OSI Model and Network Layers: PECTAA Class 12 Computer Science, Punjab Board

When you send a message, it does not jump straight from your screen to your friend's. It passes through seven steps. Each step has one job. The OSI model gives those steps names.

Section 1.6 of Chapter 1 in the PECTAA Computer Science 12 book is Network Models and Layers. This note covers it for FSc Part 2 (2nd year, Class 12) students in Lahore, Faisalabad, Sargodha, Rawalpindi, Multan, Gujranwala and the other Punjab boards. Gujrat district comes under the Gujranwala board. PECTAA writes the books. Your own board sets your paper.

What to know

Network models. They explain how data moves from one device to another in an organized way. They divide communication into different layers. Each layer performs a specific task in data transmission. The book gives these reasons for layering:

• Layered models make networks easier to understand and manage.
• They help in troubleshooting network problems.
• Changes in one layer do not affect other layers.
• Layered communication improves reliability and control.
• Network models are used as standards in networking.

OSI model. OSI stands for Open Systems Interconnection. It is a standard model used to understand network communication. It has seven layers. Each layer performs a different function. It is used for learning and designing networks and it helps different systems to communicate properly.

Figure 1.13 (sender side). On the sender side the data flows from the Application layer at the top down to the Physical layer at the bottom.

The seven layers, with the book job of each:

1. 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.
2. Data Link layer. Ensures data transfer without errors between directly connected nodes. It handles framing, error detection and correction, and controls access to the shared transmission medium.
3. 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.
4. Transport layer. Provides reliable and ordered delivery of data between devices. It performs error recovery, flow control and segmentation of data into smaller units.
5. Session layer. Establishes, maintains and terminates communication sessions between applications. It manages session checkpoints and recovery in case of interruptions.
6. Presentation layer. Translates data into a format understandable by the application layer. It handles data encryption, compression and format conversion.
7. Application layer. Provides services directly to end users and applications. It supports protocols like HTTP, FTP and email services.

Key points

The book groups the layers in a tidbit box:

• Lower layers handle physical data transfer.
• Middle layers manage routing and delivery.
• Upper layers support user applications.
• Each layer works independently, and together they ensure proper communication.

The table follows the order of Figure 1.13, from the Application layer at the top to the Physical layer at the bottom. The book text describes the layers starting from the Physical layer. Write the order the question asks for.

A memory aid. Take the first letter of each layer from the top: A, P, S, T, N, D, P. You can invent a short sentence with these letters to recall the order.

Worked examples

Example 1. Which layer decides the best path for data across different networks?

1. Routing across networks is the job of the Network layer.
2. It also manages logical addressing with IP addresses.
3. The answer is the Network layer.

Example 2. A file is split into smaller units before sending. Which layer does this?

1. The book says the Transport layer performs segmentation of data into smaller units.
2. It also gives error recovery and flow control.
3. The answer is the Transport layer.

Example 3. Sort these into lower, middle and upper layers: Physical, Network, Application.

1. Physical is a lower layer, for physical data transfer.
2. Network is a middle layer, for routing and delivery.
3. Application is an upper layer, for user applications.

Example 4. Which layer sends raw signals, and what kinds of signals does the book name?

1. The Physical layer sends raw data.
2. The signals are electrical, optical or radio.

Common mistakes in the exam

1. Writing six layers. The OSI model has seven.
2. Mixing Data Link and Network. Data Link works between directly connected nodes. Network routes across different networks.
3. Giving IP addresses to the Transport layer. Logical addressing (IP addresses) belongs to the Network layer.
4. Putting encryption only at the Application layer. The book gives encryption, compression and format conversion to the Presentation layer.
5. Forgetting what OSI stands for. It is Open Systems Interconnection.
6. Leaving out the reason for layers. Mention easier management, troubleshooting and independence of layers.

Exam technique

• For a "list the layers" question, write a numbered list and add the one-line job from the book.
• For a short question, give the full form of OSI, then say it has seven layers.
• Draw the stack of seven boxes with an arrow showing the data direction.
• Check your board's notice for the sections in your paper.

Practice questions

1. Write the full form of OSI. Answer: Open Systems Interconnection.
2. How many layers does the OSI model have? Answer: Seven.
3. Which layer defines cables, connectors and voltage levels? Answer: The Physical layer.
4. Which layer establishes, maintains and terminates sessions? Answer: The Session layer.
5. Give two benefits of using a layered model. Answer: It is easier to understand and manage, and it helps in troubleshooting. Changes in one layer do not affect the other layers.

Ready to practise this? Continue in 2nd Year Computer Science.