ADSL technology and application Fu Wave 01-6-6 15:33:58
With the explosive development of the Internet, commercial applications and multimedia services such as Internet are also rapidly promoted. If you want to enjoy a variety of services on the Internet, users must access the network in some way. In order to realize the digitization, broadband, improve user Internet speed, fiber-to-housing (fTTH) is an inevitable direction of the future development of the user network, but because the cost of fiber network is too high, most of the years in the next decades The user network will continue to use the existing copper lines, so in recent years, a number of transitional broadband access network technologies, including N-ISDN, Cable Modem, ADSL, etc., ADSL (asymmetric digital users) Loop) is one of the most prospects and competitiveness, which will dominate in the next ten years or even decades.
At present, the ADSL's boom sweeps around the world, and the PC industry leader Mic (Microsoft, Intel, Compaq), and the world's 3COM, Cisco, Alcatel, Paradyne, etc., and is committed to the development of ADSL; Many telecommunications companies around the world have also promoted their respective ADSL services. In 2001, the ADSL widespreadly moved to the public.
The main features of ADSL
DSL (Digital Subscriber Line) is a transfer technology combination with copper line lines for transmission media, including HDSL, SDSL, VDSL, ADSL, and RADSL, etc., generally referred to as XDSL. Their main difference is to reflect the difference between signal transmission speed and distance and the different aspectability of the upstream rate and downward rate.
1. Technical performance analysis
The existing user loop is mainly composed of UTP (unshielded twisted pair). UTP's attenuation of the signal is mainly related to the frequency of the transmission distance and the signal. If the signal transmission exceeds a certain distance, the transmission quality of the signal will be difficult to guarantee. In addition, the bridge tap on the line will also increase the attenuation of the signal. Therefore, the line attenuation is the main factor affecting the performance of ADSL. ADSL is subsequently transmitted by unsmmonting, using frequency division multiplexing techniques (or echo cancellation techniques) to reduce the effects of string sounds, thereby implementing high-speed transmission of signals.
Attenuation and toe are two standard damage to determine the performance of ADSL. The higher the transmission rate, the larger the impact on the signal, so the effective transmission distance of ADSL is shortened as the transmission rate increases. Although the effective transmission distance of VDSL is shorter than ADSL, the VDSL can be used for the interior office or home of the building in the building or home in VDSL fiber to the building (FTTB).
If the length of the ADSL access network is 5.5km, it can cover more than 80% of the existing telephone users; if the line length is 3.7km, it can cover more than 50% of the existing users, and the dispersion user other than the user cell can be based on Optical hub nodes are connected to the network.
The string noise is usually stable, so it is easier to study and overcome it; and the impact noise is random in terms of frequency, cycle, phase, etc., which is difficult to model and research.
2. Modulation technology
There are three types of ADSL modulation technologies that are widely used: QAM (Quadature Ampli-Tude Modulation), DMT (Discrete MultiTone), where DMT Modulation Technology is developed by ANSI standardization group T1E1.4 The standard is adopted. However, due to the shortest launch time of this standard, there is still a considerable number of ADSL products to use QAM or CAP modulation techniques.
● QAM modulation technology
The QAM modulator is divided into two channels in the bit / symbol encoder in the bit / symbol encoder (the rate is 1/2), respectively, and the output is multiplied by a pair of orthogonal modulation components. QAM encoding has the advantages of sufficient use of bandwidth and anti-noise ability than other modulation techniques. QAM The main problem for ADSL is how to accommodate the difference in performance between different telephone lines. To achieve more ideal work characteristics, the QAM receiver requires an input signal having the same spectrum and phase characteristics for the transmitting end for decoding. The QAM receiver utilizes an adaptive equalizer to compensate for the distortion generated during transmission, so using QAM The complexity of the ADSL system is mainly from its adaptive equalizer.
● CAP modulation technology
CAP modulation technology is developed based on QAM modulation technology. It can be said that it is a variant of QAM technology.
The main technical difficulty of CAP technology for ADSL is to overcome the interference of the proximal string to signal. This problem can generally be solved by using a proximal accent enchance or proximal string equalizer.
● DMT modulation technology
The main principle of DMT modulation techniques is to divide the frequency band (0 ~ 1.104MHz) into 256 orthogonal subchannels indicated by the frequency (4kHz bandwidth each sub-channel), the input signal passes through the bit allocation and cache, and the input data is divided into bits. Block, after the TCM encoding, 512-point discrete Fourier reverse transform (IDFT) converts the signal to the time domain, at this time, the bit block will be converted to 256 QAM sub-characters, and then the cyclic prefix is added to each bit. Eliminate code interference), the signal is sent to the channel via the data module transform (DA) and the transmit filter, and the signal is received in the reverse order in the reverse order.
Since the US ADSL National Standard (T1.413) is recommended to use DMT technology, in the next few years, there will be more and more ADSL modems adopt DMT technology.
Figure 1 ADSL Access Model Figure 2 Schematic diagram of personal user ADSL installation
Comparison of other access devices
1. Comparison with Cable Modem
ADSL technology has considerable advantages over Cable Mode. Cable Modem's HFC access solution uses a layered tree structure, its advantage is that the bandwidth is relatively high (10 Mbps), but this technology itself is a rough bus type network, which is limited to non-users to share limited users. Bandwidth, when the user is booted on a line, its speed will slow down. Furthermore, the relevant information indicates that in most cases, the HFC program must take into account existing cable TV programs, and take up some bandwidth, only a part remains for other data signals, so the theoretical transmission rate of Cable Modem can only reach One half. Foreign company experiments show that its rate is reduced to 1M ~ 2Mbps, and more common is 400k to 500kbps.
In whole, even in the ideal state, the HFC is only equivalent to a 10Mbps shared bus type Ethernet, and the ADSL access scheme is more advanced on the network topology, because each user has a separate line and ADSL Bureau. The end is connected, and its structure can be seen as a star structure, and its data transmission bandwidth is exclusive by each user.
2. Comparison with ordinary dial modem and n-isdn
● The speed of ordinary dial modem is up to 56kbps, and the rate of N-ISDN is 128kbps. In contrast, the rate advantage of ADSL is self-evident.
● The ADSL is more attractive as compared to ordinary dial modem or ISDN: it transmits data and speech signals on the same copper wire, and the data signal does not pass the telephone switch device, and the load of the telephone switch is not Need dialing, always online, belongs to the network. This means that the use of ADSL Internet access does not need to pay additional telephone bills.
ADSL access model
The ADSL's access model is mainly composed of a central switchboard module and a remote module.
The central switching bureaucratic module includes ADSL MODEM and access multiplexing systems in central position, and ADSL MODEM in a central location is called ATU-C (ADSL TRANSMISION UNIT-CENTRAL). Access Multiplexing System The center MODEM is usually combined into a known access node, also known as "DSL Access Multiplexer). The remote module consists of a user ADSL Modem and a filter, and the user-end ADSL MODEM is often referred to as ATU-R (ADSL TRANSMISION UNIT-Remote).
ADSL device installation
ADSL installation includes local line adjustment and user-end device installation. In terms of localization, the service provider is connected to the ADSL local device by the service provider, only 2 ~ 3 minutes; the user-end ADSL installation is also very simple and convenient, as long as the telephone line is connected to the filter, the filter and ADSL MODEM is connected to a two-core telephone line, and the ADSL MODEM is connected to the computer's network card to complete the hardware installation, and then set the IP, DNS and gateway parameters in the TCP / IP protocol. The installation work has been completed. The use of ADSL is more simple. Since ADSL does not need to dial, it is always online, and users can enjoy high-speed online surfing services, and they can call at the same time.
The ADSL installation of the local area network users is not very different from the stand-alone user, just add a hub, use the direct network cable to connect the hub and the ADSL MODEM, as shown in Figure 3.
ADSL application
Since ADSL is designed to be on demand for video programs in the initial stage of development, asymmetive and high-speed downlink channels, with the rapid development of the Internet, ADSL has more vital advantage as a high-speed access Internet technology, which is The existing Internet online provides multimedia services.
At present, ADSL mainly provides Internet high-speed broadband access, users can enjoy a variety of broadband multimedia services for free through ADSL access, access to the appropriate sites, such as online viewing, network TV, etc.
ADSL personal users have a fixed static IP address, and personal users can build personal homepages, so that other users on the viewing are their own style; ADSL LAN users have 4 fixed static IP addresses, apply for ADSL LAN form Companies can set up company's website on China's public multimedia network (view hearing), providing WWW, FTP, E-mail, etc .; ADSL services have sufficient bandwidth for local area network users sharing, users can pass the proxy server as the whole Company LAN users provide Internet services.
Since ADSL is based on ATM trunk network transmission, an ATM or IP-based VPN (virtual private network) service can also be provided.
With the further promotion of ADSL technology, ADSL access will also provide services such as telematics, remote teaching, and remote visual conferences.
ADSL foreground
Many experts in the industry have firmly believe that ADSL-based XDSL technology will eventually become the winner of copper twisted pair. The user currently uses ordinary dialing Modem and N-ISDN technology to gradually transition to broadband access methods such as ADSL, and Finally realize fiber access.
