OSI Model - Open Systems Interconnection

The OSI Model (Open Systems Interconnection) is a conceptual reference framework that describes how data is transmitted across a network, dividing the process into 7 distinct layers.

What is the OSI Model?

The OSI Model is an international standard developed by ISO (International Organization for Standardization) that defines a conceptual framework for network communication. Each layer has specific responsibilities and communicates with adjacent layers.

The 7 Layers of the OSI Model

Layer 7: Application (Application Layer)

Function: Interface between user and network

Characteristics:

  • Services: Network services for applications
  • Protocols: HTTP, FTP, SMTP, DNS, Telnet
  • Functions: Authentication, compression, encryption
  • Examples: Web browsers, email clients

Data: APDU (Application Protocol Data Unit)

Layer 6: Presentation (Presentation Layer)

Function: Data format and conversion

Characteristics:

  • Encoding: ASCII, Unicode, EBCDIC
  • Compression: Data size reduction
  • Encryption: Data security
  • Conversion: Data format between systems

Data: PPDU (Presentation Protocol Data Unit)

Layer 5: Session (Session Layer)

Function: Session control between applications

Characteristics:

  • Establishment: Session creation
  • Maintenance: Active session management
  • Termination: Session closure
  • Synchronization: Checkpoints

Data: SPDU (Session Protocol Data Unit)

Layer 4: Transport (Transport Layer)

Function: Reliable data transfer

Characteristics:

  • Segmentation: Data division into segments
  • Flow control: Traffic management
  • Error control: Detection and correction
  • Multiplexing: Multiple connections

Protocols: TCP, UDP, SCTP Data: Segments (TCP) / Datagrams (UDP)

Layer 3: Network (Network Layer)

Function: Routing and logical addressing

Characteristics:

  • Routing: Route determination
  • Addressing: IP addresses
  • Fragmentation: Packet division
  • Congestion control: Traffic management

Protocols: IP, ICMP, OSPF, BGP Data: Packets

Function: Reliable transfer between adjacent nodes

Characteristics:

  • Physical addressing: MAC addresses
  • Error detection: CRC, checksums
  • Access control: CSMA/CD, Token Ring
  • Synchronization: Data frames

Protocols: Ethernet, PPP, HDLC Data: Frames

Layer 1: Physical (Physical Layer)

Function: Bit transmission through medium

Characteristics:

  • Electrical signals: Voltages, currents
  • Physical media: Cables, fiber optic, wireless
  • Encoding: Bit representation
  • Synchronization: Transmission clocks

Protocols: Ethernet, WiFi, Bluetooth Data: Bits

OSI vs TCP/IP Comparison

OSI Model (7 layers)

Application     ←→  Application
Presentation    ←→  Application
Session         ←→  Application
Transport       ←→  Transport
Network         ←→  Internet
Link            ←→  Link
Physical        ←→  Link

TCP/IP Model (4 layers)

Application     ←→  HTTP, FTP, SMTP, DNS
Transport       ←→  TCP, UDP
Internet        ←→  IP, ICMP
Link            ←→  Ethernet, WiFi

Data Flow in OSI

Encapsulation (Sending)

User Data
    ↓ (Layer 7) + Application Header
APDU
    ↓ (Layer 6) + Presentation Header
PPDU
    ↓ (Layer 5) + Session Header
SPDU
    ↓ (Layer 4) + Transport Header
Segment
    ↓ (Layer 3) + Network Header
Packet
    ↓ (Layer 2) + Link Header + Trailer
Frame
    ↓ (Layer 1) + Physical Signals
Bits

Decapsulation (Receiving)

Bits
    ↓ (Layer 1) - Physical Signals
Frame
    ↓ (Layer 2) - Link Header - Trailer
Packet
    ↓ (Layer 3) - Network Header
Segment
    ↓ (Layer 4) - Transport Header
SPDU
    ↓ (Layer 5) - Session Header
PPDU
    ↓ (Layer 6) - Presentation Header
APDU
    ↓ (Layer 7) - Application Header
User Data

Protocols by Layer

Application Layer

  • HTTP/HTTPS: Web browsing
  • FTP: File transfer
  • SMTP: Email
  • DNS: Name resolution
  • Telnet: Remote terminal
  • SSH: Secure terminal

Presentation Layer

  • SSL/TLS: Encryption and authentication
  • JPEG, PNG: Image compression
  • MPEG: Video compression
  • ASCII, Unicode: Text encoding

Session Layer

  • NetBIOS: Network sessions
  • RPC: Remote procedure calls
  • SQL: Database sessions
  • PPTP: VPN tunnels

Transport Layer

  • TCP: Reliable transfer
  • UDP: Fast transfer
  • SCTP: Multiple stream transfer
  • DCCP: Congested data transfer

Network Layer

  • IP: Internet Protocol
  • ICMP: Control messages
  • OSPF: Internal routing
  • BGP: External routing
  • RIP: Routing information protocol
  • Ethernet: Local networks
  • PPP: Point-to-point connections
  • HDLC: Data link control
  • Frame Relay: WAN networks
  • ATM: Asynchronous transfer mode

Physical Layer

  • Ethernet: Copper cabling
  • WiFi: Wireless networks
  • Bluetooth: Short-range connections
  • Fiber optic: Optical transmission
  • DSL: Telephone lines

OSI Model Advantages

✅ Modularity: Each layer has specific responsibilities
✅ Standardization: Facilitates interoperability
✅ Troubleshooting: Facilitates problem diagnosis
✅ Development: Allows independent development by layers
✅ Scalability: Easy to add new technologies

OSI Model Disadvantages

❌ Complexity: More complex than TCP/IP
❌ Implementation: Not fully implemented
❌ Overhead: More overhead than TCP/IP
❌ Flexibility: Less flexible than TCP/IP

Practical Use Cases

1. Network Problem Diagnosis

Problem: Cannot access a website
Diagnosis by layers:
- Layer 7: Does the browser work?
- Layer 6: Are there encryption problems?
- Layer 5: Is the session established?
- Layer 4: Does TCP/UDP work?
- Layer 3: Is IP configured?
- Layer 2: Does Ethernet work?
- Layer 1: Is the cable connected?

2. Network Design

Architecture by layers:
- Layer 7: Application servers
- Layer 6: Presentation servers
- Layer 5: Session servers
- Layer 4: Load balancers
- Layer 3: Routers
- Layer 2: Switches
- Layer 1: Cables and physical media

3. Security by Layers

Security at each layer:
- Layer 7: User authentication
- Layer 6: Data encryption
- Layer 5: Session control
- Layer 4: Application firewalls
- Layer 3: Network firewalls
- Layer 2: VLANs, port security
- Layer 1: Physical access control

Analysis Tools by Layer

Application Layer

  • Wireshark: Protocol analysis
  • tcpdump: Packet capture
  • netstat: Network connections
  • ss: Socket information

Network Layer

  • ping: Connectivity test
  • traceroute: Packet route
  • ip: Network configuration
  • route: Routing tables
  • arp: ARP table
  • ifconfig: Network interfaces
  • ethtool: Ethernet information
  • iwconfig: Wireless interfaces

References

Glossary

  • Encapsulation: Process of adding headers to data
  • Decapsulation: Process of removing headers from data
  • PDU: Protocol Data Unit
  • SDU: Service Data Unit
  • SAP: Service Access Point
  • Primitive: Operation between adjacent layers