Introduction
A computer network lets two machines that may sit on opposite sides of the planet, built by different vendors, running different software, exchange data reliably. That is only possible because everyone agrees on a common set of rules called protocols, and those protocols are organized into layers. A reference model is a blueprint that says how many layers there are, what each layer does, and how they talk to their neighbours.
Two reference models dominate the exam:
- The OSI (Open Systems Interconnection) model — a 7-layer theoretical standard from ISO.
- The TCP/IP model — a 4-layer (sometimes shown as 5) practical model that the real Internet actually runs on.
For BCS, Bank IT, and IT-officer exams this is one of the highest-yield topics in Computer Networks. Almost every exam asks the order of the layers, "which layer does X", the PDU name at a layer, an example protocol/device at a layer, or an OSI-vs-TCP/IP comparison. Master this and you bank several guaranteed marks.
Learning Objectives
After studying this lesson you will be able to:
- Explain why layered architecture is used and what "protocol" and "interface" mean.
- Name the 7 OSI layers in order (both top-down and bottom-up) and state each layer's job.
- Name the 4 TCP/IP layers and map them onto the OSI layers.
- Identify the correct PDU (Protocol Data Unit) at each layer — data, segment, packet, frame, bit.
- Match common protocols, devices, and addresses to their layers.
- Describe encapsulation and decapsulation as data moves down and up the stack.
- Compare OSI and TCP/IP and answer any exam question distinguishing them.