The OSI Model o The Open Systems Interconnection(OSr) model is defined by International Standard Organization(ISO)in 1984 o to open communication between different systems without changing the underlying architecture(software and hardware) D An international standard that referenced by most network vendors for their products and services o Best tools for teaching people about sending and receiving data on a network Consist of seven layers: Physical (layer 1), data link(layer 2) Network(layer 3), Transport (layer 4), Session (layer 5) Presentation(layer 6)and Application(layer 7). Each of the layer illustrates a particular network function this separation of networking functions is called layering 哈工大计算机学院李全龙 Netork Application Development 1: Introduction 16
哈工大计算机学院 李全龙 Network Application Development 1: Introduction 16 The OSI Model The Open Systems Interconnection (OSI) model is defined by International Standard Organization (ISO) in 1984. To open communication between different systems without changing the underlying architecture (software and hardware). An international standard that referenced by most network vendors for their products and services Best tools for teaching people about sending and receiving data on a network. Consist of seven layers: Physical (layer 1), data link (layer 2), Network (layer 3), Transport (layer 4), Session (layer 5), Presentation (layer 6) and Application (layer 7). Each of the layer illustrates a particular network function. This separation of networking functions is called layering
7 layers of OSI model Layer 7: Application O Advantages of multi-layered model Layer 6: Presentation o Reduce complexity-simplified Layer 5 Session teaching and learning g Standardize interface Layer 4: Transport o Allow communication between Layer 3: Network different network hardware and software Layer 2: Data link o Prevent the change of one layer Layer 1: Physical from affecting the other layers 口 Accelerate Evolution Some Characteristics for the 7-layer model o Peer-to-peer process- layer n of one computer(device)can communicate with layer n on another computer(device) through protocols of that layer 口 a layer provide service for the layer above计t 哈工大计算机学院李全龙 Netork Application Development 1: Introduction 17
哈工大计算机学院 李全龙 Network Application Development 1: Introduction 17 7 layers of OSI model Application Presentation Session Transport Network Data link Physical Layer 7: Layer 6: Layer 5: Layer 4: Layer 3: Layer 2: Layer 1: Advantages of multi-layered model Reduce complexity - simplified teaching and learning Standardize interface Allow communication between different network hardware and software Prevent the change of one layer from affecting the other layers Accelerate Evolution Some Characteristics for the 7-layer model Peer-to-peer process - layer N of one computer (device) can communicate with layer N on another computer (device) through protocols of that layer. A layer provide service for the layer above it
Data flow and Peer communication in p SI layers PC Intermediate PC 2 node link 7 Application Application-layer protocol Application 7 6 Presentation+ Presentation-layer protocol →| Presentation6 Session Session-layer Session5 5432 protocol Transport Transport-laye → Transport4 protocol Network Network t Network3 Data link+--+Data link Data link+-- Data link2 1 Physical +Physical Physical Physical 1 Physical medium Physical medium Data flow e Peer communication within layer For PC1 sends a message to PC2 哈工大计算机学院李全龙 Netork Application Development 1: Introduction 18
哈工大计算机学院 李全龙 Network Application Development 1: Introduction 18 Data flow and Peer communication in OSI layers Application Presentation Session Transport Network Data link Physical 7 6 5 4 3 2 1 Network Data link Data link Physical Physical Application Presentation Session Transport Network Data link Physical 7 6 5 4 3 2 1 PC 1 Intermediate PC 2 node link link For PC1 sends a message to PC2 Physical medium Physical medium Transport-layer protocol Session-layer protocol Presentation-layer protocol Application-layer protocol Data flow Peer communication within layer
Wrapping of data with necessary protocol information before network transit (adding header or trailer or both PC PC 2 User data 7 Application Application7 User data A-PDU 6 Presentation Presentation 6 A-PDU P-PDU SH sEssion Session5 P-PDU S-PDU 4 Transport Transport S-PDu T-PDU 3 Network Network3 T-PDU N-PDU DH 2 Data link Data link2 N-PDU 101010101010111110000 Physical Physical 10101010101011111110000000101010 Network 哈工大计算机学院李全龙 Netork Application Development 1: Introduction 19
哈工大计算机学院 李全龙 Network Application Development 1: Introduction 19 Data Encapsulation Application Presentation Session Transport Network Data link Physical 7 6 5 4 3 2 1 PC 1 PC 2 User data AH PH SH AH PH SH TH NH DT AH PH SH TH NH DH 10101010101011111110000000101010 A-PDU S-PDU P-PDU T-PDU N-PDU Application Presentation Session Transport Network Data link Physical 7 6 5 4 3 2 1 User data AH PH SH AH PH SH TH AH PH SH TH NH DT AH PH SH TH NH DH 10101010101011111110000000101010 A-PDU S-PDU P-PDU T-PDU N-PDU Network Wrapping of data with necessary protocol information before network transit (adding header or trailer or both)
Why data Encapsulation aTo add control information in form of header or trailer or both to the data being encapsulated in order to ensure accurate and secure communication The data after encapsulated is called protocol data unit(PDU aNote: Headers are added to the data at layers 6, 5, 4, 3, and 2 Trailers are usually added only at layer2 The control information falls into the three categories a Address: The address of the sender and/or receiver may be indicated a Error-detecting code: some sort of frame check sequence a Protocol control: additional information is included to implement the protocol functions such as: priority, quality of service and security 哈工大计算机学院李全龙 Netork Application Development 1: Introduction 20
哈工大计算机学院 李全龙 Network Application Development 1: Introduction 20 Why Data Encapsulation ? To add control information (in form of header or trailer or both) to the data being encapsulated in order to ensure accurate and secure communication. The data after encapsulated is called protocol data unit (PDU) The control information falls into the three categories: Address: The address of the sender and/or receiver may be indicated. Error-detecting code: some sort of frame check sequence Protocol control: additional information is included to implement the protocol functions such as: priority, quality of service and security Note: Headers are added to the data at layers 6,5,4,3, and 2. Trailers are usually added only at layer2