计算机网络科大精品课程ch1-2.ppt

Computer Networks -1-Part 2 COMPUTER NETWORKSChapter 01Introduction (2) Shoubao Yang Computer Science Department Tel: 3601540 E-mail: syang@ 1/~syang 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 The OSI Model 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?) The Data Link Layer Data link layer is to transform a raw transmission facility into a line that appears free of undetected transmission errors to the network layer Observation: We need to at least detect bit transmission errors, send bits in frames that add redundancy to detect something went wrong Examples: Add a parity bit to every 7 transmitted bits: 1 says there were odd number of 1’s; 0 says there were an even number of 1’s Add a checksum (cyclic redundancy check) that should match the bits before it Also: Provide the mechanisms so that fast senders don’t overwhelm slow receivers (flow control) OSI Data Link Layer Observation: We also need to specify how a number of computers can share a common channel (e.g. wire), that is medium access control sublayer (MAC): 1. Specifies how