Understanding the OSI Model: The Foundation of Network Communication

The OSI Model or Open Systems Interconnection model describes the seven (7) layers that computer systems use to communicate over a network. It was the first standard model for network communications, adopted by all major computer and telecommunication companies in the early 1980s. It was introduced in 1983 by representatives of major computer and telecommunication companies and was adopted by ISO as an international standard in 1984.

The Characteristics of the OSI Model

The Seven Layers of the OSI Model and Functions

1. Physical Layer - The physical layer is responsible for the physical cable or wireless connection between network nodes. It defines the connector, the electrical cable or wireless technology connecting the devices, and is responsible for transmission of the raw data, which is simply a series of 0s and 1s, while taking care of bit rate control. Examples of items in this layer are ethernet cable, fiber optic cable, network cards (LAN and WLAN) and cable connectors.

   
   

2. Data Layer - The data link layer establishes and terminates a connection between two physically connected nodes on a network. It breaks up packets into frames and sends them from source to destination. This layer is composed of two parts—Logical Link Control (LLC), which identifies network protocols, performs error checking, and synchronizes frames, and Media Access Control (MAC) which uses MAC addresses to connect devices and define permissions to transmit and receive data. The primary examples are MAC addresses or physical address of a network card, as well as network switches.

   
   

3. Network Layer - The network layer has two main functions. One is breaking up segments into network packets and reassembling the packets on the receiving end. The other is routing packets by discovering the best path across a physical network. The network layer uses network addresses which is called Internet Protocol Addressing either IPV4 or IPV6 to route packets to a destination node. In this layer, routers and IP addresses play a crucial role in establishing communication and transmission.

   
   

4. Transport Layer - The transport layer takes data transferred in the session layer and breaks it into “segments” on the transmitting end. It is responsible for reassembling the segments on the receiving end, turning it back into data that can be used by the session layer. The transport layer carries out flow control, sending data at a rate that matches the connection speed of the receiving device, and error control, checking if data was received incorrectly and if not, requesting it again. In addition, each segment, or data unit, has a source and destination port number, as well as a sequence number. The port number ensures that the segment reaches the correct application. The sequence number ensures that the segments arrive in the correct order. Examples of known protocols that exists in this layer are UDP and TCP. The former (TCP) favors data quality over speed, whereas the latter (UDP) favors speed over data quality.

   
   

5. Session Layer - The session layer creates communication channels, called sessions, between devices. It is responsible for opening sessions, ensuring they remain open and functional while data is being transferred, and closing them when communication ends. The session layer can also set checkpoints during a data transfer—if the session is interrupted, devices can resume data transfer from the last checkpoint. The Session layer determines which data packets belong to which files, as well as where these packets go. Also, the Session layer establishes, maintains, and ends communication between devices pertaining to the Syn-Ack method or usually called the “three-way handshake” or TCP handshake.

   
   

6. Presentation Layer - The presentation layer prepares data for the application layer. It defines how two devices should encode, encrypt, and compress data so it is received correctly on the other end. The presentation layer takes any data transmitted by the application layer and prepares it for transmission over the session layer. This layer receives application data from the Application layer, translates it into binary, and compresses it, then encrypts that data as it leaves the network. Further, the Presentation layer translates application formatting to network formatting and vice versa which allows the different layers to understand each other.

   
   

7. Application Layer - The application layer is used by end-user software such as web browsers and email clients. It provides protocols that allow software to send and receive information and present meaningful data to users. A few examples of application layer protocols are the Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Post Office Protocol (POP), Simple Mail Transfer Protocol (SMTP), and Domain Name System (DNS). Most user interactions are within this layer. Web browsers and other internet-connected applications (like Skype or Outlook) use Layer 7 or Application protocols. For example, a user is communicating with another user using an email application like Gmail or Outlook, these email services use specific protocols SMTP, IMAP or POP3 which have corresponding port numbers. Also, for web browsing, HTTP corresponds to port 80 while HTTPS is port 443.

Understanding the OSI Model is fundamental for anyone interested in networking. By breaking down network communication into these seven layers, it becomes easier to troubleshoot and understand how data moves across a network. Stay tuned for more posts where we dive deeper into each layer and explore practical applications and real-world examples.