Physical layer: Describes the properties associated with the transport media interface
The mechanical properties specify the shape and size of the connectors used for the interface, the number and arrangement of leads, fixing and locking devices, etc. The electrical characteristics indicate the range of voltages present on each line of the interface cable. A functional characteristic indicates the meaning of the voltage present at a level on a line. Process features indicate the sequence of possible events for different functions.
Memory points: Extract one word for each characteristic, namely “Mechanical Qigong process”
Basic knowledge of data communication
Digital and analog signals
Digital signal: Or discrete signal, the way data is stored in general computers,
In the transmission is often used to represent different discrete values of waveform, such as high and low level, one represents 0, one represents 1.
Analog signal: or continuous signal, the voice of ordinary people is analog signal. Analog signals can be converted into digital signals by sampling and quantization.
Symbol refers to a fixed time signal waveform (digital pulse), representing different discrete values of the basic waveform, digital communication in the digital signal meter
The unit of measurement. The signal within this duration is called the k-base symbol, and the duration is called the symbol width. When there are M discrete states of symbols (M is greater than 2),
In this case, the code element is M base.
channel
Simplex communication – there can be communication in one direction and no interaction in the other. Half duplex communication – both parties can send messages, but not both at the same time (and therefore not both at the same time). Full duplex communication – both sides of communication can send and receive messages at the same time
coding
Zero coding
1 up, 0 down, 0 down
Non-return to zero coding
The high one, the low zero, the second half does not return to zero
Manchester code
Front high and back low ->1/0
Front low back high ->0/1
Each bit of the Manchester code occupies only half of the clock cycle. When transmitting 1, it is high in the first half of the clock cycle and low in the second half;
It’s the opposite when you transmit zero. In this way, every clock cycle must have a jump, this jump is the bit synchronization signal.
Differential Manchester code
Jump to 0, not 1
Nye’s criterion and Shannon’s theorem
Nye’s criterion: Limit bit transmission under ideal low pass (no noise, bandwidth limited) conditions
The transmission rate is 2W Baud, where W is the channel bandwidth in Hz and V is the distance per symbol
The number of scattered levels (i.e., how many codes there are).
Limit data transfer rate under ideal low communication channel =2Wlog2V (b/s)
Shannon’s theorem: in bandwidth-constrained and noisy channels, in order not to generate errors, information
The data transfer rate has a upper limit.
Channel limit data transmission rate =Wlog2(1+S/N) (b/ S)
Signal-to-noise ratio (dB) =10log10(S/N)
