XDSL technology and its application

Abstract: xDSL is a modulation technology that connects the two ends of the twisted pair copper wire to the xDSL modem to transmit high-speed data using its high-frequency broadband feature. This article comprehensively describes the xDSL technology and its application to achieve high-speed user line.

1. Import xDSL technology

In recent years, the rapid development of Internet and Intranet has seen a surge in demand for fixed-connection high-speed user lines. xDSL is a general term for various types of DSL, including ADSL, RADSL, VDSL, SDSL, IDSL, and HDSL. It is a new transmission technology, which uses high frequency and corresponding modulation technology on existing copper telephone lines, that is, the signal processing technology that adds or obtains more digital data to the analog line is used to obtain the high transmission rate (the theoretical value can reach 52 Mbps ). The biggest difference between various DSL technologies is the difference in signal transmission rate and distance, and the difference in the symmetry between upstream and downstream channels.

XDSL has the following features:

1) working in different frequency bands from voice, basically does not affect the normal use of the phone, the bandwidth occupied by voice is 0-4 KHz, and the xDSL modulation frequency band is 4.4 KHz-1 MHz.

2) A high data transfer rate can be provided within a certain distance between the Office and the home.

3) the specific conditions, weather, and fragment dialog of copper wires may affect transmission performance.

2. xDSL technology classification

In xDSL, "x" indicates any character or string. According to the modulation method, the differences between the obtained signal transmission rate and distance, and whether the uplink and downlink rates are the same can be divided into two types: Rate symmetric type and rate asymmetric type.

1) the rate-symmetric xDSL has multiple forms, such as HDSL and SDSL.

* HDSL adopts two pairs of twisted-pair copper wires for Bidirectional rate symmetric communication, that is, it provides equal bandwidth for upstream and downstream communication, with a rate of T1/E1 and an effective transmission distance of 5 km. It is suitable for applications connected to PBX systems, digital local loops, Internet service providers, and campus networks.

2) asymmetric xDSL values include ADSL and VDSL.

* The downstream speed of ADSL is very high. The user's downstream speed can reach 1.5 to 8 Mbps, while the upstream speed in the reverse direction is 16 to 640 kbps. The maximum transmission distance is 5.5, which is suitable for Internet businesses with large downstream data volumes. Recently, the rate-adaptive ADSL (RADSL) is introduced. It overcomes the shortcomings of ADSL in Interrupting Communication under strong noise conditions and can automatically reduce the rate and maintain the communication connection. The ability to dynamically adjust the user's access rate based on the quality of twisted pair copper wires and the distance between transmission, which allows users to connect different copper wires at different rates, maximize the use of existing communication resources.

* Similar to ADSL, VDSL (a high-speed digital user line) is also a technology with asymmetric uplink and downlink transmission rates. VDSL uses a telephone line to obtain a downlink transmission rate of 13-1.5 Mbps. the uplink speed is 2.3-1.5 Mbps, and the transmission distance does not exceed km. It is mainly used in video and multimedia scenarios.

3. Functions of the separator

XDSL technologies, such as SDAL, ADSL, and VDSL, can provide both telephone and high-speed data services. Therefore, a separator should be connected to both ends of the existing twisted pair wires, separate the low-frequency bands below 4 kHz that carry audio signals and the high-frequency bands used for xDSL Modem modulation. The splitter is actually a device synthesized by a low-pass filter and a high-pass filter. Passive devices are usually used to simplify the design and avoid the trouble of feeding.

The internal structure of xDSL Modem is almost the same as that of analog Modem such as V634. It mainly consists of the analog front end (analog frone end) for processing D/A transformations, the digital signal processor (DSP) for modulation/Demodulation processing, and the digital sending power and transmission error reduction, the digital interface for error correction is composed of "Grid encoding" and "intertwined processing.

Most xDSL Modem products on the Exchange Board have multiplexing functions. signals sent from each xDSL line are reused in DSLAM and forwarded to routers and other devices on the backbone network through high-speed interfaces, this configuration can save the router port and simplify the wiring. Currently, several xDSL lines have been bundled into a 10-BASE-T product, and all the data on the switch is integrated into a 155 Mbit/s ATM port.

4. xDSL modulation method

XDSL uses 2B1Q, CAT, and DMT modulation/demodulation methods. They all use high frequency bands. High-frequency signal loss is high and easily subject to noise interference. Therefore, the higher the speed, the closer the transmission distance. In addition, the transmission rate is also related to the path and quality of twisted pair wires.

1) 2B1Q: 2B1Q is a modulation method for transmitting data by changing the amplitude of the Rectangular Wave. Its amplitude is divided into four levels, which can transmit 2 bits of data at a time. XDSL, such as IDSL, HDSL, and SDSL, uses this modulation method.

2) CAP method: CAP is a modulation method based on orthogonal amplitude modulation. Upstream and downstream signals are modulated on different carriers, and both rate-symmetric and non-symmetric xDSL can be used. V.34 and other analog Modem also use QAM. The difference between it and CAP lies in the band used by it. V.34 Modem only uses 4 KHz, while the CAP in ADSL mode uses the 30KHz-1Mhz band. The higher the frequency, the smaller the waveform cycle, so it can increase the rate of the modulated signal (that is, the data transmission rate ). In CAP, "Carrierless (no-carrier)" means that the part of the generated carrier (circuit and DSP firmware module) is not independent. It is integrated with the modulation/Demodulation part to make the structure more refined.

3) DMT mode: DMT uses 256 QAM modulation, which is usually used for ADSL and other non-symmetric Modem. In ADSL, one carrier modulation occupies 4 kHz bandwidth. For 256 carriers to undergo QAM modulation, Fuli transformation is required. DMT allocates modulation data for each 4 kHz bandwidth, which can reduce the amount of data allocated to the noise band. Therefore, DMT has a strong anti-noise performance. DMT can also be divided into the frequency division multiplexing (OFDM) and echo offset modes. OFDM divides the frequency bands based on upstream and downstream channels. The uplink and downlink frequency bands of the ECHO offset method overlap, and the efficiency is higher than that of OFDM, but the processing is complicated.

5. Application Scope of xDSL

1) Access Line Used as a leased line network: The leased line provides symmetric upstream and downstream communication services. Therefore, IDSL, SDSL, and HDSL Modem can be used, the two-way transmission rate of the terminal is 128 kbit/s-2Mbit/s through a serial port such as V.35 or X.21.

2) Access Line for the Internet: in the Internet, the downstream data volume of Web and other customer/server services is much larger. Therefore, the downstream high-speed ADSL Modem can be used.

3) used as the access line of the ATM.

6 conclusion

As Internet bandwidth continues to increase, people are increasingly concerned about the network transmission bottleneck from high-speed networks to remote offices and home users, xDSL technology is designed to overcome the user's "last mile" transmission bottleneck. It uses existing telephone lines and special modulation technologies to greatly increase transmission rates, compared with other access methods, xDSL has maintained a high cost-performance ratio. Therefore, xDSL technology is being adopted by more and more users.
Article entry: csh responsible editor: csh