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Short note: OSI architecture

Short answer

The OSI reference model (ISO 7498) is a seven-layer framework for data exchange between open systems. Its layers are physical, data link, network, transport, session, presentation and application; each layer encapsulates data with headers, and lower three layers are network-oriented while upper four are end-to-end.

Write a short note on OSI (Open System Interconnection) architecture in data communication system.

The OSI (Open Systems Interconnection) reference model, published by ISO (ISO 7498, 1984), is a seven-layer framework that describes how data is exchanged between open systems from different makers. Each layer does a defined job, uses the services of the layer below and serves the layer above. Peer layers on two machines communicate using protocols; actual data passes down the sender's stack, over the medium and up the receiver's stack.

 Host A                       Host B
+----------------+    +----------------+
| 7 Application  |<-->| 7 Application  |
| 6 Presentation |<-->| 6 Presentation |
| 5 Session      |<-->| 5 Session      |
| 4 Transport    |<-->| 4 Transport    |
| 3 Network      |<-->| 3 Network      |  (also in
| 2 Data link    |<-->| 2 Data link    |  routers)
| 1 Physical     |<-->| 1 Physical     |
+-------+--------+    +-------+--------+
        +====== medium =======+

Functions of the layers

  1. Physical: transmits raw bits; defines connectors, voltages, bit timing, line coding, data rate (RS-232, V.24, X.21). Unit: bit.
  2. Data link: framing, MAC addressing, error detection (CRC) and correction by retransmission, flow control between adjacent nodes (HDLC, LAPB, PPP, IEEE 802; LLC and MAC sublayers). Unit: frame.
  3. Network: routes packets from source to destination host across networks; logical addressing, routing, congestion control, internetworking (IP, X.25). Unit: packet.
  4. Transport: reliable end-to-end delivery of whole messages; segmentation/reassembly, port addressing, end-to-end error and flow control, multiplexing (TCP, UDP, TP0–TP4). Unit: segment.
  5. Session: sets up, manages and ends dialogues; dialogue control (who talks when) and synchronisation checkpoints for recovery.
  6. Presentation: data representation – code conversion (ASCII/EBCDIC), encryption, compression (ASN.1, JPEG, TLS).
  7. Application: network services for users – file transfer (FTAM, FTP), e-mail (X.400, SMTP), directory (X.500), virtual terminal, web.

Layers 1–3 are network-oriented (also run in routers/switching nodes); layers 4–7 are end-to-end, running only in hosts.

Encapsulation

Each layer adds a header (data link also adds a trailer) to the data it receives, and the receiver strips them in reverse order:

L7        AH|Data
L4     TH|...|Data          segment
L3  NH|TH|...|Data          packet
L2 DH|NH|TH|...|Data|DT     frame
L1 0101101001...            bits

Merits

  • Breaks a complex job into small, independent layers; change in one layer does not affect others.
  • Open, vendor-independent standards allow interoperability.
  • Good tool for teaching, design and troubleshooting.

The practical Internet uses TCP/IP, whose four layers map onto OSI (application ≈ 5–7, transport = 4, internet = 3, network access = 1–2).

✓ Answer checked by Claude Opus 5.5 (high effort)

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