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Table of contents
1. Channel multiplexing technology
2. Wavelength division multiplexing
2. Digital transmission system
1. Channel multiplexing technology
Channel multiplexing: Multiple senders use the same channel to transmit information. The sender uses a multiplexer to combine different messages for transmission, and the receiver uses a demultiplexer to separate the messages.
1. Frequency division multiplexing, time division multiplexing and statistical time division multiplexing
1.1 Frequency Division Multiplexing FDM: Each user is assigned a frequency band, which is always occupied during communication. Users occupy different frequency bands at the same time.
1.2 Time division multiplexing TDM: Divide time into equal-length frames, and each user occupies one of the fixed sequence number gaps in each frame. Users occupy the same frequency band at different times. Due to the bursty nature of computer data, the channel utilization rate of time division multiplexing is relatively low.
1.3 Statistical time division multiplexing STDM: an improved time division multiplexing, also known as asynchronous time division multiplexing. STDM does not allocate time slots fixedly, but dynamically allocates time slots on demand.
2. Wavelength division multiplexing
Wavelength division multiplexing WDM is frequency division multiplexing of light. Dozens or more optical carrier signals can be multiplexed on one optical fiber. Optical signals will attenuate after being transmitted for a certain distance, so they need to be amplified by optical fiber amplifiers for continued transmission.
3. Code division multiplexing
Code Division Multiplexing CDM: Different users use different code types and communicate in the same frequency band at the same time. An important feature of the CDMA system is that the chip sequences assigned to each station by this system must not only be different , but also must be orthogonal to each other . In practical systems, pseudo-random code sequences are used.
For example, the vector S is (- 1- 1- 1+1+1- 1+1+1), and the vector T is (- 1- 1+1- 1+1+1+1 - 1), which is equivalent to The chip sequence at station T is 00101110. Substituting the component values of vectors S and T into equation (2-3), it can be seen that the two chip sequences are orthogonal. Not only that, the inner product of vector S and the vector of the one's complement of each station's chip is also 0. Another important point is that the normalized inner product of any chip vector and the chip vector itself is 1:
The normalized inner product value of a chip vector and the vector of the chip's complement is -1. This can be clearly seen from equation (2-4), because all the terms of the sum become -1.
Now assume that there are many stations communicating with each other in a CDMA system. What each station sends is the product of data bits and the chip sequence of its own station. Therefore, it is the chip sequence of its own station (equivalent to sending bit 1) and A combined sequence of the one 's complement of the chip (equivalent to sending bit 0), or nothing is sent (equivalent to sending no data).
Code division multiplexing is actually a kind of spread spectrum communication. CDM is commonly used in wireless LANs.
2. Digital transmission system
Digital communication is better in terms of transmission quality and economy than analog communication.
Optical fiber is the most important transmission media for long-distance trunk lines.
Synchronous digital sequence SDH and synchronous optical fiber network SONET are currently the most important international standards for digital transmission. Abbreviation SONET/SDH standard