1, the basic concept of the physical layer
Consider only the physical layer transport bit stream, not including the cable and other transmission media (Layer 0 may be considered), the shielding difference of transmission media, transmission media of different standards define different.
The main task: to determine the characteristics of the transmission media interface.
Mechanical properties: how cable crystal above the first row. RJ45 cable, RJ11 telephone line, RS232 serial port.
Electrical Characteristics: voltage range of the respective cable lines. Ethernet does not exceed 3v, POE highest 48v, serial 3-15v,
Feature: what sense voltage represents.
Process characteristics: the order and meaning of different events.
2, the basics of data communication
The basic flow of the communication system: the input information - far point - the input data - the transmitter - sends a signal - transmission system
The term: a signal (electrical, magnetic), a digital signal an analog signal vs,
Three communication methods: simplex, half-duplex, full-duplex (two channels must).
Related concepts channels:
a, a baseband signal: signal, a computer like the character image files from the source of
B, a baseband signal containing low-frequency portion of the DC part is often not transmitted, the modulation need
C, baseband modulation: After coding, or converted baseband signal, from the more recent digital channel
d, bandpass modulation: an analog signal by a carrier, called the bandpass signal.
Baseband modulation encoding used: 1001100111-- high and low frequency. Non-zero-zero self-synchronization capability not. Mochesite Manchester and differential transmission requires twice the bandwidth.
Bandpass substantially transference: FM amplitude and phase. Alter the characteristics of the wave.
3, the channel capacity limit.
Factors: frequency range, signal to noise ratio. Transmission frequency is usually difficult. SNR is the mean power / noise signal. Shannon formula.
4, the physical layer transmission media
Guided: wired, non-guide: the radio.
UTP: cable, most commonly, to a few tens of kilometers. It may be a metal shield layer. See cross-section, two twisted pairs. Lines and divided into three categories Category 5, Category 5 cable is relatively dense tangle
Coaxial cable: Immunity is better, cable TV, cable bandwidth depends on the quality of
Fiber: high optical frequency, the bandwidth is relatively high. Vs multimode singlemode, multimode attenuation more powerful, two market close price. They are more expensive.
Via satellite, delay is probably a few hundred milliseconds.
5, the channel multiplexing
Users communicate using a shared channel, reducing the cost.
a, frequency division multiplexing, to each user a different frequency band, a carrier 2.4G, 5G
b, time division multiplexing, single occupancy period of time, will result in a waste of resources. Because bursty computer data.
Statistical time division multiplexing, dynamic time respectively
C, WDM: an optical frequency division multiplexing, a multi-mode fiber.
d, code division multiplexing: the use of different patterns, such as the station S 1 is 00011011, 11100100 is 0, the other station B is 1,0 additional coding, different needs of different stations, but also mutually orthogonal. Normalized inner product.