THE OSI MODEL

1
THE OSI MODEL
ISO is the organization.OSI is the model.
ISO?International Standards Organization OSI?Open
Systems Interconnection
2
THE MODEL
7 6 5 4 3 2 1
Application
Presentation
Session
Transport
Network
Datalink
Physical
3
Layered Architecture
Device A
Device B
Intermediate node
Intermediate node
Peer-to-peer protocol (7 th layer)
7
7
Application
Application
7-6 Interface
7-6 Interface
Peer-to-peer protocol (6 th layer)
6
6
Presentation
Presentation
6-5 Interface
6-5 Interface
Peer-to-peer protocol (5 th layer)
5
5
Session
Session
5-4 Interface
5-4 Interface
Peer-to-peer protocol (4 th layer)
4
4
Transport
Transport
4-3 Interface
4-3 Interface
3rd
3rd
3rd
3
3
Network
Network
Network
Network
3-2 Interface
3-2 Interface
2nd
2nd
2nd
2
2
Datalink
Datalink
Datalink
Datalink
2-1 Interface
2-1 Interface
1st
1st
1st
1
1
Physical
Physical
Physical
Physical
Physical communication
4
Peer-to-peer Processes
Interface between Layers
5
Organization of the Layers
Layer 1 , 2 and 3 ?physical, data link and
network Are network support layer
????????????????????????? ??????????????????????
????? ? ??????? (electrical Specifications,physic
al connection, physical addressing,transport
timeing)
Layer 5 , 6 and 7 ?session , presentation and
application The user support layer
interoperability among unrelated software system
Layer 4 ?transport layer ????????????????????????
layer ??????? ????????? (hardware ,software)
6
An exchange using the OSI model
7
7
L7 data
L7 data
6
6
L7 data
H6
L7 data
H6
5
5
L6 data
H5
L6 data
H5
4
4
L5 data
H4
L5 data
H4
3
3
L4 data
H3
L4 data
H3
2
2
L3 data
H2
T2
L3 data
H2
T2
1
1
010101010101101010000010000
010101010101101010000010000
Transmission medium
7
Function of the layers
Physical layer
From data link layer
To data link layer
L2 data
L2 data
Physical layer
Physical layer
10101000000010
10101000000010
Transmission medium
8
???????????????????????????????????? physical
layer

• Line configuration
• Data transmission mode
• Topology
• Signal
• Encoding
• Interface
• Medium

9
Data Link layer
From Network layer
To Network layer
L3 data
L3 data
Data Link layer
Data Link layer
Frame
Frame
H2
T2
H2
T2
10101000000010
10101000000010
To Physical layer
From Physical layer
10
Specific responsibilities of the data link layer
include the following

• Node to node delivery
• Addressing
• Access control
• Flow control
• Error handling
• Synchronization

LLC?Logical Link Control MAC?Media Access Control
11
Example Data link layer
10
28
53
65
87
T2
Data
10 87
Trailer
Source address
Destination address
12
Network layer
From transport layer
To transport layer
L4 data
L4 data
Network layer
Network layer
Packet
Packet
H3
H3
L3 data
L3 data
To data link layer
From data link layer
13
Specific responsibilities of the network layer
include the following

• Source-to-destination delivery
• Logical addressing
• Routing
• Address transformation
• Multiplexing

14
Example Network layer
E
A
10
87
Bus
20
F
G
R
99
45
T
Ring
N
71
33
R
Z
66
P
95
77
Bus
15
Transport layer
From session layer
To session layer
L5 data
L5 data
Transport layer
Transport layer
L4 data
L4 data
L4 data
L4 data
L4 data
L4 data
To network layer
From network layer
16
Specific responsibilities of the transport layer
include the following

• End-to-end massage delivery
• Service-point (port) addressing
• Segmentation and reassemble
• Connection control

17
Example Transport layer
A
B
Transport layer
Network layer
Data link layer
Physical link
18
Session layer
From presentation layer
To presentation layer
L6 data
L6 data
Session layer
Session layer
H5
H5
syn
syn
syn
syn
syn
syn
L5 data
L5 data
To transport layer
From transport layer
19
Specific responsibilities of the session layer
include the following

• Session management
• Synchronization
• Dialog control
• Graceful close

20
Presentation layer
From application layer
To application layer
L7 data
L7 data
Presen- tation layer
Presen- tation layer
Encode,encrypted,and Compresses data
H6
Encode,encrypted,and Compresses data
H6
L6 data
L6 data
To session layer
From session layer
21
Specific responsibilities of the presentation
layer include the following

• Translation
• Encryption
• Compression
• Security

22
Application layer
User
User
FTAM
FTAM
Application layer
Application layer
L7 data
L7 data
To presentation layer
From presentation layer
23
Specific responsibilities of the application
layer include the following

• Network virtual terminal
• File access transfer,and management
• Mail services
• Directory services

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Summary of Layer Function
Application
To allow access to network resources
To translate,encrypt, and compress data
Presentation
To establish,manage,and terminate session
Session
To provide reliable end to end message Delivery
and error recovery
Transport
To move packets from source to Destination to
provide internetworking
Network
To organize bit into frames to provide Node to
node delivery
Data Link
To transmit bits over a medium to Provide
mechanical and electrical specification
Physical