5.2 OSI Reference Model

The OSI Reference Model breaks down network communication into seven layers, from physical connections to user applications. Each layer provides specific services, enabling modular design and standardized communication between different network components.

Understanding the OSI model helps grasp how data moves through networks, from user input to transmission and back. It's a crucial framework for network professionals, providing a common language and structure for discussing and troubleshooting network issues.

OSI Reference Model

Seven layers of OSI model

• Application Layer (Layer 7)
  • Directly interacts with user applications (web browsers, email clients)
  • Provides application-specific protocols (HTTP, FTP, SMTP, DNS)
  • Enables communication between applications on different devices
• Presentation Layer (Layer 6)
  • Translates data between application and network formats
  • Performs data encryption, decryption, and compression
  • Ensures compatibility between different data representation formats (ASCII, Unicode)
• Session Layer (Layer 5)
  • Creates, maintains, and terminates sessions between applications
  • Implements dialog control and synchronization points (checkpoints)
  • Enables efficient recovery of data in case of session interruption
• Transport Layer (Layer 4)
  • Segments and reassembles data for efficient transport
  • Offers connection-oriented (TCP) and connectionless (UDP) services
  • Implements error checking, flow control, and congestion control mechanisms
• Network Layer (Layer 3)
  • Assigns logical addresses (IP addresses) to devices
  • Determines optimal path for data packets using routing protocols (OSPF, BGP)
  • Enables communication between devices on different networks
• Data Link Layer (Layer 2)
  • Organizes data into frames for transmission over physical medium
  • Performs error detection and correction using checksums and cyclic redundancy checks
  • Manages access to shared media using MAC addresses and protocols (Ethernet, PPP)
• Physical Layer (Layer 1)
  • Defines physical characteristics of network hardware (cables, connectors, wireless channels)
  • Converts digital data into electrical, optical, or radio signals for transmission
  • Specifies voltage levels, timing, and other physical parameters

Services between OSI layers

• Physical Layer provides reliable bit transmission to Data Link Layer
• Data Link Layer offers error-free frame delivery to Network Layer
• Network Layer enables logical addressing and routing for Transport Layer
• Transport Layer ensures end-to-end data delivery and integrity for Session Layer
• Session Layer manages application sessions and synchronization for Presentation Layer
• Presentation Layer handles data formatting and encryption for Application Layer
• Application Layer delivers application-specific services to end-users

Data encapsulation in OSI model

1. Application Layer creates user data
2. Presentation Layer formats and encrypts application data
3. Session Layer adds session header to presentation data
4. Transport Layer segments session data and adds transport header (segment)
5. Network Layer encapsulates segments with network header (packet)
6. Data Link Layer frames packets with data link header and trailer (frame)
7. Physical Layer converts frames into bits for transmission over physical medium

Decapsulation reverses the process at the receiving end:

1. Physical Layer receives bits and passes them to Data Link Layer
2. Data Link Layer removes frame header and trailer, passes packet to Network Layer
3. Network Layer removes packet header, passes segment to Transport Layer
4. Transport Layer removes segment header, passes data to Session Layer
5. Session Layer removes session header, passes data to Presentation Layer
6. Presentation Layer decrypts and formats data for Application Layer
7. Application Layer receives original user data

OSI model vs TCP/IP suite

• Both use layered approach to network communication
• OSI has 7 layers, TCP/IP has 4 layers
  • TCP/IP Application Layer combines OSI Application, Presentation, and Session Layers
  • TCP/IP Network Access Layer combines OSI Data Link and Physical Layers
• OSI is a conceptual reference model, TCP/IP is an implemented protocol suite
• Layer equivalence:
  • TCP/IP Application Layer = OSI Layers 5-7
  • TCP/IP Transport Layer = OSI Layer 4
  • TCP/IP Internet Layer = OSI Layer 3
  • TCP/IP Network Access Layer = OSI Layers 1-2