The OSI Model Prof. Choong Seon HONG

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.

3.1 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

Model (cont’d) The OSI Model

OSI Layers

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

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

Model (cont’d) OSI 모델을 이용한 교환

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

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)

Physical Layer (cont’d)

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)

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

Data Link Layer Data Link Layer

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

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

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

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

Network Layer (cont’d)

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

Network Layer (cont’d) Example 3.2

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.

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

Transport Layer (cont’d)

Transport Layer (cont’d) Example 3.3

Session Layer is the network dialog controller Dialog unit

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

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.

Presentation Layer (cont’d)

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

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

Application Layer (cont’d)

Application Layer (cont’d)

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

Summary of Layer Functions