Network Reference Models - Core Concepts

• Network Reference Models


  Conceptual models that provide abstract representations of how data is transmitted over a network
• The OSI and TCP/IP Network Reference Models are two important conceptual models
• Network Reference Models


  • Described by a "stack" of layers
  • Each layer is dependent on its adjacent layers
  • They have an implied directionality - you go down the stack or you go up the stack
• Encapsulation


  Data moving down the stack (sending data) undergoes encapsulation, where each layer of information is wrapped in additional details and passed to the next
• Decapsulation


  Data moving up the stack (receiving data) undergoes decapsulation, where each layer will interpret and remove (strip) this additional information until only the original data remains
• OSI Model


  • Created by ISO in the 1970's
  • Has 7 layers
  • Bottom 4 are "media" layers, top 3 are "host" layers
• OSI Model Layers


  • Application
  • Presentation
  • Session
  • Transport
  • Network
  • DataLink
  • Physical
• Application Layer


  Handles the services and programs that use the network to transmit and receive data
• Presentation Layer


  Handles things like compression/decompression, encryption/decryption, and encoding/decoding
• Session Layer


  Responsible for setting up, maintaining, and tearing down sessions, in addition to performing functions like authentication and authorization
• Transport Layer


  • Handles ports, breaking large data transfers into pieces for delivery, and reconstituting them at the other end
  • Protocols used: TCP and UDP
• Network Layer


  Transmits data between devices in different networks, handles IP addresses and routing
• DataLink Layer


  Transmits data between nodes on a network, handles MAC addresses and switching
• Physical Layer


  Transmits data over a physical medium, such as a cable or over-the-air
• The OSI model is primarily theoretical, but its layers are often referenced when discussing networking devices and their capabilities
• Uses of the OSI Model


  • Comprehensive theoretical framework for understanding network communications
  • Logical map of data flow that helps for isolating causes of network problems during troubleshooting
  • A common language for discussing network systems and devices that operate on specific layers of data
• TCP/IP Model


  • A practical model that maps more closely to communications in modern networks
  • Combines several OSI layers into fewer layers
• TCP/IP Model Layers


  • Network Access
  • Internet
  • Transport
  • Application
• The TCP/IP model forms the basis of the Internet and other large-scale networks
• The TCP/IP model provides a simplified, more practical approach to network communications, relative to the OSI model
• The TCP/IP model serves as the universal standard for network communication, given its wide adoption and implementation by a wide variety of devices and applications
• Both the OSI and TCP/IP models are useful, and often used in combination rather than exclusively
• Understanding the relationships between the layers is more important than memorizing the layers themselves