The OSI Model Prof Choong Seon HONG Kyung

The OSI Model Prof. Choong Seon HONG Kyung Hee University 1

OSI Reference Model q OSI : Open System Interconnection by ISO q Basic Reference Model : ISO-7498 q Purpose of OSI Model ~ is to open communication between different systems without requiring changes to the logic of the underlying hardware and software. Kyung Hee University 2

3. 1 The Model q. OSI Model ~ is layered framework for the design of network systems that allows for communication across all types of computer systems q. Layered Architecture ~ shows the layers involved when a message is sent from device A to device B Kyung Hee University 3

Model (cont’d) q The OSI Model Kyung Hee University 4

OSI Layers Kyung Hee University 5

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

Model (cont’d) q. 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 Kyung Hee University 7

Model (cont’d) q. OSI 모델을 이용한 교환 Kyung Hee University 8

3. 2 Functions of the Layers q물리 계층(Physical Layer) q데이터 링크 계층(Data Link Layer) q네트워크 계층(Network Layer) q전송 계층(Transport Layer) q세션 계층(Session Layer) q표현 계층(Presentation Layer) q응용 계층(Application Layer) * Please Do Not Touch Steve’s Pet Alligator Kyung Hee University 9

Physical Layer q 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) Kyung Hee University 10

Physical Layer (cont’d) q. Physical Layer Kyung Hee University 11

Physical Layer (cont’d) q 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) Kyung Hee University 12

Data Link Layer qis responsible for delivering data units(group of bits) from one station to the next without errors. q. 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 Kyung Hee University 13

Data Link Layer q Data Link Layer Kyung Hee University 14

Data Link Layer (cont’d) q 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) l Kyung Hee University Headers contain bits to alert the receiving station that a frame is arriving 15

Data Link Layer (cont’d) q Data Link Layer(예 3. 1) (For error detection) Kyung Hee University (Physical address) 16

Network Layer qis 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. q provides two related services. Switching Routing Kyung Hee University 17

Network Layer (cont’d) q. Switching ~ refer to temporary connections between physical links, resulting in longer links for network transmission. (ex: telephone conversation) qrouting ~ means selecting the best path for sending a packet from one point to another when more than one path is available Kyung Hee University 18

Network Layer (cont’d) q Network Layer Kyung Hee University 19

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

Network Layer (cont’d) q. Example 3. 2 Kyung Hee University 21

Transport Layer qis 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. Kyung Hee University 22

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

Transport Layer (cont’d) q Transport Layer Kyung Hee University 24

Transport Layer (cont’d) Example 3. 3 Kyung Hee University 25

Session Layer qis the network dialog controller Dialog unit Kyung Hee University 26

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

Presentation Layer qensures interoperability among communicating devices. qis 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. Kyung Hee University 28

Presentation Layer (cont’d) q. Presentation Layer Kyung Hee University 29

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

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

Application Layer (cont’d) q Application Layer Kyung Hee University 32

Application Layer (cont’d) Kyung Hee University 33

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

Summary of Layer Functions Kyung Hee University 35