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ADSL technology can use the existing local copper wire for signal transmission, and its highest rate is 9Mbps for downlink signal (from end office to user) and 1Mbps for uplink signal (from user to end office). There are a large number of users in the existing local telephone copper wire network. ADSL can make full use of the existing local telephone copper wire and effectively protect the original network resources.
In recent years, the Internet has developed at an amazing speed, with a huge number of netizens and a wide distribution. However, the user access rate provided by the existing user network is too low to meet the needs of the rapid development of the Internet, which provides an opportunity for the development of ADSL. Because the local copper wire has connected all Internet users, once ADSL technology is mature, it will become the first choice for telephone companies and Internet users.
First, the principle and technical performance of ADSL
The existing subscriber loop is mainly composed of UTP (unshielded twisted pair). The attenuation of UTP to signal is mainly related to transmission distance and signal frequency. If the signal transmission exceeds a certain distance, the transmission quality of the signal will be difficult to guarantee. In addition, the bridge tap on the line will also increase the attenuation of the signal.
Therefore, line attenuation is the main factor affecting ADSL performance. ADSL uses frequency division multiplexing technology (or echo cancellation technology) to separate uplink and downlink channels through asymmetric transmission, so as to reduce the influence of crosstalk and realize high-speed signal transmission.
In order to utilize multiple channels, ADSL modems divide the bandwidth of available telephone lines in two ways: frequency division multiplexing or echo cancellation. As shown in figure 1-2, FDM divides the frequency band into uplink and downlink. Downlink channels are time-division multiplexed into one or more high-speed channels and low-speed channels; The uplink channel will also be multiplexed into the corresponding low-speed channel.
Echo cancellation technology makes the overlapping parts of uplink and downlink channels cancel each other in frequency band. Local echo cancellation technology can effectively separate the uplink and downlink channels and reduce the influence of crosstalk on the channels, thus realizing high-speed signal transmission. This technology has been applied to the modem products of V.32 and V.34 protocols.
Figure1-2 Schematic Diagram of FDM Technology and Echo Cancellation Technology
Attenuation and crosstalk are two standard defects that determine the performance of ADSL. The higher the transmission rate, the greater their influence on the signal, so the effective transmission distance of ADSL is shortened with the increase of transmission rate. The attached table lists the relationship between UTP downlink rate and effective transmission distance of Line 24 under normal circumstances. The VDSL in the table is a very high-speed ADSL. Although the effective transmission distance of VDSL is shorter than ADSL, VDSL can be used to connect offices or families in buildings after FTTC or FTTB.
If the length of ADSL access network is 5.5km, it can cover more than 80% of existing telephone users; If the line length is 3.7km, it can cover more than 50% of existing users, and scattered users outside the user cell can access the network through the hub node based on optical fiber. Crosstalk noise is usually stable, so it is easier to study and overcome. However, the impact noise is random in frequency, period and phase, so it is difficult to model and study it.
Second, the access mode and access model of ADSL system
The following is the functional module diagram of ADSL system connection mode.
Figure 1-3 ADSL system connection mode function module diagram
Server: application server internet: IP internet core network: backbone network (usually ATM backbone network)
ADSL: The office part is ADSL office equipment, and the user part is ADSL user equipment.
Existing copper wire: ordinary twisted-pair copper wire connecting the client and the office.
The access model of ADSL is mainly composed of central exchange office module and remote module.
Figure 1-4 ADSL access mode
The central exchange module includes ADSL modem and access multiplexing system in the central location. The ADSL modem in the central position is called ATU-C (ADSL Transmission Unit-Central). The modems in the center of an access multiplexing system are usually combined into an access node, which is also called "DSL access multiplexer".
The remote module consists of a subscriber ADSL modem and a filter. The subscriber ADSL modem is usually called ATU-R (ADSL Transmission Unit-Remote).
Figure 1-5 Connection diagram of ADSL user equipment at user end
Third, ADSL modulation and demodulation technology
At present, there are three widely used ADSL modulation technologies: QAM (Quadrature Amplitude Modulation -Tu Demodulation), CAP (Carrier Sampling-Phase Modulation) and DMT (Discrete Multi-Tone), among which the national standard formulated by ANSI standardization group T65438+E 1.4 adopts DMT modulation technology. However, due to the short introduction time of the standard, there are still a considerable number of ADSL products using QAM or CAP modulation technology.
3. 1 QAM modulation technology
The schematic diagram of QAM modulator is that the transmitted data is divided into two paths (the original rate is 1/2) in the bit/symbol encoder, multiplied by a pair of orthogonal modulation components respectively, and summed and output. Compared with other modulation techniques, QAM coding has the advantages of making full use of bandwidth and strong anti-noise ability.
Fig. 1-6 16-QAM modulation schematic diagram
As shown in figure 1-6. In the QAM modulation of 16-QAM, two bits are encoded to represent the phase change, and the other two bits are used to represent the amplitude change, so they are represented by four bits. The main problem of using QAM in ADSL is how to adapt to the huge performance difference between different telephone lines. In order to obtain ideal operating characteristics, the QAM receiver needs an input signal with the same spectrum and phase characteristics as the transmitter to decode, and the QAM receiver uses an adaptive equalizer to compensate the distortion caused by the signal in the transmission process, so the complexity of the ADSL system using QAM mainly comes from its adaptive equalizer.
3.2 CAP modulation technology (carrier-free AM/PM)
CAP modulation technology is developed on the basis of QAM modulation technology. It can be said that it is a variant of QAM technology, and its modulator principle is shown in Figure 2. Input data is sent to an encoder, in which M-bit input bits are mapped to k = 2m different complex symbols An = An=an+jbn, and k-CAP line coding is formed by k different complex symbols. The encoded an and bn are sent to in-phase and quadrature digital shaping filters respectively, and then summed and sent to D/A converter. Finally, the signal is sent out through a low-pass filter. The main technical difficulty of ADSL using CAP technology is to overcome the interference of near-end crosstalk to signals, which can generally be solved by using near-end crosstalk canceller or near-end crosstalk equalizer.
3.3 DMT modulation technology
The main principle of DMT modulation technology is to divide the frequency band (0- 1. 104MHZ) into 256 orthogonal sub-channels expressed by frequency (each sub-channel occupies 4KHZ bandwidth). After bit allocation and buffering, the input data is divided into bit blocks, coded by TCM, and then transformed by 5 12 point discrete Fourier transform (IDFT). At this point, the bit block will be converted into 256 QAM sub-characters. Then, each bit block will be added with cyclic prefix (to eliminate inter-symbol interference), and the signal will be sent to the channel through data mode conversion (DA) and transmission filter. At the receiving end, it will be received and decoded in the reverse order.
Figure 1-7 DMT modulation technology example
In the above figure, the bandwidth of 1 MHz is divided into 256 4 KHz subbands. Each sub-band is modulated by single carrier modulation technology at the transmitter, and each sub-band is received at the receiver, and its 256 carriers are integrated and demodulated. With the recommendation of using DMT technology in American ADSL national standard (T 1.4 13), more and more ADSL modems will adopt DMT technology in the next few years.
Many industry experts firmly believe that xDSL technology based on ADSL will eventually become the winner on copper twisted pair. At present, users who use ordinary dial-up modems and N-ISDN technology will gradually transition to ADSL and other broadband access methods, and finally realize optical fiber access.
Fourthly, the installation of ADSL equipment.
ADSL installation includes local line adjustment and user equipment installation. On the local side, the service provider can connect the user's original telephone line to ADSL local equipment in 2~3 minutes; The ADSL installation on the user side is also very simple and convenient. As long as the telephone line is connected to the filter, a dual-core telephone line is connected between the filter and ADSL modem, and a crossover network cable is connected between ADSL modem and computer network card, the hardware installation can be completed, and then the IP, DNS and gateway parameters in TCP/IP protocol are set, and the installation work is completed. It is easier to use ADSL. Because ADSL doesn't need to dial, in always online, users can enjoy high-speed Internet access and make phone calls just by connecting ADSL power supply.
Figure 1-8 ADSL local line adjustment and user equipment installation
There is not much difference between ADSL installation of LAN users and stand-alone users. Just add another hub and connect the hub with ADSL MODEM with a direct network cable, as shown in the following figure:
Figure 1-9 ADSL equipment installation for LAN users
Due to the complexity of ADSL system technology, the current production cost is still very high. However, according to the current development trend, it is estimated that by 200 1, ADSL will be monolithic and the chip price will be greatly reduced. In addition, ADSL access network will become the main broadband access network technology by then.