Introduction to modulation methods and application technologies of three XDSL technologies

With the rapid development of the Internet, communication technology is experiencing a huge revolution, because the number of users accessing the network rapidly increases. As a network connected to the end users in the communication network, the access network has become a hot topic in network technology.

Currently, the main access technologies include hybrid fiber/coaxial (HFCs) access technology, optical fiber access technology, copper wire access technology and wireless access technology. Copper wires are connected to existing telephone lines as transmission media, and advanced digital signal processing technology and modulation and demodulation methods are used to improve Transmission Accuracy and distance. It mainly includes HDSL (high-speed digital user loop), ADSL (Asymmetric Digital user loop), and VDSL (ultra-high speed digital user loop ). The dual-stranded copper wire using XDSL technology is configured with a separator for separating audio and high-frequency signals. The audio signals below 4 kHz are separated from the high-frequency signals after xDSL modulation by the filter, enables simultaneous transmission of voice and high-speed data. Because XDSL technology uses twisted pair copper wires for transmission, it can save line costs and be economical and easy to use. Therefore, it can be used as an advanced technology for broadband access. The following describes the modulation methods and application technologies of these three XDSL technologies.

I. HDSL

HDSL uses echo suppression, adaptive filtering, and high-speed digital processing technology, and line encoding uses 2B1Q codes. Two or three pairs of modules (local machines) and network terminal modules (User Machines) are used. The transmission rate of each pair of twisted pair wires is 1168kb/S. The two pairs provide 2 Mb/S E1 services. The HDSL no-relay transmission distance is 3-5 km.
T1E1.4/94-006 developed by American National Standards Association (ANSI) and European Telecommunications Standards Association (ETSI) the proposed DTR/DM-0.3036 defines the electrical and physical properties, frame structure, transmission mode and communication rules of HDSL. Because echo suppression adaptive balancing technology is used, anti-interference capability is enhanced, inter-code interference is overcome, and long-distance non-relay transmission can be realized. The HDSL system is placed on the switch terminal and the user end. The system consists of a transceiver, A multiplexing and ing part, and an E1 interface circuit. The transceiver consists of two parts: sending and receiving. It is the core of the HDSL system. The sending part converts the input HDSL single-channel code stream through line encoding, and then undergoes D/A switching and waveform formation and processing. The transmitting amplifier is amplified and then sent to the external line. The receiver uses the echo canceller to offset the leaked signal sent from the reflected signal that is not matching the impedance. After balanced processing, the original data signal is restored, the code stream is converted to HDSL through line decoding, and then sent to multiplexing and ing for processing. The echo eliminator and the balancer are used as the System Adaptive Adjustment and tracking of external feature changes, and the system parameters are dynamically adjusted to optimize the system transmission performance.

In the HDSL system, the reuse and ing are used to reuse and dereuse data streams. The sending part divides the 2 Mb/S PCM bitstream into two parts, respectively adding overhead bits of the HDSL frame structure to the transmission bitstream of the hdsl frame and sending it to the line through the transmitter. Correspondingly, the receiving part removes the overhead bits in the HDSL frame structure from the received two HDSL streams of 1168kb/S, and then reuses them into a 2 Mb/S PCM bitstream, the G.703 interface sent to FCM 2 Mb/S through the interface circuit. The Interface part converts and adapts the HDSL system and PCMG.703 interface, completes the conversion between the HDB3 code stream and the NRZ code stream, and achieves electrical and impedance adaptation. There are two main types of HDSL line codes: 2B1Q and CAP. 2B1Q is a 4-level pulse amplitude modulation code. Each symbol bit represents two bits. Four levels + 3, + 1,-1, and-3 represent 10, 11, 01, and 00 respectively, this improves the bit rate of transmission. CAP is a non-carrier amplitude-phase modulation. The data is superimposed after two orthogonal signals are modulated separately, and the carrier that does not carry useful information is blocked. The two types of encoding are very useful for improving the transmission efficiency. At present, many chips and products have been released to implement HDSL digital modulation and signal processing.

In addition to access networks, HDSL can also be used for DDN node relay, wireless paging relay, mobile base station relay, lan interconnection, conference television, and high-speed data transmission, provides low-cost E1 channels for enterprises and enterprises. The HDSL transmission technology can be combined with the video compression coding MPEG technology to transmit the broadband services of the machine frequency, such as the video recorder frequency signal and the multimedia conference and television system signal. According to H.261 related recommendations, it can be used as a 2 Mb/S Conference television transmission and access system. In addition, it can also be used for transmission of multimedia services such as videophone, Home Shopping, remote diagnostics, and distance education.

Ii. ADSL

ADSL is another broadband access technology developed in recent years. It uses twisted-pair copper wires to provide users with bandwidth information services with asymmetry in two directions. The downlink speed is as high as 6-9 Mb/S, and the transmission distance is 3-5 km. Typical downlink rates include T1, E1, DS2 (6.312 Mb/s), and E2 (8.448 Mb/s ). The uplink speed is low, which varies with the products of different companies, usually 16-640kb/s.

ADSL simultaneously transmits high-speed downstream data, low-speed data, and analog phones over a pair of telephone lines. Signals are divided and reused to occupy different frequency bands. Low-Frequency segments transmit voice messages. Intermediate Narrow-Band channels transmit upstream channel data and control information; other high-frequency segments transmit downstream channel data, images, or high-speed data.

ADSL uses DMT or CAP modulation. DMT is a discrete multi-frequency modulation. It divides all the frequencies into 256 sub-channels, and dynamically allocates the input data to them based on the instantaneous attenuation, group delay, and noise characteristics of the sub-channels. Adaptive distribution of sub-channel rates, and closure of sub-channels drowned by narrowband noise to achieve optimal use of transmission lines, minimizing the error code and noise, and improving the system transmission capacity. In addition, ADSL uses new developments in adaptive filtering, such as grid encoding and RS code, to improve noise resistance, and uses asymmetric echo cancellation technology to eliminate echo interference. The ADSL system sets up the ADSL modem on the local terminal and the client side respectively, and uses DMTFFT to implement signal processing and modulation and demodulation. The telephone service that comes from the telephone network is connected to the ordinary telephone line through the ordinary telephone service (POTS) Separator in passive coupling mode. The Exchange Board (LEX) node is divided and inserted into the voice through the ADSL modem and POTS splitter, and transmitted to the user end together with the high-speed data through the twisted-pair copper wire, and then through the user end separator and ADSL modem, send the information to your PC or TV Terminal.

According to the T1.431 recommendation of ECSA (exchange carrier Standards Association), ADSL can provide high-speed ticket and duplex channels. The express ticket channel uses the downlink channel to transmit 6 Mb/S data streams, 4 MPEG-1 images can be simultaneously transmitted, or 1-2 MPEG-2 images with better quality, or two 3 Mb/S digital broadcast TVs. The full-duplex ADSL channel can provide 160 kb/S and 576kb/S data streams for access to the ISDN network or high-speed link. ITU-T proposed three kinds of ADSL standard: ADSL-1, ADSL-2 and ADSL-3 transmission rate were 1.544 Mb/S, 3.152 Mb/S and 6.32 Mb/S transmission distance were 5.5 km, 3.6 km and 2.5 km, respectively, can be used to transmit MPEG-1 and MPEG-2 images.

Using the existing twisted pair wire transmission broadband service, ADSL technology can protect investment and reduce costs, especially suitable for a large number of scattered residential users. It is widely used for Internet access, remote LAN access, VOD, remote teaching, multimedia retrieval, and other broadband services. At present, ADSL technology and its applications are constantly developing, and the corresponding ADSL modulation and processing chips are also pushed to the market in large numbers, telecom equipment manufacturers also produce different types of ADSL modulation and demodulation products and actively explore the market, so that the ADSL system is continuously applied in the access network.

Iii. VDSL

Similar to ADSL, VDSL is also asymmetric. That is, it uses twisted-pair copper wires to provide asymmetric broadband services in both upstream and downstream directions. However, the transmission rate of VDSL is nearly 10 times higher than that of ADSL, and the transmission distance is much shorter than that of ADSL. The downlink transmission rate of VDSL can reach 13-55 Mb/s, the distance between non-relay transmission is 300 Mb/S, And the uplink transmission rate is 1.5-1.5 Mb/s, which varies with the products of different companies. VDSL also uses frequency division multiplexing to transmit upstream and downstream signals of telephone and VDSL in different frequencies. Low-frequency transmission of ordinary telephone, ISDN services, intermediate frequency band transmission of uplink channel digital control information, high-frequency transmission of downlink channel images or high-speed data information. The sending end modulated various business signals to different frequencies and transmitted to the receiving end through twisted pair wires. After receiving signals are demodulated and filtered, various types of signals are separated.

VDSL mainly uses DMT and discrete wavelet multi-frequency modulation (DWMT ). DWMT adopts wavelet orthogonal transformation, which has better performance than DMT. The power of the sub-channels of DWMT transformed by wavelet is more than 99.99% concentrated on the main valve, thus the signal-to-noise ratio is greatly improved. The VDSL system is similar to ADSL. VDSL Modem is configured on the local end and the user end, and the telephone service is added to the channel through the POTS separator and coupler, HDTV digital images or multi-channel MPEG-encoded images are sent to the user end through the VDSL downlink channel. VDSL can also work with the optical fiber access network to provide broadband access for enterprise users and families as a wiring between The ONU (optical network unit) and users.

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