1. The OSI Model
Prof. Choong Seon HONG

2. OSI Reference Model OSI : Open System Interconnection by ISO
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.

3. 3.1 The 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 (cont’d)
The OSI Model

5. OSI Layers

6. Model (cont’d) Peer-to-peer process
~ process on each machine that communicate at a given layer
Interfaces between Layers
~ defines what information and services a layer must provide for the layer above it

7. Model (cont’d) 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 (cont’d)
OSI 모델을 이용한 교환

9. 3.2 Functions of the Layers
물리 계층(Physical Layer)
데이터 링크 계층(Data Link Layer)
네트워크 계층(Network Layer)
전송 계층(Transport Layer)
세션 계층(Session Layer)
표현 계층(Presentation Layer)
응용 계층(Application Layer)
* Please Do Not Touch Steve’s Pet Alligator

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. Physical Layer (cont’d)

12. Physical Layer (cont’d)
고려사항 (considerations)
회성 구성(Line configuration)
데이터 전송 모드(Data transmission mode) : one way or two way
접속형태(Topology) : directly, or through an intermediary
신호방식(Signals): 신호유형
부호화(Encoding): 신호에 의한 data 표현
인터페이스(Interface)
전송매체(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. Data Link Layer
Data Link Layer

15. Data Link Layer (cont’d)
Specific responsibilities
node-to-node 전달(delivery): station-to-station
주소지정(Addressing)
액세스 제어(Access control) : for avoiding collision
흐름 제어(Flow control) : for avoiding overwhelming the receiver
에러 처리(Error handling)
동기화(Synchronization)
Headers contain bits to alert the receiving station that a frame is arriving

16. Data Link Layer (cont’d)
(For error detection)
(Physical address)

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. Network Layer (cont’d)
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 to another when more than one path is available

19. Network Layer (cont’d)

20. Network Layer (cont’d)
Specific responsibilities
Source-to-destination delivery(packet)
Logical addressing
Routing
Address transformation
Interpreting logical addresses to find their physical equivalents

21. Network Layer (cont’d)
Example 3.2

22. 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.

23. Transport Layer (cont’d)
Specific responsibility
End-to-end message delivery
Service-point(port) addressing
delivery of a message to the appropriate application on a computer running multiple applications
Segmentation and reassembly
Connection control
Deciding whether or not to send all packets by a single path

24. Transport Layer (cont’d)

25. Transport Layer (cont’d)
Example 3.3

26. Session Layer
is the network dialog controller
Dialog unit

27. Session Layer (cont’d)
Specific responsibility
Session management
Dividing a session into subsessions by the introduction of checkpoints and separating long messages into shorter message using dialog units
Synchronization
Deciding in what order to pass the dialog units to the transport layer
Dialog control : Deciding who sends, and when
Graceful close
Ensuring that the exchange has completed appropriately before the session closes

28. 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 is necessary for transmission efficiency.

29. Presentation Layer (cont’d)

30. Presentation Layer (cont’d)
Specific responsibility
Translation
Changing the format of a message to mutually acceptable one
Encryption
For security purposes
Compression
For efficiency
Security
Validating passwords and log-in codes

31. Application Layer
Enables the user, whether human or software, to access the network.
Provides user interfaces and support for services.
No headers or trailers are added at this layer

32. Application Layer (cont’d)

33. Application Layer (cont’d)

34. Application Layer (cont’d)
Specific services
Network virtual terminal
File access, transfer, and management
Mail services
Directory services

35. Summary of Layer Functions