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E. Fraccaroli and D. Quaglia
Fig. 3.5 Comparison
between the original ISO/OSI
Reference Model and the
Simplified ISO/OSI
Model [4]
Physical
Data Link
Network
Transport
Session
Presentation
Application
ISO/OSI
Model
Physical
Data Link
Network
Transport
Application
Simplified ISO/OSI
Model
1
2
3
4
5
6
7
Layer
Physical
Data Link
Network
Transport
Application
Physical
Data Link
Network
Physical
Data Link
Network
Transport
Application
Media
Layer
Host
Layer
End Node
Intermediate
System
End Node
5-pdu
4-pdu
3-pdu
2-pdu
bit
3-pdu
2-pdu
bit
Host
Router
Host
Fig. 3.6 Upper part: PDU exchange at the different layers of the Simplified ISO/OSI Model: two
end nodes and an intermediate system are represented. Lower part: in the IP context end nodes are
called hosts and intermediate systems are called routers
messages named Protocol Data Unit (PDU). The noun PDU is the ISO/OSI term,
while the following synonyms are often used in standard-specific contexts: packet,
datagram, frame, and segment.
Figure 3.7 details PDU exchange from node’s perspective. User’s data comes
from the upper layer of the transmitting end node and goes down through the layers.
At each layer the corresponding communication entity performs a communication