The physical layer considers how to transmit data bitstreams on the transmission medium connected to various computers, rather than referring to specific physical devices or specific transport media connected to the computer. Its role is the difference between shielding devices. Data (DATA) The difference between the signal (Signal). The data is an entity that carries information. The signal is the electrical or electromagnetic performance of the data. Channel: Media indicating information to a direction. (Abstract concept?) A communication circuit often contains a send channel And a receiving channel. Two types of channels: analog channels and digital channel baseband (BROADBAND) Baseband: Digital signal 1 or 0 is represented by two different voltages, then transferred to the line. Broadband: will The baseband signal is modulated by the frequency division multiplexed analog signal problem: how can the physical medium such as a cable for baseband transmission, can it be used as a broadband transmission? What is the difference between the baseband and broadband implementation? One , The highest symbol transmission rate (Naire criterion) on the channel (Nai Shi Code) The highest cryptographic transmission rate of low-communication channel = 2W baud is to say, the highest cryptographic transmission rate of the ideal low communication channel for each bandwidth is 2 per second. Code element. Ideal with the highest symbol transmission rate of the communication channel = 1W baud bits and baud distinction: Can regard Potter as a Box, Bit as apple. On the transmission channel (some specification), several boxes At this time, the transmission rate of the Box is different from the transmission rate of the APPLE. Second, the limit information transmission rate (Shannon formula) C = W log2 (1 S / N) B / S Understand what is ideal, low communication channel, what is ideal with communication channel. Transport medium 1 Guide Transport Media: Twisted pair (UTP and STP) proximal crosstalk is an important indicator of twisted pair performance. The coupling phenomenon from a pair of conductor signals on another pair of conductors called near-end crosstalk. Coaxial cable understands Manchester coding and differential Manchester code, are they solved? How to solve it? Cable 2 Non-directed transport medium: The spread of satellite channels is large and does not mean that the delay of transmitting data with satellite channels is large. Analog Transmission and Digital Transmission 1 Analog Transmission System. FDM frequency division multiplexing 2 modulator digital - analog demodulator simulation - Digital Synchronous Communication Asynchronous Communication 3 Digital Transmission System Raid Code Modulation PCM System Time Double Reuse TDM Channel Multiplexing Technology
