【计算机网络课程课件@中科大】01-2introduction.pdfVIP

COMPUTER NETWORKS Chapter 01 Introduction (2) Shoubao Yang Computer Science Department Tel: 3601540 E-mail: syang@ 1/~syang 2012-8-18 Computer Networks -1-Part 2 1/44 1.4 Reference Models 1.4.1 OSI Reference Model  1983, International Standards Organization (ISO) proposed the “ISO OSI (Open Systems Interconnection) Reference Model”, OSI-rm  The principles of layering A layer should be created where a different abstraction is needed Each layer should perform a well-defined function The function of each layer should be chosen towards defining internationally standardized protocols The layer boundaries should be chosen to minimize the information flow across the interfaces The number of layers should be large enough that distinct functions and small enough that the architecture does not become unwieldy 2012-8-18 Computer Networks -1-Part 2 2/44 The OSI Model 2012-8-18 Computer Networks -1-Part 2 3/44 The Physical Layer  Essence: Describes the transmission of raw bits in terms of mechanical and electrical issues: Example: Connect two computers by means of a wire: Setting -3V to -12V on the wire corresponds to a binary 1; +3V to +12V is a binary 0 The wire is not to be longer than 15 meters You may change the voltage at most 20,000 times per second (Question: what’s the transfer rate?) 2012-8-18 Computer Networks -1-Part 2 4/44 The Data Link Layer  Data link layer is to transform a raw tran