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Concept Breakdowns

OSI Model Layer Functions and Protocol Data Units

These flashcards cover the seven OSI model layers — their functions, associated protocols, and protocol data units (PDUs) — a foundational topic tested on CompTIA Network+, Security+, and CCNA exams. Students often confuse layer responsibilities (session vs. transport) or forget which PDU belongs to which layer. These cards build precise recall of each layer's role, from physical bit transmission up to application-level data formatting, using the patterns examiners actually test.

Interactive Deck

5 Cards
1
Front

What are the seven OSI layers in order?

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1
Back

From bottom to top:

  1. Physical — bits over medium
  2. Data Link — frames, MAC addressing
  3. Network — packets, IP routing
  4. Transport — segments, end-to-end delivery (TCP/UDP)
  5. Session — session management
  6. Presentation — encoding, encryption, compression
  7. Application — user-facing protocols (HTTP, FTP, DNS)
2
Front

What PDU does each OSI layer use?

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2
Back
  • Layer 7–5 (Application/Presentation/Session): Data
  • Layer 4 Transport: Segment (TCP) or Datagram (UDP)
  • Layer 3 Network: Packet
  • Layer 2 Data Link: Frame
  • Layer 1 Physical: Bit
3
Front

What is the difference between Layer 3 and Layer 4?

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3
Back

Layer 3 (Network): Routes packets between networks using logical IP addresses. Protocols: IP, ICMP, OSPF.

Layer 4 (Transport): Delivers data end-to-end between processes using port numbers. Protocols: TCP (reliable) and UDP (unreliable).

4
Locked

At which OSI layer does a switch operate?

5
Locked

What mnemonic helps remember OSI layers top-to-bottom?

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Frequently Asked Questions

How many layers does the OSI model have?

The OSI model has 7 layers, numbered 1 (Physical) through 7 (Application). A common memory aid for bottom-to-top order is Please Do Not Throw Sausage Pizza Away — Physical, Data Link, Network, Transport, Session, Presentation, Application.

What is the difference between the OSI and TCP/IP models?

The OSI model has 7 layers and is a conceptual framework used for teaching and troubleshooting. The TCP/IP model has 4 layers (Network Access, Internet, Transport, Application) and maps to real protocols. OSI Layers 5–7 collapse into TCP/IP Application; OSI Layers 1–2 collapse into Network Access.

Why do CompTIA and Cisco exams test the OSI model so heavily?

The OSI model provides a universal troubleshooting framework. Identifying which layer a problem occurs at (e.g., Layer 1 — check the cable) narrows the diagnosis instantly. Both CompTIA Network+ and CCNA allocate significant marks to OSI layer identification and PDU naming because it underpins all network analysis.