The OSI Model: A Conceptual Framework for
Networking
1 Introduction
The OSI (Open Systems Interconnection) model, developed by the International Or-
ganization for Standardization (ISO), is a conceptual framework that standardizes the
functions of a networking system to enable interoperability among diverse communication
systems using standard protocols. It divides network communication into seven abstrac-
tion layers, each with specific responsibilities. Data flows from one device to another by
traveling down the layers on the sender’s side (encapsulation) and up the layers on the
receiver’s side (decapsulation).
2 History
The OSI model was initiated in the late 1970s to unify competing networking protocols,
such as those from ARPANET and proprietary systems like IBM’s SNA. First outlined
in 1978 by Hubert Zimmermann, it was formalized in 1984 as ISO 7498 through col-
laboration between ISO and CCITT (now ITU-T). While influential in education and
troubleshooting, it has been largely superseded in practice by the TCP/IP model but
remains a foundational reference for understanding network architecture.
3 The Seven Layers
The OSI model consists of seven layers, numbered from 1 (closest to the physical medium)
to 7 (closest to the user). Each layer provides services to the one above it and uses services
from the one below, with data units (PDUs) transforming as they pass through (e.g., bits
at Layer 1 become frames at Layer 2). The following table summarizes the layers, their
Protocol Data Units (PDUs), and key functions:
This layered approach facilitates troubleshooting, scalability, and interoperability, though
real-world protocols like TCP/IP map imperfectly to it, often combining Layers 5–7 into
a single Application layer.
1
Layer Name PDU Key Functions
7 Application Data Interfaces with end-user applica-
tions for services like file transfer
or email (e.g., HTTP, FTP). Pro-
vides network services directly to
software.
6 Presentation Data Handles data translation, encryp-
tion/decryption, compression,
and format conversion (e.g.,
ASCII to EBCDIC) to ensure
compatibility between systems.
5 Session Data Manages sessions/connections
between applications, including
setup, coordination, and termina-
tion of communication dialogues.
4 Transport Segment Ensures end-to-end reliable data
delivery, including segmentation,
error recovery, flow control, and
multiplexing (e.g., TCP, UDP).
3 Network Packet/Datagram Routes data across networks,
handles logical addressing (e.g.,
IP addresses), fragmentation, and
congestion control.
2 Data Link Frame Provides node-to-node transfer,
error detection/correction, and
flow control; subdivided into
MAC and LLC sublayers (e.g.,
Ethernet, Wi-Fi).
1 Physical Bit/Symbol Transmits raw bits over physi-
cal media; defines hardware specs
like voltage, connectors, and
signaling (e.g., Ethernet PHY,
USB).
Table 1: The Seven Layers of the OSI Model
