7 Layers Of The OSI Model

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written by craig beam posted on November 10, 2021

The 7 Layers Of The OSI Model

The Open Systems Interconnection (OSI) model offers 7 layers that computer systems leverage to communicate over a network. OSI model was first introduced in 1983 as a combined effort of representatives of major computer and telecom networks and was adopted in 1984 as an international standard by ISO. It is the first standard model adopted by the largest computer and telecommunications companies for their network communication needs.

Today, the OSI-7 layer model is widely used across the IT and communications industries to visualize and design how networks operate. It is also a critical tool that helps isolate and troubleshoot a range of networking problems. This blog explains the OSI model and how to remember its 7 layers easily.

The 7 Layers of the OSI Model Explained

As mentioned earlier, the OSI model offers seven layers: the physical, data link, network, transport, session, presentation, and application:

Layer 7- Application Layer

This is the layer closest to the end-user. The application layer is typically used by end-user software such as email clients and web browsers. It comes with protocols that enable software to send and receive information while presenting meaningful data to the end-users. Notably, applications don’t reside at the application layer. Instead, this layer is designed to facilitate communication via lower layers, so connections with applications on the other end are established. Some of the common examples of application layer protocols include:

  • Domain Name System (DNS)
  • Hypertext Transfer Protocol (HTTP)
  • Simple Mail Transfer Protocol (SMTP)
  • File Transfer Protocol (FTP)
  • Post Office Protocol (POP)

Layer 6- The Presentation Layer

The presentation layer is the OSI Model layer that readies data for the application layer. In essence, the presentation layer outlines how two devices should encrypt, compress, and encode data to receive it in the appropriate format on each other’s end. It takes the data transmitted by the application layer and transforms it into a form that the application layer can accept, thus providing freedom from compatibility issues. Some of the layer six presentation layer examples include:

  • SCII
  • EBCDIC
  • TIFF
  • GIF,
  • PICT
  • JPEG
  • MPEG
  • MIDI.

 Layer 5- The Session Layer

Whenever two networked devices such as computers need to communicate with each other, a session is created. This session is typically done at the session layer. Layer 5 session layer is the layer that establishes, manages, and terminates connections between applications. In a nutshell, this layer help set up, coordinate, and terminate conversions. It can also set checkpoints during data transfer to ensure devices resume data transfer from the last checkpoint should there be interruptions on exchanges and dialogues between applications on each end. Layer 5 session examples include NFS, NetBios names, RPC, SQL.

Layer 4- The Transport Layer

The transport layer coordinates data transfer between end systems and hosts. It determines the much data to send, where it goes, at what rate, and more. The transport layer typically takes data transferred in the session layer and categorizes it to a segment on the transmitting end. It reassembles segments on the receiving end and transforms them into data that the session layer can use. This layer is also responsible for end-to-end error recovery and flow control. It checks whether the data was received correctly and, if not, requests it again. Layer 4 transport examples include SPX, TCP, and UDP.

Layer 3: the Network Layer

The network layer has two main roles. It helps break up segments into network packets before reassembling them on the receiving end. It also routes packets by discovering the ideal path across a physical network. The network layer leverage network addresses, mostly IP addresses, to route packets to a destination node. Apart from routing and forwarding, this layer also does other functions, including internetworking, addressing, error handling, packet sequencing, and congestion control. Typical examples of layer 3 networks are:

  • IP
  • AppleTalk DDP
  • IPX

 Layer 2- The Data Link Layer

The data link layer offers node-to-node data transfer between two nodes that are directly connected. This layer establishes and terminates a connection between two physically connected nodes on the network. It is designed to break packets into suitable frames before sending them from source to destination. This layer is made up of two parts: the Logical Link Control (LLC) and the Media Access Control (MAC). The Logical Link Control (LLC) identifies network protocols, checks for errors, and synchronizes frames. On the other hand, the Media Access Control (MAC) leverages MAC addresses to connect devices and define permission for the transmission and receiving of data.

Layer 1- The Physical Layer

At the bottom of the OSI model is the physical layer. This layer provides hardware means of sending and receiving data on a carrier. This may include cable tubes, radio frequency links, layouts of voltages, pins, and other physical needs. It is responsible for wireless or physical cable connections between network nodes. It also defines the connectors, the electrical cable, or wireless technology that connects the devices. Typically, the physical layer is the first layer where professionals check whenever a networking issue occurs. Pros check whether the cables are correctly connected and if the power plug has not been pulled off the router, switch, or computer.

How to Memorize the OSI Model 7 Layers

Experts have come up with several mnemonic tricks to help IT professionals memorize the OSI Model. These mnemonics can be handy if you are sitting for a college or certification test. Here are a few mnemonic tricks to aid your memory. Keep in mind the first letter of each word represents the first letter of an OSI model layer. (From layer 7 to Layer 1):

  • All Pros Search Top Notch Donut Places
  • All People Seem To Need Data Processing

Why You Need to Master the 7 OSI Layers

The OSI model is a great referencing tool guiding vendors and developers, so they create digital communication products and software programs that will interoperate. It also helps professionals provide an explicit comparison among various communication tools. If you are an IT professional, you need to know all the 7 layers of the OSI Model as it forms an integral part of your certification process. You also need to remember the OSI model when pitching about the layers your product works with. Once you understand the OSI model and its layers, it becomes easy to comprehend the protocols and devices that can interoperate with each other whenever new technologies are introduced.

If you have further questions or need help with OSI Model, don’t hesitate to contact MicroXpress. We provide a range of managed IT solutions that gives you the freedom to work anytime and anywhere. Contact us today to learn more about our solutions.