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Common multiplexing technology

Source: Shangpin China | Type: Maintenance | Time: June 23, 2014
In order to improve the utilization of transmission lines and save channel resources, channel multiplexing technology is widely used in wide area networks (WAN).

Multiplexing refers to a mechanism that two or more terminal devices share a common channel through a multiplexer when sending data, as shown in Figure 2-13.

Multiplexing technologies include: frequency division multiplexing, time division multiplexing, code division multiplexing and wavelength division multiplexing. Next, Shangpin China Website construction The company mainly introduces two technologies, frequency division multiplexing and time division multiplexing, which are widely used.

1. Frequency division multiplexing

Frequency Division Multiplexing (FDM) can be used when the available bandwidth of the transmission medium exceeds the total bandwidth required for the signal to be transmitted. When FDM is used, the signal input to the multiplexer can be either digital or analog.
 
Frequency division multiplexing

As shown in Figure 2-14, in the telephone communication system, the bandwidth of each telephone signal is 300 ^ - 3400Hz. When each signal source is input into the multiplexer, the multiplexer modulates each signal to different frequency bands of the physical channel spectrum (called subchannels) through the frequency band transmission technology (spectrum shifting). The frequency is respectively migrated from 4000H: as the standard bandwidth to 60-108kHz:. In order to ensure that the bandwidth of each signal does not overlap and a certain protection band is reserved, each signal takes its own carrier frequency as the center, and carries out data transmission on the sub channels of its own bandwidth, realizing channel multiplexing.
Frequency division multiplexing
Frequency division multiplexing requires that the total bandwidth should be greater than the sum of the bandwidth of each subchannel. The frequency band signals of all sub channels are superimposed and transmitted into the common channel, and the frequency band signals of each sub channel are separated by a filter at the outlet of the signal.

In practical application, the signal transmission of cable television station is to use frequency division multiplexing technology to transmit the signals of many channels through one line, and users can choose to watch any of them ADSL broadband access technology also uses frequency division multiplexing technology to separate low-frequency signals and high-frequency signals transmitted by ordinary telephone lines. The low-frequency part below 3400Hz is used for telephone communication, and the high-frequency part above 3400H is used for network communication.

Frequency division multiplexing (FDM) has the advantages of high channel multiplexing rate and convenient branching. It is a multiplexing technology commonly used in analog communication at present. The main problem of frequency division multiplexing is still the mutual interference between signals.

2. Time division multiplexing

Time Division Multiplexing (TDM), if the data transmission rate of the transmission medium exceeds the total data transmission rate of the digital signal to be transmitted, the time division multiplexing technology can be used, that is, a physical channel is divided into several time slices according to time and allocated to multiple signal sources in turn. Each time slice is occupied by a multiplexed signal.

Time division multiplexing technology is adopted. The signal input to the multiplexer is generally digital signal, but it can also be analog signal.

As shown in Figure 2-15, there are four terminal digital signals at the transmitting end, and data needs to be transmitted to the receiving end through a common transmission channel. Here, 4 is used as a fixed time slot (1 time slot is 1 B) to allocate each cycle. Each digital signal is time switched by the multiplexer, and each character occupies a time slot to form a transmission unit, namely "frame". In each time division period, because each digital signal is fixed on its own time slot, when there is no data transmission at the time slot receiver, the corresponding time slot will be wasted, and thus the utilization rate of channel resources will be reduced.
Time division multiplexing
In practical applications, such as PCM communication (Pulse Code Modulation: converting continuous analog signals into discrete digital signals, which is commonly used in digital audio) is the use of time-division multiplexing technology. The telephone network (PSTN) is also based on time-division multiplexing (TDM) technology to digitize its backbone network.
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