Chapter 1 · Computer Networks

1.6Network Models and Layers

Why networks split communication into layers, and what each of the seven OSI layers does.

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Full Stack Developer · AI Engineer · Reviewed 21 Oct 2026 · 3 min read

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Sending a message across a network is a big job. A network model breaks that job into small steps. Each step is called a layer, and each layer has one task. This topic explains why layers help, and then walks through the seven layers of the OSI model.

Layers make a network easier to understand and to manage. When something breaks, you can check one layer at a time, so finding the problem is easier. Network models are also used as standards, so devices from different makers can work together. The book's DO YOU KNOW? box sums it up: models turn complex communication into simple steps.

Network Layers

Think of the layers as a staircase. Data goes down the stairs one step at a time, and every step has its own job.

This design keeps a network simple and organized. A change in one layer does not affect the other layers. Layering also gives better reliability and more control.

OSI Model (Open Systems Interconnection)

The best-known layered model is the OSI model. It is used to learn about networks and to design them, and it helps different systems talk to each other properly.

Figure 1.13 in the book shows the sender's side. Data starts at the Application layer at the top and moves down through each layer to the Physical layer. From there it leaves on the cable towards the receiver.

The seven OSI layers on the sender side, from the Application Layer at the top down to the Physical Layer, with each layer's job in the book's words

Functions of OSI Model Layers

The book numbers the layers from the bottom (1) to the top (7):

No.	Layer	Its job, in short
1	Physical Layer	sends raw data as electrical, optical, or radio signals; covers cables, connectors and voltage levels
2	Data Link Layer	error-free transfer between directly connected nodes; framing and error detection
3	Network Layer	routes data across different networks; logical addressing (IP addresses); best path
4	Transport Layer	reliable and ordered delivery; error recovery, flow control, segmentation
5	Session Layer	starts, keeps and ends sessions between applications; checkpoints and recovery
6	Presentation Layer	translates data for the application layer; encryption, compression, format conversion
7	Application Layer	services for end users; protocols like HTTP, FTP and email

The book's TIDBIT groups them in a simple way. The lower layers move the data physically. The middle layers deal with routing and delivery. The upper layers serve the user's applications. Each layer works on its own, but together they make communication work.

Practice this topic

All MCQs of this chapter with answers

Short questions

1.What is the OSI model?2 marks
Answer
From the book: "The OSI model stands for Open Systems Interconnection. It is a standard model used to understand network communication. The OSI model has seven layers. Each layer performs a different function. This model is used for learning and designing networks. It helps different systems communicate properly." The book's Summary (p.17): "The OSI model is a seven-layer model that explains how data travels from one device to another in a network."
2.What are network models? Why are they used?3 marks
Answer
From the book: "Network models explain how data moves from one device to another in an organized way. These models divide communication into different layers." They are used because "Layered models make networks easier to understand and manage. They also help in troubleshooting network problems. Network models are used as standards in networking."
3.What are network layers?2 marks
Answer
From the book: "Network layers divide the communication process into smaller parts. Each layer has a specific responsibility. Data passes through these layers step by step." "Changes in one layer do not affect other layers. Layered communication improves reliability and control."
4.Write the names of the seven layers of the OSI model.2 marks
Answer
From the book's list, from 1 to 7: Physical Layer, Data Link Layer, Network Layer, Transport Layer, Session Layer, Presentation Layer, Application Layer. Figure 1.13 shows them in the opposite order on the sender side, from Application at the top down to Physical.
5.What are the functions of the physical layer?2 marks
Answer
From the book: "Physical Layer: Sends raw data as electrical, optical, or radio signals over the medium. It defines hardware aspects such as cables, connectors, voltage levels, and data transmission rates."
6.What is the role of the network layer?2 marks
Answer
From the book: "Network Layer: Responsible for routing data from source to destination across different networks. It manages logical addressing (IP addresses) and determines the best path for data delivery."
7.What does the transport layer do?2 marks
Answer
From the book: "Transport Layer: Provides reliable and ordered delivery of data between devices. It performs error recovery, flow control, and segmentation of data into smaller units."
8.How do the lower, middle, and upper layers of the OSI model share the work?3 marks
Answer
From the book's TIDBIT: "Lower layers handle physical data transfer. Middle layers manage routing and delivery. Upper layers support user applications. Each layer works independently. Together, they ensure proper communication."

Long questions

1.What is the OSI model? Explain its layers and their functions.8 marks
Answer
Cover the sub-headings "OSI Model (Open Systems Interconnection)" and "Functions of OSI Model Layers", using the book's text:
1. Definition (verbatim): "The OSI model stands for Open Systems Interconnection. It is a standard model used to understand network communication. The OSI model has seven layers. Each layer performs a different function. This model is used for learning and designing networks. It helps different systems communicate properly." Summary (p.17): "The OSI model is a seven-layer model that explains how data travels from one device to another in a network."
2. The seven layers and their functions (verbatim, the book's numbering):
  
  "Physical Layer: Sends raw data as electrical, optical, or radio signals over the medium. It defines hardware aspects such as cables, connectors, voltage levels, and data transmission rates."
  
  "Data Link Layer: Ensures error-free data transfer between directly connected nodes. It handles framing, error detection/correction, and controls access to the shared transmission medium."
  
  "Network Layer: Responsible for routing data from source to destination across different networks. It manages logical addressing (IP addresses) and determines the best path for data delivery."
  
  "Transport Layer: Provides reliable and ordered delivery of data between devices. It performs error recovery, flow control, and segmentation of data into smaller units."
  
  "Session Layer: Establishes, maintains, and terminates communication sessions between applications. It manages session checkpoints and recovery in case of interruptions."
  
  "Presentation Layer: Translates data into a format understandable by the application layer. It handles data encryption, compression, and format conversion."
  
  "Application Layer: Provides services directly to end users and applications. It supports protocols like HTTP, FTP, and email services for communication and data exchange."
3. Diagram: draw Figure 1.13, "OSI Model on the Sender Side Showing Data Flow from the Application Layer to the Physical Layer" (seven boxes from Application at the top to Physical at the bottom, with downward arrows).
4. Close with the TIDBIT: "Lower layers handle physical data transfer. Middle layers manage routing and delivery. Upper layers support user applications. Each layer works independently. Together, they ensure proper communication."
Aim for 250-350 words.
2.What are network models and network layers? Explain how layered models make networks easier to understand and manage.6 marks
Answer
Cover the 1.6 opening paragraph and the sub-heading "Network Layers", using the book's text:
1. Network models (verbatim): "Network models explain how data moves from one device to another in an organized way. These models divide communication into different layers. Each layer performs a specific task in data transmission."
2. Why layered models help: "Layered models make networks easier to understand and manage. They also help in troubleshooting network problems. Network models are used as standards in networking." DO YOU KNOW?: "Network models are used to understand complex network communication in simple steps."
3. Network layers (verbatim): "Network layers divide the communication process into smaller parts. Each layer has a specific responsibility. Data passes through these layers step by step."
4. Benefits of layering: "This structure makes network design simple and organized. Changes in one layer do not affect other layers. Layered communication improves reliability and control."
5. Example of a layered model: the OSI model, which "has seven layers" (Figure 1.13).
Aim for 150-250 words.

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