ADSL Technology Detailed

I. Overview
ADSL is an asymmetric version of DSL that uses digital coding to obtain maximum data transfer capacity from existing copper telephone lines without interfering with conventional voice services on the same line. The reason for this is that it transmits data in frequencies other than telephone voice transmissions. That is, users can call or send faxes while surfing the internet, which will not affect the quality of the call or reduce the speed of downloading Internet content.
ADSL can provide end users with 8Mbps downlink transmission rate and 1Mbps uplink transmission rate, compared to the traditional 28.8K analog modem nearly 200 times times faster. This is unmatched by ISDN (Integrated Business Data Network) with a transmission rate of up to 128Kbps. ADSL has a unique advantage over a cable modem: it provides a dedicated line service for users of a single telephone line, while a cable modem requires a large number of users within a system to share the same bandwidth. Although cable modems have a lower rate of downlink than ADSL, the performance of cable modems will be greatly reduced, given that more and more users will be online at the same time in the future. In addition, the uplink rate of the cable modem is usually lower than ADSL. It is not to be overlooked that at present, there are nearly 750 million copper telephone line users worldwide, while the number of households enjoying cable modem services is only 12 million.
ADSL Design aims to have two functions: high-speed data communication and interactive video. Data communication functions are available for Internet access, corporate remote computing, or dedicated network applications. Interactive video includes video on Demand (VOD), movies, games, etc. that require high-speed network video communication. At present, ADSL only supports the interface with T1/E1, in the future can go to the desktop.
The lower transmission rate is influenced by many factors, such as distance, cable size and interference, from 10Kbps to 640Kbps, the following table is the standard rate of transfer at a certain distance:

Second, the standard ADSL
ADSL has been the CAP and DMT two standards, cap by the at&t Paradyne design, and DMT by the Amati communications company invented, the difference is the way to send data. ANSI Standard T1.413 is based on DMT, DMT has become an international standard, and Cap is a great decline. Recently, many of the g.lite standards are very good, but DMT and g.lite two kinds of standards have their own strengths, respectively, applicable to different fields. DMT is a full speed ADSL standard, which supports the high speed downlink/uplink rate of 8mbps/1.5mbps, but the DMT requires a pots separator to be installed on the client side, while the G.lite standard is low and the downlink/uplink rate is 1.5mbps/512kbps. However, since the complex pots separator is omitted, users can buy CPE directly from the store, just like using a normal modem, and then they can simply install it themselves. In the area of application, DMT may be more suitable for small or family offices (SOHO); G.lite is more suitable for ordinary home users.
1. CAP (carrierless amplitude/phase modulation)
A cap is a proprietary modulation of the At&t Paradyne, which is modulated to a single carrier channel and then sent along a telephone line. The signal is compressed before it is sent and reorganized at the receiving end.
2, DMT (discrete multi-tone)
The data is divided into several sub carrier channels, the quality of each channel is tested, and then a certain number of bits is given. DMT creates these channels with discrete fast Fourier transforms.
DMT uses the familiar mechanism to create a connection between modems. When two DMT modems are connected, they try the highest possible rate. Depending on the noise and attenuation of the line, two modems may successfully connect at the highest rate or gradually reduce the rate until both sides are satisfied.
3, G.lite
Just as N1 standards and interoperability testing have driven the ISDN market, customers and vendors are eagerly awaiting the arrival of a DSL equipment interoperability standard. The standard, known as G.lite, also known as the consumer asymmetrical DSL (consumer ADSL), is being developed by an ADSL Working group, which includes almost all major DSL device manufacturers. However, do not confuse this with the Rockwell consumer DSL chip set that was no longer in use in the summer of 1997, or with Universal ADSL. G.lite's first edition of the working document was published at the Supercomm Trade Fair held in Atlanta in June 1998. This initial g.lite standard was first put to the vote by UAWG and then forwarded as a recommendation to ITU, the International Telecommunication Union. The ITU was expected to sign a formal g.lite standard by the end of 1998.
Some details of the future G.lite standards are clear, and CPE based on that standard may soon emerge. The G.lite Standard (ADSL) will be based on the ANSI standard "T1.413 Issue 2 DMT line Code", and the 1.5Mbps downlink speed and 384Kbps upstream speed are predetermined as their maximum speed. Speed Adaptive (rate-adaptive), which is less than the maximum speed, is also part of the standard, so the Internet service provider (ISP) can provide 256Kbps of symmetrical speed as a g.lite connection speed. However, in order to simplify equipment and supply requirements, most devices will be limited to those maximum speeds.
The 1.5Mbps speed limit seems restrictive compared to the maximum downward speed of the DSL's generally advertised 7Mbps, but it is based on empirical testing of a typical customer cabling scheme, based on the actual backbone bandwidth available through the ISP.
DSL lines require premium copper rings-which means no inductance coils, no more than 2500 feet between bridging taps, and generally no more than 18000 feet away from central bureaus. If the speed is higher, the distance requirement becomes more critical, and the lines are more susceptible to disruption by "disrupting"-ISDN and T1 lines in the same bundle as DSL lines.
While G.lite is being advertised as a "splitterless" standard, the engineering reality of the new standards means that in the beginning it may still be necessary to route the separator, filter, and even new client locations. As G.lite standards mature, people better understand these issues and better implement the vendor chip, it may be closer to becoming a true separation of standards.
Of course, even at g.lite speeds, the UARTs (Universal asynchronous receiver/transmitter) on the regular PC serial port cannot be followed. As a result, an external PC modem for a single user using serial technology will use a universal serial bus port on the PC (Universal serial buses), or an enhanced parallel port, and a router and bridge unit using Ethernet; newer chip sets, The V.90/ADSL pairing chip set, recently promoted by Rockwell, will combine the g.lite and v.90 standards on a modem to provide a connection configuration option for customers.
Bandwidth is another factor to consider. When Bellcore first published its DSL work in 1989, the goal was to use the DSL for video-on-demand services rather than purely data communications.
However, few ISPs today can truly meet the 7Mbps Internet access requirements of 1000 users. G.lite's 1.5mbps/384kbps limit is a reasonable maximum speed, however, many users are likely to choose a slower symmetry speed ...
4, the current standard
ANSI has proposed an ADSL standard T1.413,etsi (European technical Standard Institute) with a rate of up to 6.1Mbps to accommodate the needs of Europe, Called t1e1.4, the standard is extended to include client-side multiplexing interfaces, network configuration and management protocols, and other improvements.
Three, principle
ADSL with its unique modulation and demodulation hardware to connect the existing twisted-pair connection of the end, it creates a three-channel pipeline, see the following figure.

The pipeline has a high-speed channel (to the client), a medium-speed duplex channels and a pots channel (4KHz), pots channel to ensure that even if the ADSL connection fails, voice communication can still function. Both high-speed and medium-speed channels can be reused to create multiple low-speed channels.

This table is a simple reference, the actual wire speed is subject to physical cable length, size and interference factors such as the impact.
In the past few years, the telephone system has made great progress in hardware technology, but ADSL uses a very simple method to obtain an astonishing rate: compression. It uses very advanced DSP and algorithm to compress as much information as possible in the telephone line (twisted pair). (☆ Introduction to Programming Network ☆)
ADSL uses frequency Division multiplexing (FDM) or feedback suppression (Echo cancellation) to create multiple channels in a telephone line. FDM uses a down-pass data pipeline and a data pipeline to upload, and divides the down-pass pipeline by Time Division Multiplexing (TDM), and the upload pipeline is divided into multiple low-speed channels. Feedback suppression overlaps the down-pass pipeline with the upload pipeline and distinguishes the two by local feedback suppression (such as the V.34 specification). Feedback suppression, while more effective, adds complexity and cost.
ADSL multiplexing under the Messenger channel, duplex, the channel block, to each block plus error code, and then send data, the receiver based on error and block length error correction. The test shows that the ADSL modem is correct enough to handle MPEG2 and a variety of other digital video schemes.
Four, the future of ADSL
The future of ADSL may not be much different from what it is now. There are two ways to implement ADSL at present: cap and DMT, the latter has become standard. Although the CAP is not standard, it was invented by At&t Paradyne and has been widely used in communications. G.lite also has a promising future. It's hard to say what the future will look like, but one thing is certain: manufacturers and telephone companies have a bottleneck that needs to be overcome to develop their networks, and solutions must be available quickly and still be useful in the future.