2.1 The OSI Reference Model (7 Layers)
The Open Systems Interconnection (OSI) model, defined by the International Organization for Standardization (ISO), is a conceptual 7-layer architectural framework that decomposes computer communication into standardized, modular functional tiers.
Figure 2.1: The 7-layer OSI model showing top-to-bottom encapsulation during transmission and bottom-up decapsulation upon receipt.
2.2 The TCP/IP Reference Model (4 Layers)
The TCP/IP Protocol Suite (Internet protocol model) was designed by DARPA and serves as the real-world operational architecture of the Internet. Rather than 7 theoretical layers, TCP/IP condenses functionality into 4 pragmatic layers.
Figure 2.2: The four practical layers of the TCP/IP suite upon which the global Internet operates.
2.3 OSI vs TCP/IP — A Detailed Comparison
While the OSI model serves as the standard theoretical teaching tool, TCP/IP was deployed earlier in practice and became the de facto world standard.
Figure 2.3: Direct layer-by-layer correspondence between the 7-layer OSI conceptual model and the 4-layer TCP/IP Internet suite.
