· Physical Layer
2.1 Basic concept of the physical layer
The physical layer address how to transfer data on the bit stream various computer connection transmission medium, rather than to specific transmission medium.
The main task of the physical layer: determining a transmission medium interface features:
1, the mechanical properties: the interface shape, size, number of leads
2, the electrical characteristics: a predetermined voltage range
3, features: e.g. -5v predetermined represents 0, 1 + 5 represents
4, process characteristics: establish a predetermined connection-related components of each working step.
2.2 Basics of data communication
Related Terms:
One, communication
Destination communication message is transmitted.
1, data data: transport entity message.
2, the signal signal: electrical or electromagnetic performance data
a) digital signal: The values are discrete
b) an analog signal: The values are discrete
3, the symbol code: basic waveform is representative of a discrete number of different symbols.
Second, the channel
Several basic concepts channels:
Generally represents a channel communicating media information to one direction. Common communication line often contains a message transmission channel and a reception channel information.
1, one-way communication: a communication in one direction only and not in the opposite direction of the interaction.
2, two-way alternate communication: communication parties can send information, but not both simultaneously transmitted.
3, while two-way communication: the communicating parties can send and receive messages simultaneously.
Third, the baseband signal and Bandpass
1. baseband signal: the signal from the source. The data signals representing a variety of text or image file output from the computer belongs to a baseband signal. The baseband signal is sent direct expression of the signal information to be transmitted, such as sound as we speak.
2, bandpass signal: The baseband signal after carrier modulation, the frequency range signals move to a higher frequency, for transmission in the channel.
(Understanding as to why the use of bandpass signal: The baseband signal is the information we sent a start, carrying the information very accurate, no change in the close range, but in the long distances, the baseband signal is insufficient to assume responsibility, as required. it is modulated, so he used a bandpass signal, the helper signal can be transported over long distances.)
Several basic modulation method:
Amplitude modulation (AM): baseband carrier varies varies. When the baseband signal is 0, no vibration signal with an amplitude of 1, there is vibration amplitude.
Frequency Modulation (FM): When the base band 0, sparsely vibration frequency, when the strip is a base, the vibration frequency dense (is relative)
Phase Modulation (PM): 1 when the baseband wave with respect to 0 when only general.
Fourth, the common encodings
1, unipolar NRZ
2, bipolar NRZ
3, zero unipolar
4, bipolar zero
5, Manchester coding
6, differential Manchester encoding
Fifth, the channel capacity limit
Nyquist criterion: assuming ideal conditions, in order to avoid inter-symbol interference, a symbol transmission rate has an upper limit.
Tone noise ratio: Theoretical Shannon information theory deduced agricultural bandwidth limited Gaussian White Noise and has limit channel, error-free information transfer rate.
C = W log2（1+S/N）b/s
W = bandwidth of the channel
S = the average power of the signal transmission channel
N internal channel Gaussian noise power
Shannon formula shows that:
The larger channel bandwidth or signal to noise ratio of the channel, the higher limit of the transmission rate information.
As long as the information transmission rate is below the limit information transmission rate of the channel, it will be able to find some way to achieve non-discriminatory transmission.
(Subsequent updates on personal understanding)