What Is ADSL Internet?

Asymmetric Digital Subscriber Line (ADSL) is the most popular type of Digital Subscriber Line (xDSL) service. [1]

Asymmetric Digital Subscriber Line (ADSL) is the most popular type of Digital Subscriber Line (xDSL) service. [1]
The ADSL international standard was approved in 1999 and is called G.dmt. It allows up to 8 Mbps downlink speed and 1 Mbps uplink speed. This standard has been surpassed by ADSL2 released in 2002. The downlink speed has been continuously improved, and it can now reach 12 Mbps, and the uplink speed is still 1 Mbps. Now we have ADSL2 + again, which doubles the downlink speed to 24 Mbps using double bandwidth (2.2 MHz) on the twisted pair. [1]
Chinese name
Asymmetric digital subscriber line
Foreign name
Asymmetric Digital Subscriber Line
Using the circuit
Copper twisted pair
Provide multimedia services
Other technologies
Ethernet, fiber, wireless, etc.

adsl introduction

ADSL (Asymmet-ricaI DigitaI Subscriber Loop), which was born in Bell Labs in 1989, is a member of the xDSL family and is known as "the express train on the modern information highway". It is well received by customers because of its high downlink speed, high frequency bandwidth, and excellent performance. It has become a new, faster, and more efficient access method after MODEM and ISDN. It is a new high-speed broadband technology that runs on existing ordinary telephone lines. [2]
In view of the uplink and downlink transmission speed of ADSL, among the high-speed data communication and interactive video functions of ADSL, the data communication function can be Internet / Intranet access, SOHO (SmaII Office Home Office), distance education, or dedicated network applications It can also be used for interactive video, including VOD (Video On Demand), movies, games and other applications that require high-speed network video communication. [2]

adsl basic principles

The so-called asymmetry is mainly reflected in the asymmetry of the uplink rate and the downlink rate. It uses digital coding technology to obtain the maximum data transmission capacity from existing copper telephone lines without interfering with conventional voice services on the same line. The reason is that it uses a frequency other than telephone voice transmission to transmit data. Users can make phone calls or send faxes while surfing the Internet, which will not affect the call quality or slow down the download of Internet content. [2]
In fact, in ADSL transmission technology, 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:
Figure 2 ADSL channel structure
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 the voice communication can still operate normally even if the ADSL connection fails. Both high-speed and medium-speed channels can be multiplexed to create multiple low-speed channels. The key concept of ADSL is also the key that digital and analog signals can be transmitted on the telephone line at the same time, because the upstream and downstream bands are asymmetric. That is, the bandwidth transmitted from the ISP to the client (downlink channel) is relatively high, and the bandwidth from the client to the ISP (uplink channel) is relatively low. This design is on the one hand to be compatible with the existing telephone network spectrum, on the other hand it also conforms to the habits and characteristics of using the Internet (the amount of data received is much larger than the amount of data sent out). [2]

adsl main features

(1) High-speed transmission
Provide asymmetrical transmission bandwidth between uplink and downlink; [3]
(2) Internet and phone calls do not interfere with each other
The data signal and the telephone audio signal are modulated in the respective frequency bands by frequency division multiplexing. They do not interfere with each other. You can make or receive calls while surfing the Internet, avoiding the trouble of not being able to use the phone when dialing to the Internet;
(3) Exclusive bandwidth, safe and reliable
Each node uses a broadband switch to process and exchange information, and the information transfer is fast and secure.

adsl main categories

There are two more mature ADSL standards-G.DMT and G.Lite. G.DMT is full rate
Figure 3 Schematic diagram of G.DMT
ADSL standard, supporting 8Mbps / 1.5Mbps high-speed downlink / uplink rate, but G.DMT requires the user to install a POTS splitter,
More complicated and expensive; G.Lite standard rate is low, the downlink / uplink rate is 1.5Mbps / 512Kbps, but the complex POTS splitter is omitted, the cost is low, and it is easy to install. In terms of applicable areas, G.DMT is more suitable for small homes or offices (SOHO), while G.Lite is more suitable for ordinary home users.

adsl technical standards

Bell Labs has proposed a series of xDSL technologies since 1989, including from the earliest SDSL (symmetric digital subscriber line) to later VDSL (ultra high speed digital subscriber line) and ADS (asymmetric digital subscriber line). ADSL appeared earlier in various xDSL technologies, and the American National Standards Institute ANSI formulated the corresponding standard (ANSI TIE1.413) in 1995. This standard was also adopted by the International Telecommunication Union ITU, so it has been widely used. [4]

adsl transmission standard

Due to the influence of transmitting high-frequency signals, ADSL requires that the distance between the telecommunication service provider's access equipment and the user terminal cannot exceed 5 kilometers, that is, the distance between the user's telephone line and the telephone office cannot exceed 5 kilometers.
Figure 1 adsl architecture
ADSL equipment needs to follow one of the following standards in transmission:
ITU-T G.992.1 (G.dmt)
  1. dmt: full rate, 8Mbps downlink, 1Mbps uplink
ITU-T G.992.2 (G.lite)
  1. lite: Downlink 1.5Mbps, Uplink 512Kbps
ITU-T G.994.1 (G.hs)
Variable bit rate (VBR)
ANSI T1.413 Issue # 2
8Mbps downlink, 896Kbps uplink
There are also more quickly updated standards, but few telecommunications service providers use them:
ITU G.992.3 / 4
ADSL2 downlink 12Mbps, uplink 1.0Mbps
ITU G.992.3 / 4
Annex J ADSL2 downlink 12Mbps, uplink 3.5Mbps
ITU G.992.5
ADSL2 + downlink 24Mbps, uplink 1.0Mbps
ITU G.992.5
Annex M ADSL2 + downlink 24Mbps, uplink 3.5Mbps
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, and the uplink rate is low. And other interactive businesses.
Taking the ITU-T G.992.1 standard as an example, ADSL supports an uplink rate of 512 Kbps to 1 Mbps and a downlink rate of 1 Mbps to 8 Mbps on a pair of copper wires, and the effective transmission distance is within 3 to 5 kilometers. When the distance between the equipment end of the telecommunications service provider and the user terminal is less than 1.3 kilometers, a higher speed VDSL can also be used, and its speed can reach 55.2 Mbps downstream and 19.2 Mbps upstream.

adsl login standard

ADSL usually provides three network login methods:
Bridging; PPPoA (PPPoverATM, ATM-based end-to-end protocol); PPPoE (PPPoverEthernet, Ethernet-based end-to-end protocol). Bridging is to provide static IP directly, while the latter two types usually do not provide static IP, and dynamically assign network addresses to users.

adsl access method

Private line access and virtual dialing

adsl technology comparison

xDSL technology is a general term for all types of digital subscriber lines, including RADSL, SDSL, HDSL, ADSL, VDSL and IDSL. [5]
xDSL is a transmission technology based on copper telephone lines. It is also a new transmission technology. It can use higher frequencies and corresponding modulation technologies on existing copper telephone lines to increase the transmission rate. The differences between various DSL technologies are mainly reflected in the two aspects of signal transmission rate, distance, and different symmetry of the uplink and downlink channels. [5]
DSL (Digital Subscriber Line, Digital Subscriber Line) is a combination of transmission technologies using copper telephone lines as the transmission medium. It includes HDSL, SDSL, VDSL, ADSL, and RADSL, and is generally called xDSL. The main difference between them is the two aspects of the difference in signal transmission speed and distance and the symmetry of the uplink and downlink rates. [6]


ADSL technology is the most commonly used in xDSL technology and belongs to asymmetric digital subscriber lines. The transmission rates in the two directions are completely different. It uses ordinary telephone lines as the transmission medium, and uses the high-frequency band after 26Kttz, and the transmission speed is high. It can make full use of the existing local telephone network and effectively reduce installation and maintenance costs, but this technology is only suitable for applications with a downlink transmission rate of 1-2 Mbits. Telephone lines have high coverage in people's lives, and most user networks will continue to use existing copper loops for a long time to come. ADSL will occupy a certain market in the next few decades.
The international standard on ADSL was mainly formulated by ANSI. In 1994, the TIE1.4 working group adopted the first draft ADSL standard and decided to use DMT as the standard interface. The key is to support 6.144Mbit / s or higher and can transmit data. Long distance. The ANSI standard will contain an appendix specifying the European ADSL standard. Therefore, the ADSL standard formulated by ANSI is actually a quasi-international standard. CAP codes are also striving to become de facto standards.
In mid-1997, some ADSL vendors and operators began to realize that perhaps sacrificing some speeds of ADSL may speed up the commercialization process of ADSL, because the decrease in speed also means the reduction of technical complexity. The downlink speed of full-rate ADSL is 8Mbps, but a splitter must be installed on the user side. If the downlink rate of ADSL is reduced to 1.5Mbps (downlink is 1.5Mbps, uplink is 384 Kbps), then the user-side splitter can be cancelled. This means that users can install the ADSL Modem just like the ordinary analog modems in the past. There is no difference, and the on-site service of the service provider is omitted, which is very important for the promotion of ADSL.
As a result, a new version of ADSL was born, called Universal ADSL (Universal ADSL). In January 1998, some well-known manufacturers, operators and service providers in the world organized and established the Universal ADSL Working Group (UAWG) to work on the standardization of this version.
In October 1998, the ITU began discussions on 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 approved and passed the G.Lite ( (G.992.2) standard, thus clearing the way for the commercialization of ADSL.


HDSL is also called high-speed digital subscriber line, and it is the most mature one in xDSL technology. It uses relatively advanced digital signal adaptive equalization and echo cancellation technology, which requires less bandwidth and does not require repeaters. HDSL technology is symmetrical, that is, uplink communication and downlink communication provide the same bandwidth, which can replace Tl / E1 of small and medium-sized local area networks, which is cheap and easy to install.
HDSL and SDSL support symmetrical T1 / E1 (1.544Mbps / 2.048Mbps) transmission. 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. Comparatively speaking, symmetric DSL is more suitable for enterprise point-to-point connection applications, such as file transmission, video conferencing, and so on. Compared to asymmetric DSL, the market for symmetric DSL is much less. [6]


VDSL is called high-speed digital subscriber line. It is an asymmetric data transmission technology with extremely high speed. It can provide residential users and enterprise customers with high-speed network connections and integrated services. There are two main ways of applying VDSL technology. One method is to insert VDSL equipment directly 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 decided. VDSL is more economical and feasible, and it is an important supplementary method in network reform. [5]
VDSL, ADSL and RADSL are asymmetric transmissions. Among them, VDSL technology is the fastest of xDSL technology. On a pair of copper twisted pair telephone lines, the downlink data rate is 13 to 52Mbps and the uplink data rate is 1.5 to 2.3Mbps. However, the transmission distance of VDSL is only between 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 upstream rates of 640Kbps to 1Mbps and downstream rates of 1Mbps to 8Mbps, the effective transmission distance is within 3-5 kilometers. [6]


The speed range that RADSL can provide is basically the same as ADSL, but it can dynamically adjust the user's access speed based on 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 surfing on the Internet, video on demand (IAV), and remote local area network (LAN) access, because in these applications the information downloaded by the user is often more than the information uploaded (sending instructions) Much more.

adsl new development

While the ADSL market is booming, it is still a transitional technology due to its own technology constraints. The issues of speed, distance, and line rate have plagued the development of ADSL to a large extent. In order to solve these problems of ADSL, it is also constantly improving. ADSL2 technology is the new generation ADSL standard (G.992.3 / G.992.4) defined by the ITU on the basis of the first generation of ADSL, with the support of relevant operators and equipment manufacturers. Compared with ADSL, ADSL2 adds some new functions, mainly improving transmission performance, network interoperability and management functions. At the same time, it also greatly improves support for new services and applications. ADSL2 + (G.992.5) is further developed on the basis of ADSL2. ADSL2 + has all the characteristics of ADSL2, and on this basis, the available frequency band and the uplink and downlink transmission rates have been further expanded. Compatible with ADSL2. [7]
agreement type
Protocol description release time
ADSL G.992.1 G.dmt 1999
G.992.2 G.1ite 1999
ADSL 2G.992.3 G.dmt.bis 2002
ADSL2 G.992.4 G.1ite.bis 2002
ADSL2 + G.992.5 ADSL two plus 2003
ADSL2 G.992.3 Annex L Reach Extended 2003

ADSL2 adsl ADSL2

adsl features

Increase in speed and coverage
ADSL2 has better performance in speed and coverage than the first-generation ADSL. ADSL2 has a maximum downlink rate of 12 Mbit / s and an uplink maximum rate of 1 Mbit / s. ADSL2 is implemented by reducing the overhead of the frame, improving the performance of the initial state machine, and adopting a more efficient modulation method, a higher coding gain, and an enhanced signal processing algorithm.
Compared with the first-generation ADSL, on long-distance telephone lines, ADSL2 will provide 50 kbit / s rate increase on upstream and downstream lines than the first-generation ADSL. At the same speed, ADSL2 increases the transmission distance by about 180 m, which is equivalent to an increase of 6% of the coverage area.
Line diagnostic technology
For ADSL services, how to quickly locate faults is a huge challenge. In order to solve this problem, the ADSL2 + transmitter has enhanced diagnostic tools. These tools provide a means of solving problems during the installation phase, a means of monitoring during the service phase, and updates to the tools.
In order to be able to diagnose and locate faults, ADSL2 transmitters provide means for measuring line noise, loop attenuation, and SNR (signal-to-noise ratio) at both ends of the line. These measurement methods can be completed by a special diagnostic test module. collection. This kind of test can be completed even when the line quality is poor (even if ADSL cannot complete the connection). In addition, ADSL2 provides real-time performance monitoring, which can detect the quality and noise status information at both ends of the line. Operators can use this information processed by software to diagnose the quality of ADSL2 connections and prevent further service failures. It can also be used to Determine if you can provide users with a higher rate of service.
(3) Enhanced power management technology
The first generation of ADSL transmitters were also in full energy mode when no data was being transmitted. If the ADSL Modem can work in standby / sleep state, it can save considerable power for millions of Modems. In order to achieve the above purpose, ADSL2 proposes two power management modes, low-energy mode L2 and low-energy mode L3. In this way, the total energy consumption of the device can be reduced while keeping ADSL always online.
The low energy mode L2 enables the central office modem ATU-C end to quickly enter and exit the low energy mode according to the ADSL traffic flowing over the Internet. When downloading a large number of files, ADSL2 works in the all-around 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 will not affect the service, will not cause service interruption, or even a bit error.
Low energy mode L3 is a sleep mode. This mode is entered when the user is offline and there is no traffic on the ADSL line. When the user comes back online, the ADSL transceiver takes about 3 s to re-initialize and then enters 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 traffic of the line connection, the transmission power can be flexibly switched between L0, L2, and L3, and the switching time can be completed within 3 seconds to ensure that the service is not affected.
Rate adaptive technology
Cross talk between telephone lines will seriously affect the ADSL data rate, and changes in cross talk levels 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 using SRA (Seamless Rate Adaptation) technology, so that the ADSL2 system can change the connection rate without any service interruption and bit error during work. ADSL2 changes the data rate of the connection by detecting changes in the channel conditions to comply with the new channel conditions. The rate is dynamically adjusted according to the line quality. The unit is 32 kbps. The rate of change is based on the actual transmission quality of the line to reduce data packet drops and improve stutter. This change is transparent to the user.
Multi-pair binding 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 phone lines can be bundled into one ADSL link through the IMA and ADSL2 chipset, so that the downlink data rate of the line is more flexible. Sex.
Channelization technology
ADSL2 can divide the bandwidth into channels with different link characteristics to provide services for different applications. This capability enables it to support CVoDSL (Channelized Voice over DSL) and transparently transmit TDM voice over the DSL link. CVoDSL technology reserves a 64 kbit / s channel for transmitting TDM from a DSL modem to a remote office or central office. The central office access device directly sends voice 64 kbit / s signals to the circuit-switched network through PCM.
Other advantages
Improved interoperability: simplified initialization of the state machine, interoperability and improved performance when connecting ADSL transceivers from different chip vendors.
Quick start: ADSL2 provides a quick start mode, the initialization time is reduced from 10 s to 3 s for ADSL.
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 256 kbit / s.
Support for packet-based services: ADSL2 provides a packet aggregation mode transmission aggregation layer that can be used to transmit packet-based services.

adsl application

ADSL2 utilizes existing telephone copper cable resources, and can provide broadband data services using high-frequency bands while opening voice services (POTS, ISDN). The application reference model of ADSL2 is shown in Figure 1. Among them, ATU-C and ATU-R are ADSL2 transceiver units of the central office and user, respectively, and the voice and data services are separated by a splitter.
Figure 4 ADSL2 application reference model
According to the different services provided, ADSL2 includes the following four specific application forms:
Data, which only provides data services.
Data + POTS, which provides both data and ordinary telephone services.
(3) Data + ISDN, which provides both data and ISDN services.
Voice over Data, which provides voice services (VoADSL) through data channels. At this time, the voice gateway function is required to complete the conversion from voice to packet data.

ADSL2+ adsl ADSL2 +

adsl features

In addition to the technical characteristics of ADSL2, ADSL2 + also has an important feature of extending the downstream frequency band of ADSL2, thereby increasing the downstream rate on short-distance lines. The two standards of ADSL2 each specify 1.1 MHz and 552 kHz downlink frequency bands, while ADSL2 + specifies a 2.2 MHz downlink frequency band. This makes ADSL2 + the downlink speed in a short distance (within 1.5 km) has been greatly improved, which can reach more than 20 Mbit / s. The uplink rate of ADSL2 + is about 1 Mbit / 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 line pairs, its downlink operating frequency band can be set between 1.1 and 2.2 MHz to avoid interference with the 1.1 MHz downlink frequency band of ADSL, thereby reducing crosstalk and improving service. The purpose of quality.

adsl development

No matter in terms of coverage distance, outgoing line rate, downlink bandwidth or power management, fault detection, etc., ADSL2, ADSL2 + have greatly improved compared to ADSL technology, and have many new features and functions. These new features and functions will further improve network performance and interoperability, so that operators can implement new technology application deployments by upgrading existing equipment, rather than eliminating existing equipment and better supporting new applications. and service. Therefore, ADSL / ADSL2 + technology can be gradually applied where conditions permit, and the existing ADSL equipment can be upgraded to have ADSL / ADSL2 + capability. For example: ADSL2 / ADSL2 + can be used to cover users in some areas with long distances of subscriber lines; ADSL2 + can also be deployed where some bandwidth requirements are higher than the capacity provided by ADSL; ADSL2 + can also be used in areas with low outgoing rates Deployed as a solution to reduce interference between harnesses and improve line availability.
The new-generation ADSL technology is good, but after all, the standard launch time is not long, and the chips and equipment are not mature, so it is not suitable for large-scale application. In addition, since ADSL2, ADSL2 + have made major changes to the first-generation ADSL technology, especially the frame structure, operators should pay close attention to the interoperability 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. Focus on research first, conduct full network tests, and then gradually deploy to the network to lay the foundation for the upgrade of ADSL networks.

adsl application prospects

Since the introduction of the ADSL2 / 2 + standard is not long, the development of the chip has not been fully mature, and the current equipment cannot support all the functions defined by the standard. However, based on ADsL's good market performance and broad market prospects, chip developers and equipment suppliers have been accelerating the commercialization of ADSL2 / 2 +. Chinese operators have also carried out trials and commercial trials of ADSL2 / 2 +. The development trend of ADSL is ADSL2 / 2 +. However, the large-scale application of ADSL2 / 2 + needs to be comprehensively considered 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 its long-distance transmission capability to solve the technical problem that ADSL cannot achieve long-distance access. In view of the superiority of ADSL2 / 2 + technology and the increasing demand of users for higher speed and longer distance broadband access, 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 be perfected in the application. [7]


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