Abstract: xDSL technology is a modulation technology. The xDSL modem is connected to the two ends of the twisted pair copper wire, so that high-frequency broadband characteristics can be used to transmit high-speed data. This article comprehensively explains the xDSL technology and its applications that realize the user line speed.
1. Import of xDSL technology
In recent years, the Internet and intranet have developed rapidly, and the demand for fixed-connected high-speed user lines has risen a few days ago. xDSL is the general term for various types of DSL, including ADSL, RADSL, VDSL, SDSL, IDSL and HDSL. It is a new transmission technology, which uses higher frequency and corresponding modulation technology on existing copper telephone lines, that is, using signal processing technology that adds or obtains more digital data in the analog lines to obtain high transmission rates (theoretical value can reach 52Mbps). The biggest difference between various DSL technologies is reflected in the different signal transmission rates and distances, as well as the different symmetry between uplink and downlink channels.
xDSL has the following characteristics:
1) Working in different frequency bands from voice, it basically does not affect the normal use of the phone. The frequency band occupied by voice is 0-4KHz, and the xDSL modulation band is 4.4KHz-1MHz.
2) High data transmission rates can be provided within a certain distance centered on the office and home.
3) Specific conditions, weather and fragment dialogue of copper wire may affect transmission performance.
2. Classification of xDSL technology
"x" in xDSL represents any character or character string. According to the modulation method adopted, the different signal transmission rate and distance obtained and whether the uplink and downlink rates are the same can be divided into rate symmetrical and rate asymmetrical types.
1) The rate symmetrical xDSL can be found in various forms such as HDSL and SDSL.
*HDSL uses two pairs of twisted pair copper wires to achieve bidirectional rate symmetric communication, that is, provide equal bandwidth for upstream and downstream communication, with a rate of T1/E1 and an effective transmission distance of 5km. It is very suitable for connecting PBX systems, digital local loops, Internet service providers and campus networks.
2) There are several types of asymmetric xDSL values: ADSL and VDSL.
*ADSL downlink rate is very high, the user downlink rate can reach 1.5 to 8Mbps, while the uplink rate in the opposite direction is 16 to 640kbps. Its maximum transmission distance is 5.5km, which is suitable for Internet services with large downlink data. Recently, rate-adaptive ADSL (RADSL) has emerged, which overcomes the disadvantage of ADSL interrupting communication under strong noise conditions and can adaptively reduce the rate and maintain communication connections. The user's access rate can be dynamically adjusted according to the quality of twisted pair copper wires and the distance of transmission distance, which allows users to connect different copper wires at different rates to maximize the use of existing communication resources.
*VDSL (extremely high-speed digital subscriber line) is also a technology with asymmetric uplink and downlink transmission rates, just like ADSL. VDSL uses a telephone line to obtain a downlink transmission rate of 13-52Mbps, an uplink rate of 1.5-2.3Mbps, and the transmission distance does not exceed 1.5km. It is mainly used in related occasions such as video and multimedia.
3. Function of the separator
xDSL technologies such as SDAL, ADSL and VDSL can provide telephone and high-speed data services at the same time. For this reason, the splitter should be connected to both ends of the existing twisted pair cable to separate the low frequency band below 4KHz that carries the audio signal and the high frequency band for xDSL Modem modulation. A separator is actually a device synthesized by a low-pass filter and a high-pass filter. To simplify design and avoid the hassle of feeding, passive devices are usually constructed.
xDSL ModemInternal structure andV634SimulationModemAlmost the same。 It is mainly composed of an analog front end that processes D/A transformation, a digital signal processor (DSP) that performs modulation/demodulation processing, and a digital interface that reduces digital transmission power and transmission errors, and uses "grid encoding" and "interleaving processing" to achieve error correction.
Most of the xDSL Modem products on the switching office have multiplexing functions. The signals from each xDSL line are multiplexed in DSLAM and forwarded to routers and other devices on the backbone network side through a high-speed interface. This configuration can save the router's ports and simplify wiring. Currently, there are products that integrate several xDSL lines into a 10BASE-T and products that combine all data on the switch rack into 155Mbit/s ATM port.
4 xDSL modulation method
xDSL uses three modulation/demodulation methods: 2B1Q, CAT and DMT. They all use high frequency bands. High-frequency signal loss is high and is susceptible to noise interference, so the higher the rate, the closer the transmission distance is. In addition, the transmission rate is also related to twisted pair diameter and mass.
1) 2B1Q method: 2B1Q is a modulation method that transmits data by changing the amplitude of the rectangular wave. Its amplitude is divided into 4 levels and can transmit 2 bits of data at a time. This modulation method is used for the same-rate symmetric xDSL, HDSL and SDSL.
2) CAP method: CAP is a modulation method based on quadrature amplitude modulation. Up and downlink signals are modulated on different carriers, and both rate symmetrical and asymmetrical xDSL can be used. Analog Modem such as V.34 also uses QAM, and the difference between it and CAP is the frequency band it uses. V.34 Modem only uses 4KHz, while CAP in ADSL mode uses the 30KHz-1Mhz frequency band. The higher the frequency, the smaller the waveform period, so the rate of the modulated signal (i.e., the data transmission rate) can be increased. "Carrierless" in CAP means that the part that generates the carrier (the circuit and the firmware module of the DSP) is not independent. It is integrated with the modulation/demodulation part to make the structure more refined.
3) DMT method: DMT uses 256 QAM modulation, which is usually used for ADSL equal rate asymmetric modem. In ADSL, one carrier modulation occupies 4KHz bandwidth, and QAM modulation of 256 carriers requires a Fourier transform. DMT allocates the modulated data amount to each 4KHz bandwidth, which can reduce the amount of data allocated to the noisy band, so the DMT method has strong noise resistance. DMT can be divided into frequency division multiplexing method (FDM) and echo cancellation method. FDM divides frequency bands according to up and downlink channels. The up and downlink frequency bands of the echo cancellation method overlap, which is more efficient than FDM, but the processing is complicated.
5. Application scope of xDSL
1) Access line for dedicated network: Dedicated lines provide communication services with symmetrical upstream and downstream rates. Therefore, IDSL, SDSL and HDSL modem can be used. The terminal is connected to it through serial ports such as V.35 or X.21, and its bidirectional transmission rate is 128kbit/s-2Mbit/s.
2) Used as an access line for the Internet: In the Internet, the amount of downlink data browsing client/server services such as the Web is much larger, so downlink high-speed ADSL Modem can be used.
3) Used as the access line of the ATM.
6 Conclusion
With the continuous increase in Internet bandwidth, people are paying more and more attention to the "last mile" network transmission bottleneck problem from high-speed networks to remote office and home users. xDSL technology is generated to overcome the "last mile" transmission bottleneck of users. It uses existing telephone lines and uses special modulation technology to greatly improve the transmission rate. Compared with other access methods, xDSL access has maintained a high performance-price ratio. Therefore, xDSL technology is being adopted by more and more users.
Article entry: csh Editor in charge: csh
1. Import of xDSL technology
In recent years, the Internet and intranet have developed rapidly, and the demand for fixed-connected high-speed user lines has risen a few days ago. xDSL is the general term for various types of DSL, including ADSL, RADSL, VDSL, SDSL, IDSL and HDSL. It is a new transmission technology, which uses higher frequency and corresponding modulation technology on existing copper telephone lines, that is, using signal processing technology that adds or obtains more digital data in the analog lines to obtain high transmission rates (theoretical value can reach 52Mbps). The biggest difference between various DSL technologies is reflected in the different signal transmission rates and distances, as well as the different symmetry between uplink and downlink channels.
xDSL has the following characteristics:
1) Working in different frequency bands from voice, it basically does not affect the normal use of the phone. The frequency band occupied by voice is 0-4KHz, and the xDSL modulation band is 4.4KHz-1MHz.
2) High data transmission rates can be provided within a certain distance centered on the office and home.
3) Specific conditions, weather and fragment dialogue of copper wire may affect transmission performance.
2. Classification of xDSL technology
"x" in xDSL represents any character or character string. According to the modulation method adopted, the different signal transmission rate and distance obtained and whether the uplink and downlink rates are the same can be divided into rate symmetrical and rate asymmetrical types.
1) The rate symmetrical xDSL can be found in various forms such as HDSL and SDSL.
*HDSL uses two pairs of twisted pair copper wires to achieve bidirectional rate symmetric communication, that is, provide equal bandwidth for upstream and downstream communication, with a rate of T1/E1 and an effective transmission distance of 5km. It is very suitable for connecting PBX systems, digital local loops, Internet service providers and campus networks.
2) There are several types of asymmetric xDSL values: ADSL and VDSL.
*ADSL downlink rate is very high, the user downlink rate can reach 1.5 to 8Mbps, while the uplink rate in the opposite direction is 16 to 640kbps. Its maximum transmission distance is 5.5km, which is suitable for Internet services with large downlink data. Recently, rate-adaptive ADSL (RADSL) has emerged, which overcomes the disadvantage of ADSL interrupting communication under strong noise conditions and can adaptively reduce the rate and maintain communication connections. The user's access rate can be dynamically adjusted according to the quality of twisted pair copper wires and the distance of transmission distance, which allows users to connect different copper wires at different rates to maximize the use of existing communication resources.
*VDSL (extremely high-speed digital subscriber line) is also a technology with asymmetric uplink and downlink transmission rates, just like ADSL. VDSL uses a telephone line to obtain a downlink transmission rate of 13-52Mbps, an uplink rate of 1.5-2.3Mbps, and the transmission distance does not exceed 1.5km. It is mainly used in related occasions such as video and multimedia.
3. Function of the separator
xDSL technologies such as SDAL, ADSL and VDSL can provide telephone and high-speed data services at the same time. For this reason, the splitter should be connected to both ends of the existing twisted pair cable to separate the low frequency band below 4KHz that carries the audio signal and the high frequency band for xDSL Modem modulation. A separator is actually a device synthesized by a low-pass filter and a high-pass filter. To simplify design and avoid the hassle of feeding, passive devices are usually constructed.
xDSL ModemInternal structure andV634SimulationModemAlmost the same。 It is mainly composed of an analog front end that processes D/A transformation, a digital signal processor (DSP) that performs modulation/demodulation processing, and a digital interface that reduces digital transmission power and transmission errors, and uses "grid encoding" and "interleaving processing" to achieve error correction.
Most of the xDSL Modem products on the switching office have multiplexing functions. The signals from each xDSL line are multiplexed in DSLAM and forwarded to routers and other devices on the backbone network side through a high-speed interface. This configuration can save the router's ports and simplify wiring. Currently, there are products that integrate several xDSL lines into a 10BASE-T and products that combine all data on the switch rack into 155Mbit/s ATM port.
4 xDSL modulation method
xDSL uses three modulation/demodulation methods: 2B1Q, CAT and DMT. They all use high frequency bands. High-frequency signal loss is high and is susceptible to noise interference, so the higher the rate, the closer the transmission distance is. In addition, the transmission rate is also related to twisted pair diameter and mass.
1) 2B1Q method: 2B1Q is a modulation method that transmits data by changing the amplitude of the rectangular wave. Its amplitude is divided into 4 levels and can transmit 2 bits of data at a time. This modulation method is used for the same-rate symmetric xDSL, HDSL and SDSL.
2) CAP method: CAP is a modulation method based on quadrature amplitude modulation. Up and downlink signals are modulated on different carriers, and both rate symmetrical and asymmetrical xDSL can be used. Analog Modem such as V.34 also uses QAM, and the difference between it and CAP is the frequency band it uses. V.34 Modem only uses 4KHz, while CAP in ADSL mode uses the 30KHz-1Mhz frequency band. The higher the frequency, the smaller the waveform period, so the rate of the modulated signal (i.e., the data transmission rate) can be increased. "Carrierless" in CAP means that the part that generates the carrier (the circuit and the firmware module of the DSP) is not independent. It is integrated with the modulation/demodulation part to make the structure more refined.
3) DMT method: DMT uses 256 QAM modulation, which is usually used for ADSL equal rate asymmetric modem. In ADSL, one carrier modulation occupies 4KHz bandwidth, and QAM modulation of 256 carriers requires a Fourier transform. DMT allocates the modulated data amount to each 4KHz bandwidth, which can reduce the amount of data allocated to the noisy band, so the DMT method has strong noise resistance. DMT can be divided into frequency division multiplexing method (FDM) and echo cancellation method. FDM divides frequency bands according to up and downlink channels. The up and downlink frequency bands of the echo cancellation method overlap, which is more efficient than FDM, but the processing is complicated.
5. Application scope of xDSL
1) Access line for dedicated network: Dedicated lines provide communication services with symmetrical upstream and downstream rates. Therefore, IDSL, SDSL and HDSL modem can be used. The terminal is connected to it through serial ports such as V.35 or X.21, and its bidirectional transmission rate is 128kbit/s-2Mbit/s.
2) Used as an access line for the Internet: In the Internet, the amount of downlink data browsing client/server services such as the Web is much larger, so downlink high-speed ADSL Modem can be used.
3) Used as the access line of the ATM.
6 Conclusion
With the continuous increase in Internet bandwidth, people are paying more and more attention to the "last mile" network transmission bottleneck problem from high-speed networks to remote office and home users. xDSL technology is generated to overcome the "last mile" transmission bottleneck of users. It uses existing telephone lines and uses special modulation technology to greatly improve the transmission rate. Compared with other access methods, xDSL access has maintained a high performance-price ratio. Therefore, xDSL technology is being adopted by more and more users.
Article entry: csh Editor in charge: csh