Layers of the OSI Model

The OSI model, or Open Systems Interconnection model, is a conceptual framework that describes how information from a software application in one computer moves through a physical medium to the software application in another computer. The OSI model was developed by the International Organization for Standardization (ISO) in the late 1970s and early 1980s to support the emergence of diverse and interoperable computer networking methods¹.

The OSI model consists of seven layers, each of which performs a specific function in the communication process. The layers are:

1. Physical Layer

The physical layer is the lowest layer of the OSI model, and it is responsible for transmitting and receiving raw bits over a physical medium, such as a cable, a wireless channel, or an optical fiber. The physical layer defines the characteristics of the medium, such as voltage levels, modulation schemes, connectors, and pinouts. The physical layer also handles error detection and correction at the bit level, such as parity checks and cyclic redundancy checks (CRC).

Some examples of protocols and standards that operate at the physical layer are:

Ethernet
Wi-Fi
Bluetooth
USB
HDMI

2. Data Link Layer

The data link layer is the second layer of the OSI model, and it is responsible for organizing bits into frames and transferring them between nodes on the same network segment or link. The data link layer also handles error detection and correction at the frame level, such as checksums and retransmissions. The data link layer can be divided into two sublayers: the logical link control (LLC) sublayer and the media access control (MAC) sublayer.

The LLC sublayer provides a common interface for different types of data link protocols, such as Ethernet, Wi-Fi, or ATM. The LLC sublayer also performs flow control and multiplexing functions.

The MAC sublayer defines how nodes access the shared medium and avoid collisions, such as by using carrier sense multiple access with collision detection (CSMA/CD) or collision avoidance (CSMA/CA). The MAC sublayer also assigns unique addresses to each node on the network, such as MAC addresses or hardware addresses.

Some examples of protocols and standards that operate at the data link layer are:

Ethernet
Wi-Fi
Bluetooth
ATM
PPP

3. Network Layer

The network layer is the third layer of the OSI model, and it is responsible for routing packets across different networks or subnets. The network layer also handles fragmentation and reassembly of packets, as well as congestion control and quality of service (QoS) functions. The network layer uses logical addresses to identify nodes on different networks, such as IP addresses or network addresses.

Some examples of protocols and standards that operate at the network layer are:

IP
ICMP
ARP
RARP
RIP
OSPF
BGP

4. Transport Layer

The transport layer is the fourth layer of the OSI model, and it is responsible for establishing end-to-end connections between applications on different nodes. The transport layer also provides reliable or unreliable delivery of data, as well as segmentation and reassembly of data streams. The transport layer uses port numbers to identify different applications or processes on each node, such as TCP ports or UDP ports.

Some examples of protocols and standards that operate at the transport layer are:

TCP
UDP
SCTP
DCCP

5. Session Layer

The session layer is the fifth layer of the OSI model, and it is responsible for creating, maintaining, and terminating sessions between applications on different nodes. A session is a logical connection that allows bidirectional communication between applications. The session layer also provides synchronization, checkpointing, and recovery functions for sessions.

Some examples of protocols and standards that operate at the session layer are:

6. Presentation Layer

The presentation layer is the sixth layer of the OSI model, and it is responsible for translating data between different formats and encoding schemes. The presentation layer also performs encryption, decryption, compression, decompression, and other transformations on data to ensure compatibility and security between applications.

Some examples of protocols and standards that operate at the presentation layer are:

SSL/TLS
MIME
JPEG
GIF

7. Application Layer

The application layer is the highest layer of the OSI model, and it is responsible for providing services and interfaces to users and applications. The application layer defines how applications communicate with each other over a network, such as by using request-response or publish-subscribe models. The application layer also defines protocols for specific types of applications, such as web browsing, email, file transfer, remote access, etc.

Some examples of protocols and standards that operate at the application layer are:

HTTP
FTP
SMTP
POP3
IMAP
DNS
DHCP
Telnet
SSH

Layers of the OSI Model