ADSL technology Overview

ADSL Technology Introduction: ADSL is a technology that provides broadband data transmission services for homes and offices through existing common telephone lines. ADSL is an asymmetric digital signal transmission. It can provide a high-speed downlink rate of up to 10 Mbit/s on an existing copper twisted pair wire, that is, a common telephone line, much higher than the ISDN speed; the uplink speed is 1 Mbit/s, and the transmission distance is 3 km-5 km. The main feature of ADSL technology is to make full use of the existing copper network telephone line network). The installation of ADSL equipment at both ends of the line can provide users with high bandwidth services. Another advantage of ADSL is that it can coexist with a common phone on a telephone line, while receiving and dialing a common phone at the same time for ADSL transmission without affecting each other. Users can access the broadband multimedia information network and the Internet through ADSL, watch video programs, hold a video conference, and download data files at a high rate, you can also use the phone number on the same phone line without affecting other activities mentioned above.

The installation of ADSL is extremely convenient and quick. If you install ADSL on an existing telephone line, you do not need to modify the existing line unless you have installed an ADSL communication terminal on the client. The Cable MODEM technology and ADSL technology have considerable advantages. The Cable MODEM Solution uses a hierarchical tree structure, but only a few nodes are simply connected together on the tree node. Therefore, it is actually a rough bus-type network, this means that users need to share limited bandwidth with their neighbors.

Furthermore, the HPC solution must take into account the existing limited TV programs, while occupying most of the bandwidth, only a small portion of the remaining can be used to transmit other data. Therefore, the theoretical transmission rate of Cable MODEM can only reach half a half. In general, even in ideal conditions, a cable TV network is equivalent to a 10 Mbps shared-bus Ethernet. The ADSL solution is more advanced in terms of network topology, because each user has a separate line connected to the local terminal of the telephone switch. Its structure can be viewed as a star structure, in addition, its data transmission rate is exclusive to every user. In addition, it is costly and complex to re-wiring the service. However, ADSL does not need to re-wiring, reducing the cost and reducing the cost of accessing the Internet.

With ADSL, users can enjoy their resources with ease, free of expensive telephone and Internet fees. The user access network is an important part of the communication network, the window of the communication network, and the last mile of the information highway ). In order to achieve digital and broadband access for users, using optical fiber as the user line is an inevitable direction for the development of the user network. However, due to the high cost of the optical fiber user network, in the next decade to decades, most user networks will continue to use existing copper wires. In recent years, people have proposed a number of transitional broadband access network technologies, including ADSL Asymmetric Digital user loops) the two most competitive types are fiber-optic coaxial hybrid network and HFCs.

The ADSL technology can transmit signals using existing local copper wires. the maximum rate is 9 Mbps for the downlink signal from the terminal to the user, and 1 Mbps for the uplink signal from the user to the terminal. The number of users of the existing municipal copper wire network is very large, and ADSL can make full use of the existing municipal copper wire.

1. Technical Performance Analysis

Existing user loops are mainly composed of UTP unshielded twisted pair wires. The signal attenuation of UTP is mainly related to the transmission distance and the signal frequency. If the signal transmission exceeds a certain distance, the signal transmission quality will be difficult to guarantee. In addition, the bridging head on the line will also increase the signal attenuation. Therefore, line attenuation is the main factor affecting ADSL performance. Through asymmetric transmission, ADSL uses the frequency division multiplexing technology or ECHO offset technology) to separate upstream and downstream channels to reduce the impact of crosstalk, so as to achieve high-speed signal transmission.

Attenuation and Crosstalk are two standard damages that determine ADSL performance. The higher the transmission rate, the greater the impact on the signal. Therefore, the effective transmission distance of ADSL decreases with the increase of the transmission rate.

If the ADSL Access Network line is 5.5 km long, it can cover more than 80% existing telephone users. If the line length is 3.7 km, it can cover more than 50% existing users, scattered users outside the residential area can access the network through fiber-based hub nodes.

The crosstalk noise is usually stable, so it is easier to study and overcome it. However, the impact noise is random in terms of frequency, frequency, phase, and so on, and it is difficult to model and study for the phase.

2. Modulation Technology

Currently, three widely used ADSL modulation technologies are available: qadature ampli-tude modulation, CAP (carrierless ampl1_- phase modulation), and DMT (discrete multitone ), among them, DMT modulation technology is adopted by the ANSI standardization team T1E1.4 national standards. However, due to the short release time of this standard, there are still a considerable number of ADSL products using either the QAM or CAP modulation technology. The sent data is divided into two channels in the bit/Symbol encoder (each with an original rate of 1/2), multiplied by a pair of orthogonal modulation components, summed and output. Compared with other modulation technologies, QAM encoding has the advantages of making full use of bandwidth and strong anti-noise ability.

The main problem of using QAM for ADSL is how to adapt to the large performance differences between different telephone lines. To achieve ideal performance, the QAM receiver needs an input signal with the same spectrum and phase characteristics as the sending end for decoding, the QAM receiver uses an adaptive balancer to compensate for the distortion produced by the signal during transmission. Therefore, the complexity of the ADSL system using the QAM mainly comes from its adaptive balancer.

CAP modulation technology is developed based on the QAM modulation technology. It can be said that it is a variant of the QAM technology. The input data is sent to the encoder, m-bit input bits are mapped to k = 2 m different plural symbols An = an + jbn consists of K different plural symbols to form k-CAP line encoding. After encoding, an and bn are respectively sent to the same phase and orthogonal digital shaping filter, and then to the D/A converter after summation, and finally sent through the low-pass filter signal.

The main technical difficulty of CAP Technology for ADSL is to overcome the signal interference caused by near-end crosstalk. Generally, this problem can be solved by using the near-end crosstalk cancellation or near-end crosstalk balancer.

The main principle of DMT modulation technology is to divide the frequency band (0-1.104 MHZ) into 256 orthogonal subchannels with frequency instructions, each sub-channel occupies 4 kHz bandwidth ), after Bit Allocation and caching, the input data is divided into bit blocks. After TCM encoding, the 512-point Discrete Fourier inversion (IDFT) is used to convert the signal to the time domain, in this case, the bit block is converted to 256 QAM subcharacters. then, a circular prefix is added to each bit block to eliminate inter-code interference. The signal is sent to the channel through the data mode conversion (DA) and transmission filter. The Receiver performs receiving Decoding in reverse order.