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