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Module 1: Physical Layer & Data Communication

20257m

Discuss the responsibilities of the different layers present in the OSI model.

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OSI Reference Model: Layer Responsibilities

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 Responsibilities
7 Application Gives network services to user applications: email, file transfer, web access, directory services (HTTP, FTP, SMTP, DNS)
6 Presentation Translation between data formats, encryption/decryption, compression
5 Session Establishes, manages and terminates sessions; dialog control; synchronization with checkpoints
4 Transport Process-to-process (end-to-end) delivery, segmentation and reassembly, flow control, error control, connection control (TCP/UDP)
3 Network Logical addressing (IP), routing, path selection, packet forwarding, fragmentation
2 Data Link Framing, physical (MAC) addressing, error detection, flow control, access control to the medium
1 Physical Transmission of raw bits over the medium; signal encoding, data rates, voltage levels, topology, transmission mode

Key points

  • Data units: Data, Segment, Packet, Frame, Bit as it goes down.
  • Each layer adds a header (data link also a trailer) on the sender side; the receiver removes them going up (encapsulation / decapsulation).
  • Layers 1-3 are network-support layers, layers 5-7 are user-support layers, and layer 4 links the two groups.

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