3 ? OSI ?? ?? ??

1
3 ? OSI ?? ?? ??

• OSI Open System Interconnection
• Basic Reference Model ISO-7498
• Purpose of OSI Model
• is to open communication between different
  systems without requiring changes to the logic of
  the underlying hardware and software.

2
??

• 3.1 ??
• 3.2 ? ??? ??
• 3.3 ??

3
3.1 Model

• OSI Model
• is layered framework for the design of network
  systems that allows for communication across all
  types of computer systems
• Layered Architecture
• shows the layers involved when a message is
  sent from device A to device B

4
Model(contd)

• OSI Model

5
Model(contd)
Peer-to-peer protocol(7th layer)
7
7
Application
Application
Peer-to-peer protocol(6th layer)
7-6 interface
7-6 interface
6
6
Presentation
Presentation
Peer-to-peer protocol(5th layer)
6-5 interface
6-5 interface
5
5
Session
Session
Peer-to-peer protocol(4th layer)
5-4 interface
5-4 interface
4
Transport
Transport
4
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
Data Link
Data Link
Data Link
Data Link
2-1 interface
2-1 interface
1st
1st
1st
1
1
Physical
Physical
Physical
Physical
6
Model(contd)

• Peer-to-peer process
• process on each machine that communicate at a
  given layer
• Interfaces between Layers
• defines what information and services a layer
  much provide for the layer above it

7
Model(contd)

• Organization of the Layers
• Layer 1, 2, 3(network support layers)
• deal with the physical aspects of moving data
  from one device to another
• Layer 5, 6, 7(user support layers)
• allow interoperability among unrelated software
  systems
• Layer 4(transport layer)
• links the two subgroups and ensures that what
  the lower layers have transmitted is in a form
  that the upper layers can use

8
Model(contd)

• OSI ??? ??? ??

H6
H6
H5
H5
H4
H4
H3
H3
T2
H2
H2
T2
9
3.2 ? ??? ??

• ?? ?(Physical Layer)
• ??? ?? ?(Data Link Layer)
• ???? ?(Network Layer)
• ?? ?(Transport Layer)
• ?? ?(Session Layer)
• ?? ?(Presentation Layer)
• ?? ?(Application Layer)

10
?? ?(Physical Layer)

• coordinates the functions required to transmit
  a bit stream over a physical medium.
• (deal with the mechanical and electrical
  specification of the primary connections cable,
  connector)

11
?? ?(contd)

• Physical Layer

12
?? ?(contd)

• ????
• ?? ??(Line configuration)
• ??? ?? ??(Data transmission mode)
• ????(Topology)
• ????(Signals) ????
• ???(Encoding)
• ?????(Interface)
• ????(Medium)

13
??? ?? ?(Data Link Layer)

• is responsible for delivering data units(group
  of bits) from one station to the next without
  errors.
• It accepts a data unit from the third layer and
  adds meaningful bits to the beginning(header) and
  end(trailer) that contain addresses and other
  control information Frame

14
??? ?? ?(contd)

• Data Link Layer

15
??? ?? ?(contd)

• Specific responsibilities
• node-to-node ??(delivery) station-to-station
• ????(Addressing)
• ??? ??(Access control)
• ?? ??(Flow control)
• ?? ??(Error handling)
• ???(Synchronization)

16
??? ?? ?(contd)

• Data Link Layer(? 3.1)

17
???? ?(Network Layer)

• is responsible for the source-to-destination
  delivery of a packet across multiple network
  link.
• cf the data link oversees station-to-station(node
  -to-node) delivery.
• provides two related services.
• Switching
• Routing

18
???? ?(contd)

• Switching
• refer to temporary connections between physical
  links, resulting in longer links for network
  transmission.(ex telephone conversation)
• routing
• means selecting the best path for sending a
  packet from one point another when more than one
  path is available

19
???? ?(contd)

• Network Layer

20
???? ?(contd)

• Specific responsibilities
• Source-to-destination delivery(packet)
• Logical addressing
• Routing
• Address transformation
• Multiplexing

21
???? ?(contd)

• ? 3.2

22
???? ?(contd)

• ? 3.2(contd)

23
?? ?(Transport layer)

• is responsible for source-to destination
  (end-to-end) delivery of the entire message.
• cf the network layer oversees end-to-end
  delivery of individual packet.

24
?? ?(contd)

• Transport Layer

25
?? ?(contd)

• Specific responsibility
• End-to-end message delivery
• Service-point(port) addressing
• Segmentation and reassembly
• Connection control

26
?? ?(contd)

• ? 3.3

T2
T2
H2
H2
T2
H2
T2
H2
Physical link
27
?? ?(Session Layer)

• is the network dialog controller

28
?? ?(contd)

• Specific responsibility
• Session management
• Synchronization
• Dialog control
• Graceful close

29
?? ?(Presentation Layer)

• ensures interoperability among communicating
  devices.
• is responsible for the encryption and
  decryption of data for security purpose and for
  the compression and expansion of data when
  necessary for transmission efficiency.

30
?? ?(contd)

• Presentation Layer

31
?? ?(contd)

• Specific responsibility
• Translation
• Encryption
• Compression
• Security

32
?? ?(Application Layer)

• enables the user, whether human or software, to
  access the network.
• provides user interfaces and support for
  services.

33
?? ?(contd)

• Application Layer

34
?? ?(contd)

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

35
? ??? ?? ??