Multiplexing Technology __ Computer network

In the data communication system or computer network system, the bandwidth or capacity of the transmission media will often be greater than the demand for the transmission of a single signal, in order to effectively use the communication line, hope that a channel simultaneously transmission multi-channel signals, this is called Multiplexing Technology (multiplexing). Multiplexing technology can combine multiple signals to transmit on a physical channel, which can greatly save the installation and maintenance cost of cable during long-distance transmission. Frequency Division Multiplexing FDM (Frequency Division Multiplexing) and Time Division Multiplexing TDM (Time division Multiplexing) are the two most commonly used multiplexing technologies.

Frequency Division MultiplexingFrequency Division Multiplexing (FDM) divides the channel according to frequency spectrum, and the multiple roadbed band signal is modulated on different spectrum. So they do not overlap on the spectrum, that is, orthogonal on the frequency, but overlapping in time and can be transmitted simultaneously within a channel. In a frequency division multiplexing system, the signal M1 (t), M2 (t) of the transmitter end, ..., MN (t) by the respective Low-pass filters on each carrier F1 (t), F2 (t), ..., fn (t) modulation, and then by the various Bandpass filter filter out the corresponding Edge band (carrier telephone is usually using unilateral band modulation), The Frequency division multiplex signal is formed after adding. At the receiving end, the band-pass filter of each road separates each signal, and with each carrier F1 (t), F2 (t), ..., fn (t) multiplication, realize coherent demodulation, then can restore each signal, realize frequency division multiplex communication. In order to construct large capacity frequency division multiplexing equipment, the spectrum of modern High-capacity carrier series is composed of various base groups according to the modular structure. According to the International Telegraph and Telephone Advisory Committee (CCITT), the base group is divided into former group, base group, Superior and main group. ① before the group, also known as 3 Road group. It is composed of 3 words by frequency conversion. The carrier frequency of each channel is 12,16,20 khz respectively. Take the top band and get the 12~24 signal of the spectrum as the first group of khz. ② Base Group, also known as 12 Road group. It is composed of 4 former groups by frequency conversion. The carrier frequency of each front group is 84, 96,108,120 khz respectively. Remove the edge band and get the 60~108 signal of the spectrum as the base of khz. The base group can also be composed of 12 paths by one frequency conversion. ③ is superior, also called 60 Road group. It is composed of 5 groups by frequency conversion. The carrier frequency of each base group is 420,468,516,564,612 khz respectively. Remove the edge band and get the superior signal of the spectrum for 312~552 khz. ④ Main group, also known as 300 Road group. It is composed of 5 superior by frequency conversion. Each of the superior frequency of the carrier is 1364,1612,1860,2108,2356 khz. Remove the edge band and get the main group signal of the spectrum as 812~2044 khz. 3 main groups can be composed of 900-way Super host group. 4 Super Groups can be composed of 3600-way giants. The advantage of frequency division multiplexing is that the channel multiplexing rate is high, allowing multiple paths to be reused, and the shunt is also very convenient. Therefore, frequency division multiplexing has become one of the most important multiplexing methods in modern analog communication, and has been widely used in analog telemetry, cable communication, microwave relay communication and satellite communication.Time Division MultiplexingIf the media can reach the bit transmission rate than the data transmission rate required to transmit the information, then the time Division multiplexing TDM Technology, but also a physical channel is divided into several time slices in turn to be allocated to the use of multiple signals. Each time slice is occupied by a multiplexed signal, unlike FDM, which sends multiple signals at the same time. In this way, using each signal in the intersection of time, you can transmit multiple digital signals on a single physical channel. This crossover can be bit-level, or it can be a block of bytes or a larger set of information that crosses. If the multiplexer in Figure 2.12 (b) has 8 inputs and the data rate per input is assumed to be 9.616ps, then a line of up to 76.8kbps can hold 8 signal sources. The time division multiplexing four m scheme described in this diagram, also known as synchronous (synchronous) TDMA TDM, is pre-configured and fixed, so that the transmission timing of various signal sources is synchronized. In contrast, asynchronous time-division multiplexing 1DM allows for the dynamic distribution of the media to be allocated. Time-Division Multiplexing TDM is not limited to the transmission of digital signals, but also can cross-transmit analog signals simultaneously. In addition, for analog signal, time Division multiplexing and Frequency division multiplexing technology can be combined to use. A transmission system, which can be divided into many sliver channels, each channel and then use time Division multiplexing technology to subdivide. This hybrid technique can be used in broadband local area networks.