ADSL (Asymmet-ricaIDigitaISubscriberLoop: Asymmetric Digital Subscriber Line Loop), born in 1989 at Bell Labs, is a member of the xDSL family and is known as "modern Express trains on the information superhighway". It is deeply loved by customers because of its high downlink rate, frequency bandwidth, and excellent performance. It has become another new, faster and more efficient access method after MODEM and ISDN. It is a new high-speed broadband technology running on the original ordinary telephone line.

In view of the uplink and downlink transmission speed of ADSL, in the high-speed data communication and interactive video functions of ADSL, the data communication function can be Internet/Intranet access, SOHO (SmaIIOfficeHomeOffice: home office), remote education or Dedicated network applications and other applications; it can also be interactive video including VOD (VideoOnDemand: video on demand), movies, games and other applications that require high-speed network video communication.

Basic Principles

The so-called asymmetry is mainly reflected in the asymmetry between the uplink rate and the downlink rate. It uses digital coding technology to obtain the maximum data transmission capacity from the existing copper telephone line without interfering with conventional voice services on the same line. The reason is that it uses frequencies other than telephone voice transmission to transmit data. Users can make calls or send faxes while surfing the Internet, and this will not affect the call quality or reduce the speed of downloading Internet content.

In fact, in the transmission technology of ADSL, ADSL uses its unique modem hardware to connect each end of the existing twisted pair connection. It creates a channel with three channels, as shown in Figure 1. Show:

It has a high-speed downstream channel (Downstream) to the user end, an upstream channel (Upstream) and a POTS channel (4kHz), the POTS channel is used to ensure that even if the ADSL connection fails, the voice communication is still Can operate normally. Both high-speed and medium-speed channels can use multiplexing technology to create multiple low-speed channels. The key concept of ADSL is also the key to the simultaneous transmission of digital and analog signals on the telephone line, because the upstream and downstream bands are asymmetrical. That is, the transmission bandwidth from the ISP to the client (downlink channel) is relatively high, and the transmission bandwidth from the client to the ISP (uplink channel) is relatively low. On the one hand, this design is to be compatible with the existing telephone network spectrum, and on the other hand, it is also in line with the habits and characteristics of using the Internet (the amount of data received is much larger than the amount of data sent).

Main features

(1) High-speed transmission

Provide uplink and downlink asymmetric transmission bandwidth;

(2) Internet, Do not interfere with each other when making a phone call

The data signal and the phone audio signal are modulated in their respective frequency bands by the principle of frequency division multiplexing without interfering with each other. You can make or receive calls while surfing the Internet, avoiding the inability to use the phone when dialing the Internet. Trouble;

(3) Exclusive bandwidth, safe and reliable

Each node uses broadband switches to process and exchange information, and information is transmitted quickly and safely.

Main classification

There are two mature ADSL standards-G.DMT and G.Lite. G.DMT is a full-rate ADSL standard that supports high-speed downlink/uplink rates of 8Mbps/1.5Mbps. However, G.DMT requires the user to install a POTS splitter, which is more complicated and expensive; G.DMT The Lite standard rate is low, and the downlink/uplink rate is 1.5Mbps/512Kbps, but the complicated POTS splitter is omitted, the cost is lower and the installation is easy. In terms of applicable fields, G.DMT is more suitable for small homes or offices (SOHO), while G.Lite is more suitable for ordinary home users.

Technical Standards

Since 1989, Bell Labs has successively proposed a series of xDSL technologies, including from the earliest SDSL (Symmetric Digital Subscriber Line) to the later VDSL (Super High-speed digital subscriber line) and ADS (asymmetric digital subscriber line). ADSL appeared earlier in various xDSL technologies, and the American National Standards Committee ANSI formulated a corresponding standard (ANSITIE1.413) for it in 1995. This standard was also adopted by the International Telecommunication Union ITU, and therefore has been widely used.

Transmission standard

Due to the influence of high-frequency signal transmission, ADSL requires that the distance between the access equipment of the telecommunications service provider and the user terminal cannot exceed 5 kilometers, that is The distance between the user's telephone line and the telephone exchange cannot exceed 5 kilometers.

ADSL equipment needs to follow one of the following standards in transmission:

ITU-TG.992.1 (G.dmt)

  1. dmt : Full rate, downlink 8Mbps, uplink 1Mbps

ITU-TG.992.2 (G.lite)

  1. lite: downlink 1.5Mbps, uplink 512Kbps

ITU-TG.994.1 (G.hs)

Variable bit rate (VBR)


Downlink 8Mbps, uplink 896Kbps

There are some faster and updated standards, but few telecom service providers use them:

< p>ITUG.992.3/4

ADSL2 downstream 12Mbps, upstream 1.0Mbps


AnnexJADSL2 downstream 12Mbps, upstream 3.5Mbps


ADSL2+ downlink 24Mbps, uplink 1.0Mbps


AnnexMADSL2+ downlink 24Mbps, uplink 3.5Mbps

< p>ADSL is an asymmetric DSL technology. The so-called asymmetric means that the uplink rate of the subscriber line is different from the downlink rate. The uplink rate is low and the downlink rate is high. It is especially suitable for the transmission of multimedia information services, such as video on demand (VOD) and multimedia. Information retrieval and other interactive services.

Taking the ITU-TG.992.1 standard as an example, ADSL supports an uplink rate of 512Kbps~1Mbps and a downlink rate of 1Mbps~8Mbps on a pair of copper wires, and the effective transmission distance is within the range of 3~5 kilometers. When the distance between the telecommunications service provider's equipment terminal and the user terminal is less than 1.3 kilometers, a higher rate VDSL can also be used, and its rate can reach 55.2 Mbps in the downlink and 19.2 Mbps in the uplink.

Login Standard

ADSL usually provides three network login methods:

Bridge; PPPoA (PPPoverATM, ATM-based end-to-end protocol); PPPoE (PPPoverEthernet , Ethernet-based end-to-end protocol). Bridging is to directly provide static IP, while the latter two usually do not provide static IP, but to dynamically assign network addresses to users.

Access methods

Dedicated line access and virtual dialing

Technology comparison

xDSL technology is the general term for all types of digital subscriber lines , Including RADSL, SDSL, HDSL, ADSL, VDSL and IDSL, etc.

xDSL is a transmission technology based on copper telephone lines, and it is also a new transmission technology. It can use higher frequency and corresponding modulation technology on the existing copper telephone lines to improve transmission rate. The differences between various DSL technologies are mainly reflected in the signal transmission rate, distance, and the symmetry of the uplink and downlink channels.

DSL (Digital Subscriber Line) is a combination of transmission technologies that uses copper telephone lines as the transmission medium. It includes HDSL, SDSL, VDSL, ADSL, RADSL, etc., generally called xDSL. The main difference between them is the difference in signal transmission speed and distance, and the difference in symmetry between upstream and downstream rates.


ADSL technology is the most commonly used xDSL technology. It belongs to asymmetric digital subscriber lines, and the transmission rate in the two directions is completely different. It uses ordinary telephone lines as the transmission medium, using high frequency bands after 26Kttz, and the transmission speed is relatively high. It can make full use of the existing local telephone network, effectively reducing installation and maintenance costs, but this technology is only suitable for applications with a downlink transmission rate of 1-2Mbits. Telephone lines have a high coverage rate in people's lives, and most user networks will continue to use the existing copper loops for a long time in the future, and ADSL will occupy a certain market in the next few decades.

The international standard for ADSL is mainly formulated by ANSI. In 1994, the TIE1.4 working group passed the first ADSL draft standard and decided to use DMT as the standard interface. The key is to support 6.144Mbit/s or even Higher speed and can transmit longer distances. The ANSI standard will include an appendix specifying the European standard ADSL standard. Therefore, the ADSL standard established by ANSI is actually a quasi-international standard. The CAP code is also striving to become a de facto standard.

In the middle of 1997, some ADSL manufacturers and operators began to realize that perhaps sacrificing some of ADSL's speed may accelerate the commercialization of ADSL, because the decline in speed also means the complexity of the technology reduce. The downstream speed of full-rate ADSL is 8Mbps, but a splitter (Splitter) must be installed at the user end. If the downstream rate of ADSL is reduced to 1.5 Mbps (the lower line is 1.5 Mbps, the upper line is 384 Kbps), then the splitter on the user side can be cancelled. This means that users can install ADSL Modem just like ordinary analog Modem in the past, there is no difference, and the on-site service of the service provider is omitted, which is very important to the promotion of ADSL.

So, a new version of ADSL was born, called Universal ADSL (UniversalADSL). In January 1998, some well-known manufacturers, operators and service providers in the world organized and established the Universal ADSL Working Group (UAWG) to commit to the standardization of this version.

In October 1998, the ITU began to discuss the general ADSL standard and named it G.Lite. After more than half a year of waiting, on June 22, 1999, the ITU (International Telecommunication Union) finally The G.Lite (G.992.2) standard was approved, which cleared the way for the commercialization of ADSL.


HDSL is also known as high-speed digital subscriber line and is the most mature type of xDSL technology. It uses more advanced digital signal adaptive equalization and echo cancellation technology, which requires less bandwidth and does not require a repeater. HDSL technology is symmetrical, that is, upstream communication and downstream communication provide the same bandwidth, which can replace the Tl/E1 of small and medium-sized local area networks, and is cheap and easy to install.

HDSL and SDSL support symmetrical T1/E1 (1.544Mbps/2.048Mbps) transmission. Among them, the effective transmission distance of HDSL is 3-4 kilometers, and two to four pairs of copper twisted-pair telephone lines are required; the maximum effective transmission distance of SDSL is 3 kilometers, and only one pair of copper wires is required. In comparison, symmetrical DSL is more suitable for enterprise point-to-point connection applications, such as file transmission, video conferencing, and other tasks that roughly correspond to the amount of data sent and received. Compared with asymmetric DSL, the market for symmetric DSL is much smaller.


VDSL is known as a high-speed digital subscriber line. It is an asymmetric data transmission technology with a very high speed. It can provide high-speed network connections and Convergence business. There are two main ways to apply VDSL technology. One method is to directly insert VDSL equipment into the copper cable network without changing the original network structure. Another method is to replace the last part of the original copper cable line with an optical cable. The cable part is determined. VDSL is more economical and feasible, and it is an important supplementary method in network reform.

VDSL, ADSL and RADSL belong to asymmetric transmission. Among them, VDSL technology is the fastest one of xDSL technologies. On a pair of copper twisted-pair telephone lines, the downstream data rate is 13 to 52 Mbps, and the upstream data rate is 1.5 to 2.3 Mbps. However, the transmission distance of VDSL is only Within a few hundred meters, VDSL can become a cost-effective alternative to fiber to the home. Shenzhen’s VOD is implemented using this access technology; ADSL supports an uplink rate of 640Kbps to 1Mbps on a pair of copper wires, and a downlink rate of 1Mbps to 8Mbps, the effective transmission distance is within 3-5 kilometers.


The speed range that RADSL can provide is basically the same as ADSL, but it can dynamically adjust the user's access speed according to the quality of the twisted-pair copper wire and the distance of 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 local area network (LAN) access, because in these applications the information downloaded by users is often more than the information uploaded (send instructions) Much more.

New development

While the ADSL market is booming, due to its own technology constraints, it is still a transitional technology. Problems such as speed, distance and line rate are To a large extent troubled the development of ADSL. In order to solve these problems of ADSL, it is also constantly improving. The ADSL2 technology is based on the first generation of ADSL, and with the support of related operators and equipment manufacturers, the new generation of ADSL standards (G.992.3/G.992.4) defined by the ITU. Compared with ADSL, ADSL2 Some new functions have been added, mainly to improve transmission performance, network interoperability and management functions. At the same time, the support for new services and new applications has also been greatly improved. And ADSL2+ (G.992.5) is further developed on the basis of ADSL2. ADSL2+ has all the characteristics of ADSL2, and on this basis, it further expands the available frequency band and the uplink and downlink transmission rate, and at the same time To be compatible with ADSL2.

Protocol type

Protocol description

Release time








2G.992.3G.dmt .bis













⑴Improved speed and expanded coverage

ADSL2 has better performance than the first-generation ADSL in terms of speed and coverage. The maximum downlink rate of ADSL2 can reach 12Mbit/s, and the maximum uplink rate can reach 1Mbit/s. ADSL2 is achieved by reducing frame overhead and improving the performance of the initialization state machine, using more effective modulation methods, higher coding gains, and enhanced signal processing algorithms.

Compared with the first-generation ADSL, on long-distance telephone lines, ADSL2 will provide 50kbit/s more rate increments on the upstream and downstream lines than the first-generation ADSL. Under the condition of the same speed, ADSL2 increases the transmission distance by about 180m, which is equivalent to an increase of the coverage area by 6%.

⑵Line diagnosis technology

For ADSL services, how to achieve rapid fault location is a huge challenge. In order to solve this problem, the ADSL2+ transmitter has enhanced diagnostic tools. These tools provide the means to solve the problem in the installation phase, the monitoring means in the service phase, and the update and upgrade of the tools.

In order to be able to diagnose and locate faults, the ADSL2 transmitter provides means to measure line noise, loop attenuation and SNR (signal-to-noise ratio) at both ends of the line. These measurement methods can be measured by a special Diagnose the test module to complete the data collection. This kind of test can also be done in the case of poor line quality (even when ADSL cannot complete the connection). In addition, ADSL2 provides real-time performance monitoring, which can detect the quality and noise conditions at both ends of the line. Operators can use the information processed by software to diagnose the quality of the ADSL2 connection and prevent further service failures. It can also be used Determine whether a higher rate service can be provided to users.

3 Enhanced power management technology

The first-generation ADSL transmitter is also in full-energy working mode when there is no data transmission. If ADSLModem can work in standby/sleep state, then for millions of modems, considerable power can be saved. In order to achieve the above objectives, ADSL2 proposes two power management modes, low-energy mode L2 and low-energy mode L3. In this way, while maintaining ADSL "always online", it can reduce the total energy consumption of the device.

The low-energy mode L2 enables the ATU-C end of the central office modem to quickly enter and exit the low-energy mode according to the ADSL traffic on the Internet. When downloading a large number of files, ADSL2 works in the all-powerful mode to ensure the fastest download speed; when the data traffic drops, the ADSL2 system enters the L2 low-energy mode, at this time the data transmission rate is greatly reduced, and the total energy consumption is reduced. When the system is in L2 mode, if the user starts to increase data traffic, the system can immediately enter L0 mode to achieve the maximum download rate. The completion of the entry and exit of the L2 state does not affect the service, will not cause the interruption of the service, or even a one-bit error.

The low-energy mode L3 is a sleep mode. When the user is offline and there is no traffic on the ADSL line, it enters this mode. When the user is back online, the ADSL transceiver needs about 3 seconds to reinitialize and then enter a stable communication mode. In this way, the L3 mode saves the total power at both ends of the transceiver.

In short, according to the actual data flow of the line connection, the transmission power can be flexibly switched between L0, L2, and L3, and the switching time can be completed within 3s to ensure that the business is not affected.

⑷Rate adaptive technology

Crosstalk between telephone lines will seriously affect the data rate of ADSL, and the change of crosstalk level will cause ADSL to drop. AM radio interference, temperature changes, humidity and other factors can also cause ADSL to drop. ADSL2 solves these problems by adopting SRA (SeamlessRateAdaptation) technology, so that the ADSL2 system can change the connection rate without any service interruption and bit errors during operation. ADSL2 changes the data rate of the connection by detecting changes in channel conditions to meet the new channel conditions, dynamically adjusting the rate according to the line quality, using 32kbps as the unit, and changing the rate to apply the actual transmission quality of the line to reduce data packet drop and improve the phenomenon of lag , This change is transparent to the user.

⑸Multi-pair bundling technology

Operators usually need to provide different service levels for different users. By bundling multiple telephone lines together, the user's access rate can be increased. In order to achieve the purpose of bundling, ADSL2 supports the IMA standard of the ATM Forum. Two or more telephone lines can be bundled into one ADSL link through the IMA and ADSL2 chipset, so that the downlink data rate of the line has greater flexibility sex.

⑹Channelization technology

ADSL2 can divide bandwidth into channels with different link characteristics to provide services for different applications. This capability enables it to support CVoDSL (ChannelizedVoiceoverDSL) and transparently transmit TDM voice within the DSL link. CVoDSL technology reserves a 64kbit/s channel for TDM transmission from DSLmodem to remote office or central office, and the central office access equipment directly sends the voice 64kbit/s signal to the circuit-switched network through PCM.

⑺Other advantages

Improved interoperability: simplified initialization of the state machine, when connected to ADSL transceivers provided by different chip vendors, interoperability and improved performance .

Quick start: ADSL2 provides a quick start mode, the initialization time is reduced from 10s of ADSL to 3s.

Full digital mode: ADSL2 provides an optional mode, which enables ADSL2 to use the voice frequency band for data transmission, which can increase the data rate of 256kbit/s.

Support packet-based services: ADSL2 provides a packet transmission mode transmission convergence layer, which can be used to transmit packet-based services.


ADSL2 utilizes existing telephone copper cable resources to enable voice services (POTS, ISDN) and at the same time provide broadband data services using high frequency bands. The application reference model of ADSL2 is shown as in Fig. 1. Among them, ATU-C and ATU-R are the ADSL2 transceiver units of the central office and the user side respectively, and the voice and data services are separated by a splitter. According to the different services provided, ADSL2 includes the following four specific application forms:

⑴Data, which only provides data services.

⑵Data+POTS, that is, provide data and ordinary telephone services at the same time.

⑶Data+ISDN, that is, provide data and ISDN services at the same time.

⑷VoiceoverData, which provides voice services (VoADSL) through the data channel. At this time, the voice gateway function is required to complete the conversion from voice to packet data.



In addition to the technical features of ADSL2, ADSL2+ also has an important feature that extends the downstream frequency band of ADSL2, thereby increasing the short Downlink rate on the line within the distance. The two ADSL2 standards each specify 1.1MHz and 552kHz downstream frequency bands, while ADSL2+ specifies a 2.2MHz downstream frequency band. This makes the downlink rate of ADSL2+ in short distances (within 1.5km) greatly improved, which can reach more than 20Mbit/s. The upstream rate of ADSL2+ is about 1Mbit/s, which depends on the condition of the line.

Using ADSL2+ can effectively reduce crosstalk interference. When ADSL2+ is mixed with ADSL, in order to avoid crosstalk interference between pairs, the downstream working frequency can be set between 1.1~2.2MHz to avoid interference with the 1.1MHz downstream frequency band of ADSL, so as to reduce crosstalk and improve service The purpose of quality.


Whether it is in terms of coverage distance, line rate, downstream bandwidth, power management, fault detection, etc., ADSL2 and ADSL2+ have greatly improved compared with ADSL technology. , Has many new features and functions. These new features and functions will further improve the performance and interoperability of the network, so that operators can implement new technology application deployment by upgrading existing equipment, rather than obsolete existing equipment, while better supporting new applications and service. Therefore, where conditions permit, ADSL/ADSL2+ technology can be gradually applied, and existing ADSL equipment can be upgraded to enable it to have ADSL/ADSL2+ capabilities. For example: ADSL2/ADSL2+ can be used to cover users in some areas where the subscriber line is far away; and in areas where bandwidth requirements are higher than the ability to provide ADSL, ADSL2+ can also be deployed; for areas with low outgoing lines, ADSL2+ can also be deployed. Deploy as a solution to reduce the interference between the wiring harnesses and increase the line-out rate.

The new generation of ADSL technology is good, but after all, the standard launch time is not long, and the chips and equipment are immature, so it is not suitable for large-scale application. In addition, because ADSL2 and ADSL2+ have made major changes to the first-generation ADSL technology, especially the frame structure, operators should pay close attention to the interoperability issues between these technologies before large-scale deployment. As the development direction of ADSL, ADSL2/ADSL2+ should be used as a useful supplement to ADSL technology. First, focus on research, fully conduct network tests, and then gradually deploy to the network to lay a solid foundation for the upgrade of ADSL networks.

Application prospects

Because the ADSL2/2+ standard has not been launched for a long time, the development of the chip has not yet fully matured, and the current equipment cannot support all the functions defined by the standard. But based on ADsL's good market performance and broad market prospects, chip developers and equipment suppliers are already accelerating the commercialization of ADSL2/2+. Operators in my country have also carried out trials and commercial trials of ADSL2/2+ in a certain range. The development trend of ADSL is ADSL2/2+. However, the large-scale application of ADSL2/2+ requires comprehensive consideration based on many factors such as technology maturity, interoperability, and cost.

In the future, ADSL will coexist with ADSL2/2+. In the short term, ADSL2/2+ can cut into the broadband access market through the long-distance transmission capabilities it provides, so as to solve the current technical problem that ADSL cannot achieve long-distance access. In view of the superiority of ADSL2/2+ technology itself and the increasing demand for higher speed and longer-distance broadband access by users, ADSL2/2+ is expected to replace the first generation of ADSL as the mainstream broadband access technology in the future, and ADSL2 /2+ technology and equipment itself will also be improved day by day in application.

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