Tham khảo tài liệu 'kỹ thuật truyền số liệu 3', giáo dục - đào tạo, cao đẳng - đại học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | CHAPTER 3 The OSI Model Established in 1947 the International Standards Organization ISO is a multinational body dedicated to worldwide agreement on international standards. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection OSI model. An open system is a model that allows any two different systems to communicate regardless of their underlying architecture. Vendor-specific protocols close off communication between unrelated systems. The purpose of the OSI model is to open communication between different systems without requiring changes to the logic of the underlying hardware and software. The OSI model is not a protocol it is a model for understanding and designing a network architecture that is flexible robust and interoperable. ISO is the oiL ani ation. OSI is the model. THE MODEL The Open Systems Interconnection model is a layered framework for the design of network systems that allows for communication across all types of computer systems. It consists of seven separate but related layers each of which delines a segment of the process of moving information across a network see Figure . Understanding the fundamentals of the OSI model provides a solid basis for exploration of data communication. Layered Architecture The OSI model is built of seven ordered layers physical layer I data link layer 2 network layer 3 transport layer 4 session layer 5 presentation layer 6 and application layer 7 . Figure shows the layers involved when a message is sent from device A to device B. As the message travels from A to B it may pass through many intermediate nodes. These intermediate nodes usually involve only the first three layers of the OSI model. In developing the model the designers distilled the process of transmitting data down to its most fundamental elements. They identified which networking 43 44 CHAPTER 3 THE OS I MODEL Figure The OSI model Figure OSI layers Device A Device B Intermediate 6 5 4 3
