It is worth noting that the physical layer does not only refer to specific transmission media, such as optical fibers, cables, etc.
Data Communication System Model
A data communication system can be divided into three parts, the sending device, the transmission system, and the destination system
Analog signal, the signal between the modem and the telephone end office.
digital signal, from PC to modem
modulation strategy
Usually, we can't directly transmit the bitstream to another place over a long distance. If you try to transmit the bitstream directly, it will be lost beyond recognition after reaching a certain distance, which is what we call signal attenuation.
Therefore, the clever ancestors proposed two modulation strategies, one is band-based modulation, and the other is band-pass modulation.
Band base modulation
To convert the bit stream by encoding, we know that we can use positive electricity to represent and negative electricity to represent 0, which constitutes the bit stream we know, 110100001, etc., then we use graphics to describe
From the above picture, we can see that if we transmit the bit stream, then, after long-distance transmission, we cannot know at all, how many 0s and 1s are there, and in the case of continuous 0 or 1, even more Indistinguishable.
So there are the following three encoding mechanisms, we can see that whether it is 0 or 1, it has an action, so that we can know how many 0s and how many 1s there are.
Manchester encoding, jump down represents 1, jump up represents 0.
Differential Mann-Ster coding, the start bit has a transition to represent 0, and no transition means 1.
Bandpass modulation
Through band-pass modulation, the bit stream is converted into wave transmission. Then, at this time, we can use electromagnetic waves. Electromagnetic waves are a kind of light waves, and the propagation speed of light can be imagined. In communication, we can To further increase the propagation rate, how is bandpass modulation achieved?
Amplitude modulation, 1 modulates with a continuous sine wave, and 0 does not modulate.
Frequency modulation, 1 has a larger wave frequency and 0 has a smaller frequency.
Phase modulation, 0 uses a sine wave, 1 uses the opposite sine wave to modulate.
signal to noise ratio
Noise exists in all electronic equipment and communication channels. When the noise reaches a certain level, it will affect the propagation of the signal. The ratio of the signal evaluation power to the noise evaluation power is calculated as 10log2(S/N) . On this basis, Shannon A formula is proposed, which indicates the limit information transmission rate of the channel, C=Wlog2(1+S/N) (b/s) .
Therefore, all major modem operators are trying to get closer to this rate, so some people have come up with a way to make each symbol carry more information by encoding, such as 101011000, if I want to transmit this bit stream, just There are 9 different frequencies of waves to use, but if we group 101, 011, 000, this allows each symbol to carry more information.
transmission medium
Commonly used transmission media are twisted pair, coaxial cable, optical cable.
A twisted pair consists of two mutually insulated copper wires placed side by side, just like the structure of DNA, which is formed by a helix, and the higher the degree of twist, the stronger the anti-interference ability, the better the quality, and the faster the transmission.
Fiber optic cables are widely used, made of quartz glass, and if the angle of refraction is large enough, total reflection will occur.
Unguided Transmission Media
Relay is carried out through multiple transmitting stations, and microwaves are sent, and the microwaves are not easily distorted during the transmission process, or satellite communication is used.
Channel Multiplexing Technology
If ABC wants to communicate with DEF, we do not have to establish three separate communications, but see that we can transmit data for multiple users through one communication channel.
Here, frequency division multiplexing and time division multiplexing can be used.
Frequency division multiplexing allows multiple users to share the same line and occupy different bandwidth resources at the same time.
In the process of transmission, each user has one frequency per person, sums the waves of all users together, and then sends them until the waves do not interfere with each other. For example, if we throw two stones in the water, two The propagation of waves does not interfere with each other, and multiplexing is performed in one section and demultiplexing in the other section. Frequency division multiplexing is most commonly used in telephones.
Time division multiplexing , the same line, periodically transmits data for multiple users. For example, user 1 wants to transmit AAA, and user 2 wants to transmit CCC. Then, if time division multiplexing is used, it may be ACACAC, but it may exist, there are only 2 In the case of A or an A, if you still use time division multiplexing, is it a waste of an opportunity for multiplexing? At this time, there is statistical time division multiplexing. Statistical time-division multiplexing is to add a mark to the data during the transmission process, and add a mark to which interface is transmitted, and then after the receiving end receives the signal, it identifies the mark and sends the signal corresponding to the mark to the corresponding interface.
Time division multiplexing, each transmission is in sequence, even if it is not transmitted at the same time, it also occupies a position.
Wavelength division multiplexing , which can be understood in the same way as frequency division multiplexing, and this technology also uses the method of amplifying the signal. There may be several amplifiers between two receiving stations.
Code division multiplexing , which is widely used in China Unicom, for example, we also usually call CDMA, the multiplexing technology, in fact, is to split a bit into multiple bits to represent, we will split such a The unit is called a chip, and this sequence of chips can only be operated with the receiver to get the correct information.
We represent the 1 in the bit stream as a sequence 00011011, and the 0 as the complement 11100100. We represent this station as station S. , the receiving end also uses this sequence 00011011, we will use -1 to represent 0, the sequence obtained at this time is -1-1-111-111 and 111-1-11-1-1, the receiving end, -1-1 -111-111, at this time, if we want to send 1 to the receiver, then we need to send 1-1-111-111. If we want to send 0, we only need to send 111-1-11-1-1. The reason is that the inner product of the transmitted sequence and the receiving end, and then divided by the number of bytes sent is exactly 1, and when 0 is sent, the inner product of the transmitted sequence and the receiving end is exactly -1, and We use -1 to represent 0. If there are other sending stations, the inner product of the transmitted sequence and the receiving end must be 0. (The so-called inner product is, for example, (1,1,-1) and one's complement (-1,-1,1), [1x(-1) + 1x(-1) + (-1) x 1]/ 3 = -1)
The disadvantage of code division multiplexing, one bit should be represented by m bits. If there are too many mobile phones using CDMA technology, in order to distinguish, the more sequence digits are used to represent one bit.
After learning code division multiplexing, will you think of eavesdropping technology ? If you want to hear someone else's message, you just need to make a segment that is the same as his chip sequence, so that when you receive the message, you can also receive the message.
ADSL broadband access technology
Through statistics, it is found that users upload less and download more, which divides the communication frequency band into two parts, one for uploading and the other for downloading.
But at that time, there was no uploading function, it was all used for downloading, so the part of uploading you would be used by the people behind to spread the Internet signal, which became ADSL network communication.
