Which of the following best describes the purpose of encapsulation across layers in the OSI model?

• A. To add headers to data at each layer
• B. To compress data for efficiency
• C. To encrypt data as it moves downward
• D. To generate error-correcting codes

Answer: (A)

Encapsulation across the OSI layers is about wrapping data with the appropriate control information as it moves down the stack. Each layer takes the data it received from the layer above and adds its own header (and sometimes a trailer) before handing it to the layer below. That header carries fields the next layer needs to function—source and destination addresses, length, protocol identifiers, sequencing, and similar control information. The result is a stack of protocol data units that can be transmitted and then properly interpreted on the receiving side when decapsulation occurs.

This approach supports modular design: each layer provides services to the one above while relying on the layer below to handle transmission. While features like compression, encryption, or error detection may be implemented within certain layers or payloads, the core idea of encapsulation is the addition of headers at each layer to enable proper handling and delivery of the data.