How Data is Sent Over the Internet and the OSI Model

Introduction

• OSI model (Open Systems Interconnect model) is a theoretical framework for networking.
• It splits network communication into 7 abstraction layers.
• Related to how data is transmitted across the internet.

OSI Model Layers

1. Physical Layer

• Bottom-most layer.
• Responsible for transmitting raw bits of data across a physical connection.

2. Data Link Layer

• Organizes raw bits from the physical layer into frames.
• Ensures frames are delivered to the correct destination.
• Ethernet operates in this layer.

3. Network Layer

• Responsible for routing data frames across different networks.
• IP (Internet Protocol) operates in this layer.

4. Transport Layer

• Manages end-to-end communication between nodes.
• TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) operate in this layer.
  • TCP:
    • Provides reliable, end-to-end communication.
    • Divides data into manageable segments with sequence numbers.
    • Uses error checking to ensure data integrity.
  • UDP:
    • Simpler and faster than TCP.
    • Less reliable as it lacks error-checking and reassembly.

5. Session Layer, 6. Presentation Layer, 7. Application Layer

• Less useful in practice and often considered as a single application layer.
• Application protocols like HTTP are considered layer 7 protocols.

Data Transmission Example

• Sending an HTTP request to a web server.
  • Application Layer: HTTP header added to data.
  • Transport Layer: TCP header added, encapsulated into TCP segments.
    • Headers contain source port, destination port, and sequence number.
  • Network Layer: IP header added, encapsulated into IP packets.
    • Headers contain source and destination IP addresses.
  • Data Link Layer: MAC header added, encapsulated into frames.
    • Headers contain source and destination MAC addresses of routing devices (next hop).
  • Physical Layer: Frames are transmitted as raw bits.
• Receiving process reverses these steps, stripping headers layer by layer.

Real-World Use and Educational Purpose

• OSI model is primarily educational.
• Layers don't perfectly match real-world networking but are used as shorthand by vendors and providers.
  • Example: Cloud load balancers categorized as L4 (TCP level) or L7 (application protocol level like HTTP).

Conclusion

• OSI model provides a way to conceptualize networking.
• Important for understanding how data is transmitted and received.
• Networking products are often described using OSI model terminology.
• Further learning resources available through books and newsletters.