The OSI reference model, encapsulation and decapsulation

The seven layers

From the top (closest to the user) down:

Memory aid (top down): All People Seem To Need Data Processing

Why layers?

• Each layer provides services to the layer above and uses the layer below; it only needs to talk to its peer layer on the other device.
• A layer can be changed (for example, Wi-Fi instead of Ethernet) without changing the others.
• Vendors can build compatible products to agreed standards.
• Problems are easier to locate and teach.

Encapsulation and decapsulation

Encapsulation happens on the sending device as data moves down the layers: each layer adds its own header (and the data link layer also adds a trailer, often a checksum) around the data it receives from above.

Decapsulation happens on the receiving device as data moves up: each layer reads and removes its own header and passes the rest up.

In a diagram, the transport layer wraps the data into a segment; the network layer wraps the segment into a packet; the data link layer wraps the packet into a frame; the physical layer sends the frame as bits.

Worked example

A web page is sent from a server to your browser. Describe what happens at layers 4, 3 and 2 on the server.

1. Layer 4 (transport): the data is split into segments, each with a header holding the source and destination port numbers and a sequence number.
2. Layer 3 (network): each segment gets an IP header with source and destination IP addresses, making a packet.
3. Layer 2 (data link): each packet gets a header with MAC addresses and a trailer for error checking, making a frame.

Answer: Segments, then packets, then frames: one header added per layer.

Check yourself

1. How many layers does the OSI model have?
   Show answer

   7. Seven layers, from physical (1) to application (7).

2. Which OSI layer is responsible for routing packets between networks using IP addresses?
   • Data link layer
   • Transport layer
   • Session layer
   • Network layer
   Show answer

   Network layer. Layer 3 handles logical addressing and routing.

3. Which OSI layer deals with data format, encryption and compression?
   • Physical layer
   • Presentation layer
   • Network layer
   • Data link layer
   Show answer

   Presentation layer. Layer 6 presents data in a form both ends understand.

4. At which layer are MAC addresses used to deliver frames on a local link?
   • Application layer
   • Transport layer
   • Data link layer
   • Presentation layer
   Show answer

   Data link layer. Layer 2 uses MAC addresses and frames.

5. What is encapsulation?
   • Each layer adds its own header to the data as it passes down the stack on the sending device
   • Each layer removes its header on the receiving device
   • Encrypting data with a public key
   • Splitting a network into subnets
   Show answer

   Each layer adds its own header to the data as it passes down the stack on the sending device. Going down: wrap; going up: unwrap (decapsulation).

6. On the receiving device, in which direction does data travel through the layers during decapsulation?
   • Down, from the application layer to the physical layer
   • Sideways, between peer devices only
   • Up, from the physical layer to the application layer
   • It skips straight to the application layer
   Show answer

   Up, from the physical layer to the application layer. Each layer removes its header and passes the rest upward.

7. Which is a benefit of a layered model?
   • One layer's protocol can be replaced without changing the other layers
   • Data no longer needs addressing
   • Every layer must be written by the same company
   • It removes the need for error checking
   Show answer

   One layer's protocol can be replaced without changing the other layers. Layers are independent behind fixed interfaces.

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Sources

• Pearson Edexcel International Advanced Subsidiary and Advanced Level in Computer Science specification (2026), Issue 1
• Pearson Edexcel IAL Computer Science sample assessment materials (Unit 3 paper, resource booklet and mark scheme)

Last updated 8 November 2026. Spotted a mistake? Tell us.