1/9
The OSI Layers
Surasak Sanguanpong
nguan@ku.ac.th
http://www.cpe.ku.ac.th/~nguan
Last updated: 27 June 2002
Applied Network Research Group
Department of Computer Engineering, Kasetsart University
2/9
OSI Reference Model
Layer 7
Application
z
Layer 6
Presentation
Layer 5
Session
The Open Systems
Interconnection (OSI)
model was developed by
the ISO as
z
Layer 4
Transport
z
Layer 3
Network
Layer 2
Data Link
z
a model for computer
communications
architecture
a framework for developing
protocols standards
OSI model consists of
seven layers
Layer 1
Physical
Applied Network Research Group
Department of Computer Engineering, Kasetsart University
1
3/9
Layering
Application Programs
Layer 7
Application
User-oriented
functions
Layer 6
Presentation
Layer 5
Session
Layer 4
Transport
Communications
functions
Applied Network Research Group
Layer 3
Network
Packet
Layer 2
Data Link
Frame
Layer 1
Physical
Bit
Department of Computer Engineering, Kasetsart University
4/9
Why Seven Layers?
Applications
Applications
Applications
Physical
Transport
Presentation
Need at least Physical
and Application layers
Network
Session
Datalink
Transport
Physical
Applications
Datalink
Physical
Datalink concerns all the
link independent details for
different link technology
Applied Network Research Group
Both end-to-end and
hop-by-hop actions are
required
Transport layer handles
end-to-end functions such
as flow control
Network layer handles
routing in hop-by hop
fashion
Network
Datalink
Physical
Separate some
application functions to
Presentation and
Session Layers
Department of Computer Engineering, Kasetsart University
2
5/9
Layer
Boundary
Service #M
Service #3
Service #2
Layer N
Service #1
Service Access Point (SAP)
z
Each layer has its own
functionality
z
When a layer needs
functionality of another
layer, it must issue a
request.
z
At each layer boundary,
there is at least one point
of services called SAP
Layer N-1
= Service Access Point
Applied Network Research Group
Department of Computer Engineering, Kasetsart University
6/9
Virtual Communications
Applications
Presentation
Session
Transport
Network
Datalink
Physical
Application protocol
Presentation protocol
Session protocol
Transport protocol
Network Access Protocol
Datalink protocol
Physical protocol
Applications
Presentation
Session
Transport
Network
Datalink
Physical
Physical Media
Applied Network Research Group
Department of Computer Engineering, Kasetsart University
3
7/9
Protocol Data Unit
protocol data unit (PDU) = combination of data from
the next higher and control information
Data
Applications
Application PDU
Presentation
Presentation PDU
Session
Session PDU
Transport PDU
Transport
Network PDU
Network
Datalink
Datalink PDU
Physical
AH Data
Encapsulation
PH AH Data
SH PH AH Data
TH SH PH AH Data
NH TH SH PH AH Data
DH NH TH SH PH AH Data DT
DH NH TH SH PH AH Data DT
Additional of control information to
data is referred to encapsulation
Applied Network Research Group
Department of Computer Engineering, Kasetsart University
8/9
Decapsulation
Data
Decapsulation
In each layer,
corresponding
header/trailer
has been removed
Applied Network Research Group
AH Data
Applications
PH AH Data
Presentation
SH PH AH Data
TH SH PH AH Data
NH TH SH PH AH Data
Session
Transport
Network
DH NH TH SH PH AH Data DT
Datalink
DH NH TH SH PH AH Data DT
Physical
Department of Computer Engineering, Kasetsart University
4
9/9
OSI and TCP/IP
OSI
TCP/IP
Application
Presentation
Application
Session
Transport
Transport
Network
Internet
Data Link
Network Access
Physical
Physical
Applied Network Research Group
Department of Computer Engineering, Kasetsart University
5