission of packets over the network.
  • Transport – The Transport Layer defines protocols for setting up the level of transmission service for applications. This layer is responsible for reliable transmission of data and the the error-free delivery of packets.
  • Application – The Application Layer defines protocols for node-to-node application communication and provide services to the application software running on a computer.

 

Similarities Between OSI and TCP / IP Network Models

  • Both the OSI & TCP/IP Reference models have layers and were a response to the need to better understand networking.
  • The layers in the models are compared with each other. The physical layer and the data link layer of the OSI model correspond to the link layer of the TCP/IP model. The network layers and the transport layers are the same in both the models. The session layer, the presentation layer and the application layer of the OSI model together form the application layer of the TCP/IP model.
  • In both OSI & TCP/IP Reference models, data is divided into packets and each packet take the routes from the source to the destination over protocols.

Differences Between OSI and TCP / IP Network Models

  • The OSI model is a generic model that is based upon functionalities of each layer. TCP/IP model is a protocol-oriented standard.
  • The OSI model distinguishes the three concepts, namely, services, interfaces, and protocols. TCP/IP does not have a clear distinction between these three.
  • The OSI model gives guidelines on how communication needs to be done, while TCP/IP protocols layout standards on which the Internet was developed. So, TCP/IP is a more practical model.
  • In the OSI model, developed first and then the protocols in each layer were developed.
  • In the TCP/IP model, it was the opposite, the protocols were developed first and then the model was developed.
  • The OSI model – seven layers, where TCP/IP model has four layers.

 

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