The OSI (Open Systems Interconnection) model is a conceptual framework used to understand how different networking protocols interact in layers to create a network. It was developed by the International Organization for Standardization (ISO) in the late 1970s and early 1980s. The OSI model divides the networking process into seven separate layers, each with a specific function:
Layer 7: Application Layer
- Closest to the end-user.
- Provides network services directly to applications such as web browsers and email clients.
- Protocols & Services: HTTP, FTP, SMTP, POP3, DNS.
Layer 6: Presentation Layer
- Transforms data into a format that can be consumed by the application layer above it.
- Handles data compression, encryption, and translation services.
- Examples: SSL/TLS, ASCII, EBCDIC, MPEG.
Layer 5: Session Layer
- Establishes, maintains, and terminates user connections.
- Manages sessions or connections between applications on different devices.
- Protocols: RPC, PPTP, SMB.
Layer 4: Transport Layer
- Ensures end-to-end communication, flow control, and error recovery.
- Segments data from the session layer and adds source and destination port numbers.
- Protocols: TCP, UDP, SCTP.
Layer 3: Network Layer
- Responsible for determining the best path to route data packets from the source to the destination.
- Deals with logical addressing (like IP addresses) and routing.
- Protocols: IP (both IPv4 and IPv6), ICMP, OSPF, BGP.
Layer 2: Data Link Layer
- Provides a means for data to be transferred from one device to another on a local network or a point-to-point connection.
- Concerned with physical addressing (MAC addresses), error detection, and framing.
- Sub-layers: Logical Link Control (LLC) and Media Access Control (MAC).
- Protocols & Technologies: Ethernet, Wi-Fi, PPP, ARP.
Layer 1: Physical Layer
- Defines the physical medium (like cables) and the characteristics of the transmission (like voltage levels).
- Concerned with bit-level transmission, signaling, and the physical connectors.
- Technologies: Ethernet cables (Cat5e, Cat6, etc.), optical fiber, USB, Bluetooth.
It’s important to note that the OSI model is largely theoretical. The TCP/IP model, which is a more practical framework and is the foundation of the internet, condenses the OSI model into four layers. However, the OSI model remains a valuable tool for understanding and teaching network operations.
Key terms in plain language
Open a term for a concise explanation of language used on this page.
Fiber Internet
Internet delivered through strands of glass using light. Fiber commonly supports high capacity, low latency, and strong upload performance, but availability must be confirmed for the exact address.
Broadband
A general term for always-on, high-speed Internet access. Broadband can be delivered over fiber, cable, DSL, fixed wireless, cellular, or satellite networks.
Bandwidth
The amount of data a connection can carry in a given time, usually measured in Mbps or Gbps. More bandwidth supports more users, devices, and simultaneous applications.
Latency
The time it takes data to travel between two points. Lower latency improves voice, video meetings, cloud applications, gaming, and other real-time services.
Dedicated Internet Access (DIA)
A business-grade Internet connection with capacity dedicated to the customer rather than shared in the same way as typical consumer broadband. It often includes symmetrical speeds and an SLA.
SD-WAN
Software-defined wide area networking. It manages multiple connections and chooses paths based on application needs, performance, and policy to improve resilience and control.