ADSL technology is a technology to realize broadband access to the Internet through asymmetric digital subscriber lines. ADSL, as a transport layer technology, makes full use of existing copper cable resources and provides bandwidth of 640kbps upstream and 8Mbps downstream on a twisted pair, thus overcoming the "bottleneck" of traditional users in the "last mile" and realizing real broadband access.
The principle of ADSL:
The traditional telephone system uses the low frequency part of copper wire (frequency band below 4kHz). ADSL adopts DMT (Discrete Multi-tone) technology, which divides the frequency band from 0Hz to 1. 1MHz of the original telephone line into 256 sub-bands with a bandwidth of 4.3kHz, of which the frequency band below 4kHz is still used to transmit POTS (traditional telephone service). The frequency band from 20kHz to 138kHz is used to transmit uplink signals, and the technology from 138kHz to 65438DMT can adjust the number of bits modulated on each channel according to the line conditions, thus making full use of the line. Generally speaking, the greater the signal-to-noise ratio of a subchannel, the more bits are modulated on that channel. If the signal-to-noise ratio of the subchannel is poor, it is discarded. At present, ADSL can reach the data transmission rate of 640kbps upstream and 8Mbps downstream.
As can be seen from the above, the original telephone signal still uses the original frequency band, while ADSL-based services use frequency bands other than voice. Therefore, the original telephone service is not affected.
Basic situation of ADSL:
With the rapid and healthy development of the Internet and the application of e-commerce, the Internet is fast, efficient and convenient, which has penetrated into thousands of households and become an indispensable part of people's lives. The application of the Internet has also developed rapidly. Users can access the Internet in many ways. Digitalization, broadband and FTTH are the inevitable development directions of future access methods. At present, due to the high cost of FTTH, most user networks will continue to use existing transitional broadband access technologies in the next few years, including N-ISDN, Cable Modem and ADSL. ADSL (Asymmetric Digital Subscriber Loop) is the most promising and competitive one in the next decade or even years.
Advantages of ADSL:
DSL (Digital Subscriber Line) is a combination of transmission technologies using copper telephone lines as transmission media, including HDSL, SDSL, VDSL, ADSL and RADSL, commonly known as xDSL. Their main differences are reflected in the difference of signal transmission speed and distance and the difference of symmetry between uplink rate and downlink rate.
HDSL and SDSL support symmetric t1/e1(1.544mbps/2.048mbps) transmission. Among them, the effective transmission distance of HDSL is 3-4 kilometers, and two sets are needed.
To four pairs of twisted-pair copper telephone lines; The maximum effective transmission distance of SDSL is 3 kilometers, and only one pair of copper wires is needed. Comparatively speaking, symmetrical DSL is more suitable for enterprise point-to-point connection applications, such as file transfer, video conferencing and other jobs with roughly the same amount of data sent and received. Compared with asymmetric DSL, the market of symmetric DSL is much smaller.
VDSL, ADSL and RADSL belong to asymmetric transmission. Among them, VDSL technology is the fastest among xDSL technologies. On a pair of twisted-pair copper telephone lines, the uplink data rate is 13 to 52Mbps, and the downlink data rate is 1.5 to 2.3 Mbps. However, the transmission distance of VDSL is only a few hundred meters, and VDSL can be used as an economical and efficient alternative to FTTH. At present, VOD (Video on Demand) in Shenzhen is realized through this access technology. ADSL supports the uplink rate of 640Kbps to 1Mbps and the downlink rate of 1Mbps to 8Mbps on a pair of copper wires, and the effective transmission distance is within the range of 3-5 kilometers. RADSL can provide the same speed range as ADSL, but it can dynamically adjust the access speed of users according to the quality of twisted-pair copper wire and the transmission distance. It is these characteristics of RADSL that make RADSL an ideal technology for high-speed internet surfing, video on demand (IAV) and remote L an access, because in these applications, users often download more information than upload (send instructions).
Comparison between ADSL and Cable Modem;
Compared with wired mode, ADSL technology has considerable advantages. Cable Modem's HFC access scheme adopts hierarchical tree structure, which has the advantage of high bandwidth (10M), but this technology itself is a rough bus network, which means that users have to share limited bandwidth with neighboring users. When the number of users on a line increases sharply, its speed will slow down. Moreover, relevant data show that HFC scheme must take into account the existing cable TV programs in most cases, occupying a part of bandwidth and leaving only a part for transmitting other data signals, so the theoretical transmission rate of cable modems can only reach half. Experiments of foreign companies show that its rate is reduced to 1M-2Mbps, and the common rate is 400K-500Kbps. On the whole, even in an ideal state, HFC is only equivalent to a shared bus Ethernet of 10Mbps * *, while ADSL access scheme is more advanced in network topology, because each user has a separate line connected to the ADSL local terminal, and its structure can be regarded as a star structure, and its data transmission bandwidth is exclusive to each user.
Comparison of ADSL with ordinary dial-up modem and N-ISDN;
A) Compared with the highest speed of ordinary dial-up modems of 56K and N-ISDN 128K, the speed advantage of ADSL is self-evident.
B) Compared with ordinary dial-up modems or ISDN, ADSL is more attractive, because it transmits data and voice signals on the same copper wire, and the data signals do not pass through telephone exchange equipment, thus reducing the load of telephone exchanges and eliminating the need for dialing. Always online belongs to private line access mode. This means that you don't need to pay extra telephone charges when using ADSL to surf the Internet.