What is the OSI model?

The OSI model (Open Systems Interconnection) is a reference seven-layer model, developed by ISO, that describes how data travels through a network from an application on one device to an application on another.

Short formulation for an interview:

OSI is a conceptual model of 7 layers that standardizes the interaction of network systems. Each layer is responsible for its part of network communication: from physical transmission to the work of applications.

OSI is NOT a protocol, but a theoretical model, on the basis of which protocols and networks are built.

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#The main idea of OSI

The OSI model answers the question:

How does data turn from packets into electrical signals and back?

It breaks the process of network exchange into 7 layers, where each layer:

• performs its own tasks,
• interacts with neighboring layers,
• hides its internal workings from other layers.

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#The 7 layers of the OSI model: a breakdown of each layer

#1. Physical Layer

Transmits raw bits over the physical medium:

• electrical signals
• optical pulses
• radio waves
• connectors, cables, frequencies

Devices: cables, connectors, hubs, transceivers Example technologies: Ethernet physical layer, Wi-Fi radio, fiber optics, USB

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#2. Data Link Layer

Transmits frames between devices within one network (LAN).

Responsible for:

• MAC addressing
• error detection (CRC)
• media access control

Sublayers:

• MAC (Media Access Control)
• LLC (Logical Link Control)

Examples: Ethernet, Wi-Fi (802.11), PPP, VLAN (802.1Q) Devices: switch

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#3. Network Layer

Moves packets between networks (routing).

Functions:

• IP addressing
• routing
• packet fragmentation

Protocols:

• IPv4
• IPv6
• ICMP
• OSPF, BGP, RIP (routing)

Devices: router

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#4. Transport Layer

Ensures reliable or fast delivery of data between hosts.

Functions:

• port numbers
• segmentation and reassembly
• delivery control

Protocols:

• TCP: reliable, with acknowledgment
• UDP: fast, without acknowledgment

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#5. Session Layer

Manages sessions between applications.

Functions:

• creating/maintaining/terminating connections
• dialog control
• synchronization

Rarely used directly in modern protocols.

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#6. Presentation Layer

Responsible for data format:

• encoding (ASCII, UTF-8)
• compression
• encryption (SSL/TLS partially belongs here)

Example: converting data into a format suitable for transmission.

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#7. Application Layer

The layer where the user and applications operate.

Protocols:

• HTTP / HTTPS
• DNS
• FTP / SFTP
• SMTP / POP3 / IMAP
• DHCP
• SNMP

These are not applications, but network protocols for applications.

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#How data passes through the OSI layers

#On the sending computer:

Application → Presentation → Session → Transport → Network → Data Link → Physical

#On the receiving computer:

Physical → Data Link → Network → Transport → Session → Presentation → Application

Each layer adds or removes its own header (encapsulation / decapsulation).

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#Comparing OSI and TCP/IP

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TCP/IP is a practical model, OSI is a theoretical one.

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#Why the OSI model is needed

• helps understand how networks work, layer by layer
• useful for diagnostics (identify at which layer the problem is)
• applicable when designing networks
• structures the work of protocols
• used in interviews and exams (Cisco CCNA, CompTIA Network+)

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#A ready answer for an interview

The OSI model is a seven-layer reference model for the interaction of network systems. Each layer is responsible for its part of data transmission, from the physical medium to applications. Layers: Physical, Data Link, Network, Transport, Session, Presentation, Application. OSI helps understand network protocols, diagnostics, and network design.

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#Cheat sheet (5 seconds)

7 OSI layers:

1. Physical: signals, cables
2. Data Link: MAC, frames, switch
3. Network: IP, routing
4. Transport: TCP/UDP
5. Session: session management
6. Presentation: encryption, encoding
7. Application: HTTP, DNS, SMTP