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This material was developed with funding
from the National Science Foundation
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OSI Fundamentals
This work is licensed with a
Creative Commons Attribution 4.0 International LicenseEndFragment
Communication always starts from a sending (SOURCE) device and ends at the receiving (DESTINATION) device. Data travels down the OSI Model layers as the SOURCE transmits to the DESTINATION, and then travels up the OSI Model as the DESTINATION receives the data.
Application Layer
Transport Layer
Data Link Layer
Presentation Layer
Physical Layer
Network Layer
SOURCE
Session Layer
OSI Model Fundamentals
<<--click the source to
begin communication
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DESTINATION
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Receiver
The OSI Model (Open Systems Interconnection Model) is a conceptual framework that defines and describes the functions of a networking system. It was the first standard model for network communications adopted by vendors in the early 80’s, so it promotes multi-vendor interoperability.
There are seven layers that make up the OSI Model. Each layer:has a specific function.requires different types of networking devices.gets packaged into different types of data units.uses rules or formal protocols to successfully and reliably transmit the data.
Sender
L2 PDU
Layer 7
Header
Layer 4
Header
Layer 1
Header
<<-click the PDU to continue
Data
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0111001010001101010001111001010111100
Click the receiver to
de-encapsulate ->>
Layer 6
Header
Layer 3
Header
Each OSI layer asks for services from the next lower layer. The lower layer encapsulates the higher layer’s data with a header. A protocol data unit (PDU) represents the unit of data with the layer’s header. PDU’s are numbered from 1 to 7 (the Physical layer is the first one). De-encapsulation is the exact reverse process. The additional information added on the sender’s side during encapsulation gets removed when it travels on the receiver’s side.
L7 PDU
<<--click the sender to
begin data encapsulation
L6 PDU
L5 PDU
Layer 5
Header
Layer 2
Header
L4 PDU
L3 PDU
Sender
