Features and applications of emerging ADSL Access Technology

China's ADSL access technology has developed very rapidly. So I have studied the features and applications of the ADSL access technology. I would like to share with you here, hoping it will be useful to you. The maximum downlink rate of ADSL2 is 12 Mb/s due to improved frame structure and optimized RS encoding. Because ADSL2 + uses a wider spectrum, there are more than 512 sub-carriers). When the distance is short, the downlink speed can reach up to 24 Mb/s. If g.992.32.16j is used, the maximum uplink rate of ADSL2/2 + can be increased from 1.5 Mb/s to about 2.3 Mb/s. ADSL2/ADSL2 + supports longer access distance. The first-generation ADSL access technology has a transmission distance of less than 5 kilometers, while ADSL2/ADSL2 + supports applications with a distance of nearly 7km. The speed can reach kb/s downstream and kb/s upstream. Higher speed and longer transmission distance are achieved through the following measures.

Improves modulation efficiency. The first-generation ADSL access technology requires that the minimum data transmitted by each sub-channel is 2 bits. If the value is less than 2 bits, the sub-channel cannot be used to send data, you must know that the signal-to-noise ratio may be very low when the transmission distance is relatively long.) the data transmitted by most subchannels in the high-frequency segment may be less than 2bit, according to the requirements of the first-generation ADSL access technology, these subchannels become unavailable. To maintain a high rate at a long distance, ADSL2 requires that each sub-channel can support 1 bit data to use many sub-channels with low signal-to-noise ratio and fail to carry 2 BIT data. In addition, ADSL2/2 + requires a four-dimensional, 16-status code (S = 1/2), which improves the modulation efficiency and enables the maximum downlink rate to reach 12 Mb/s. In the first-generation ADSL technical standard, the support for S = 1/2 is only an option, although some chip manufacturers support S = 1/2 in the first-generation ADSL access technology, however, since non-standard things fail to pass the connectivity test, there is a problem with intercommunication, and more delay channels and bearer channels are supported. The new-generation ADSL access technology supports up to four bearer channels and four delay channels, while the first-generation ADSL supports up to three bearer channels and two delay channels ).

You can configure the frame structure latency and bit error rate (BER. The first generation of ADSL access technology has a fixed error code of 10. Because the BER is fixed, the line must maintain this BER indicator from start to end. Once the BER is greater than 10, the speed must be reduced to maintain the BER level, when the rate reduction is not enough to maintain this BER, it is difficult for both sides of the transceiver to train Training), and The ADSL service will be interrupted. The error rate of the next-generation ADSL access technology can be configured in 10, 10, and 10. In this way, when the BER requirements are not high, you can choose to configure a high BER value to achieve a high rate and a longer transmission distance, reducing the frame overhead. The first-generation ADSL Access Technology imposes a fixed overhead of 32 KB/s for any data rate. The frame overhead of the next-generation ADSL access technology can be programmed between 432kb/s, so that the overhead can be set to 4kb/s for low-speed data services, let more bits be used for the transmission of the Net Load.

Optimized RS gain. The ADSL2/2 + frame generator improves the flexibility and programmability of constructing the RS code. Therefore, at low data rates, ADSL2/2 + can obtain higher encoding gain from the RS code. Improved the initial state machine. ADSL2/2 + has greatly improved the initial state machine, improving the data rate of ADSL2/2 + and wider application scope. The first generation of ADSL mainly supports two business models: ADSLoverPOTS and ADSLoverISDN. In addition to the two modes, ADSL2/ADSL2 + also supports the full digital business mode, which uses the bandwidth band of the narrowband voice business to transmit upstream data services. The first-generation ADSL technology only supports the STM synchronous transmission mode) and the ATM asynchronous transmission mode), while ADSL2/2 + supports the STM and ATM transmission modes, the PTM group transmission mode is also supported. Therefore, ADSL2/2 + not only supports ATM services, but also supports groups such as Ethernet) services, which is more flexible in terms of service support.

More powerful line diagnosis capability. Fault Locating is a very difficult problem in the application of the first-generation ADSL technology. ADSL2/2 + solves this problem well. The transceiver of ADSL2/2 + greatly enhances the fault diagnosis capability. The embedded loop diagnosis function defined in the standard can measure the line conditions at both ends of the line in a timely manner without the need for maintenance personnel to use the test equipment. Therefore, ADSL2/ADSL2 + supports the dual-end test function. The test results help to locate the fault location and cause. As long as the local terminal and the user side can perform the initialization sequence exchange process between the Training Board device and the user side device), even if the SHOWTIME stage cannot be performed, it refers to the data transmission phase after initialization and training ), you can also obtain the following line parameters through a special line test process by using the management interface to issue test commands by the network administrator: transmission Characteristics, background noise power spectrum, signal-to-noise ratio of each sub-channel, noise tolerance of each sub-channel, number of capable bits, and connection rate of the link.

Rate Adaptation capability. Telephone lines are usually bundled with many pairs of cables. The bundled cables cause crosstalk and affect the performance of ADSL Services. For the first generation of ADSL technology, crosstalk, AM radio interference, temperature changes, and moisture in the wiring harness will lead to ADSL service disconnection. ADSL2/2 + uses real-time SRA seamless Rate Adaptation) technology to solve the problem of ADSL service disconnection, without interrupting the service or generating an error code, changes the data rate in real time based on the line quality to meet the requirements of the line's BER and noise capacity.

SRA is implemented based on the mechanism of separating the modulation layer from the frame layer in the ADSL2/2 + system, in this way, the data rate parameter of the modulation layer can be modified without changing any parameters of the frame layer. Because no parameters affect the frame layer, the user terminal keeps frame synchronization, the system will not be restarted or have error codes, saving energy. The first-generation ADSL Transceiver always works in the maximum power mode, even if there is no ADSL service on the line. The maximum power mode makes the ADSL device consume extremely much power, and the heat dissipation problem of the ADSL local device and ADSLModem is difficult to solve. Therefore, the energy-saving feature is particularly important for the wide application of ADSL. ADSL2/2 + adds L2 and L3 power management modes, which can reduce the overall power consumption of the line while ensuring normal user services, it effectively solves the problem of heat dissipation for local and client devices. The power management function of ADSL2/2 + is mainly reflected in the following aspects.

In the new L2 low-power mode, when the business traffic on the line is reduced by an hour, the transmission power is reduced to exceed the required noise tolerance, reduces useless power consumption while ensuring stable business operation. In L2 state, the power consumption is only about 30% of the maximum power consumption. The new L3 low power consumption mode reduces the transmission power without user data transmission, only the power consumption required for sending necessary management messages and synchronous signals is maintained, and can be quickly restored when the number of users passes data.

Both the CO and CPE of ADSL2/2 + have the powercutback function, ranging from 0 ~ 40 dB, combined with L2 low-power status, can effectively reduce the sending power during normal operation, while the first generation of ADSL only has the powercutback function, and the maximum range is only 12 dB. Multi-line rate bundling. ADSL2/2 + supports bundling multiple telephone lines to provide a higher data rate to meet bandwidth requirements of different users. ADSL2/2 + supports repeated use of IMAATM in the ATM Forum.) Two or more pairs of copper cables can be bundled on an ADSL link, this provides a higher and more flexible downstream data rate.

Stronger interconnection capability. Through the following measures, ADSL2/2 + improves the intercommunication capability of different chip vendors and provides better performance during intercommunication: by clarifying the initial state machine and adding new features; ADSL2/ADSL2 + the ADSL Transceiver according to the function is divided into TPS-TC transmission protocol-related aggregation sub-layer), PMS-TC physical media-related aggregation sub-layer), PMD physical media-related sub-layer) and MPS-TC management protocol-related aggregation sub-layer, used for network management interface), and defines the message between the sub-layer, which helps different manufacturers of devices to achieve interoperability. In fact, the first-generation ADSL standard also includes these features, but does not define a clear hierarchy like ADSL2/ADSL2 +. When the ADSL2/ADSL2 + technology starts to use, it is estimated that it will also face interoperability issues. However, due to the above reasons, it is estimated that the problems will be much less. Quick Start. The initialization time of the first generation of ADSL access technology is greater than 10 seconds, while the fast initialization process supported by ADSL2/2 + can make the initialization time less than 3 seconds.

In addition, ADSL2/2 + has more stable operation and good spectrum compatibility capabilities. This is mainly reflected in the carrier sorting and pilot position determined by the receiver based on the channel analysis results, which can reflect the channel situation more realistically and select the best sub-carrier as the pilot, it is conducive to stable connections of the ADSL Access Technology. During the training process, the sub-carrier can be disabled to allow the receiver to test the rfi rf interference) distribution, in addition, the transmit power of each sub-carrier can be controlled through the CO-MIB, which can avoid RFI and reduce interference to peripheral equipment. OLR is reconfigured online) function enhances dynamic adaptation to line parameter changes. Supports short initialization to quickly restore connection synchronization from errors.