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Module 1: Physical Layer & Data Communication

20267m

Explain the layered architecture of the OSI Reference Model with a neat block diagram. Describe the:

  • primary functions
  • data units
  • responsibilities

of each of the seven layers.

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OSI Reference Model

The OSI model (ISO) divides network communication into seven layers. Each layer provides services to the layer above and uses services of the layer below.

SVGVector Architecture Diagram (SVG)
Sender Receiver 7 Application7 Application 6 Presentation6 Presentation 5 Session5 Session 4 Transport4 Transport 3 Network3 Network 2 Data Link2 Data Link 1 Physical1 Physical Physical medium
Layer Data unit Main functions
7 Application Data / message Interface to user applications; email, file transfer, web (HTTP, FTP, SMTP, DNS)
6 Presentation Data Translation, encryption/decryption, compression, character encoding
5 Session Data Establishes, manages and terminates sessions; dialog control; synchronization/checkpoints
4 Transport Segment (TCP) / Datagram (UDP) End-to-end delivery, segmentation and reassembly, flow control, error control, port addressing
3 Network Packet Logical addressing (IP), routing, path selection, fragmentation
2 Data Link Frame Framing, physical (MAC) addressing, error detection, flow control, media access control
1 Physical Bit Transmission of raw bits, signal encoding, voltage levels, data rate, topology, transmission medium

Key points

  • Encapsulation: each layer adds its own header (data link adds a trailer too) as data moves down; the receiver strips them going up.
  • Layers 1-3 are network-support layers; layers 5-7 are user-support layers; layer 4 links the two.
  • Peer layers communicate logically using protocols; real data flows vertically through the stack.

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