OSI Reference Model

The OSI Reference Model, or Open Systems Interconnection Reference Model, is a framework that defines the rules and procedures on how data should be sent across a network.

According to the model, which was developed by ISO (International Organization of Standardization) in 1984, there are 7 layers to the model, which is used together to form most packets sent across any network in existence.

OSI Reference Model

Layer 1 – Physical

The physical layer is the most obvious layer which covers anything physical to connect network devices. As the physical layer, it’s responsible for taking electrical signals and translating them into bits.

Layer 1 examples include cables, network cards, wireless radios, switches, routers, and more.

Layer 2 – Data-Link

The data-link layer is responsible for encapsulating, or packaging all the other layers after it, into a frame so that it may be sent across the physical layer.

The data-link layer contains two sub-layers: MAC (Media Access Control) and LLC (Logical Link Control).

1. The MAC sub-layer is used to control how devices gain access to the network and get permission to transmit which is why the MAC address is so important.
2. The LLC sub-layer is responsible for encapsulation, error checking, and frame syncing, or essentially how data is transmitted and received at layer 2.

Layer 2 examples include Ethernet, 802.11 Wi-Fi, Frame Relay, and more.

Layer 3 – Network

The network layer is responsible for controlling communications between networks. As such, it’s responsible for addressing, or assigning/identifying the host and the network the host is on, and routing, or figuring out how to get a packet from point a to point b in the most efficient way.

Layer 3 examples include IP (Internet Protocol), OSPF (Open Shortest Path First), RIP (Routing Information Protocol), and more.

Layer 4 – Transport

The transport layer is responsible for ensuring that data transferred between two devices is correct and error free.

There are two types of transmissions that occur with the transmission protocol – connection oriented transmissions and connectionless transmissions.

1. Connection oriented transmissions require that a connection be established first before data is sent.
2. Connectionless transmissions do not require a connection is established.

Layer 4 examples include TCP, UDP, and more. TCP is a connection oriented protocol, which is why it needs to go through a handshake process before data is sent. UDP is a connectionless protocol, which means data is blasted into the network without the distant end confirming data can be received.

Layer 5 – Session

The session layer is responsible for creating, maintaining, and ending sessions between network devices.

Layer 5 examples include RPC, SQL, and more.

Layer 6 – Presentation

The presentation layer is used to format data for the application layer to use. In addition, the presentation layer is also responsible for providing encryption/decryption and compression/decompression of files.

Layer 6 examples include JPG, GIF, MP3, WAV, AVI, MOV, ASCII, SSL, TLS, and more.

Layer 7 – Application

The application layer is the final interface between the application used on a system and the network. This is typically all protocols.

Layer 7 examples include HTTP, HTTPS, FTP, and more.