1. overview
ADSL is an asymmetric version of DSL, which uses digital coding technology to obtain the maximum data transmission capacity from the existing copper telephone line without interfering with the conventional voice service on the same line. The reason is that it uses frequencies other than telephone voice transmission to transmit data. In other words, users can make phone calls or send faxes while surfing the Internet, which will not affect the call quality or slow down the speed of downloading Internet content.
ADSL can provide end users with 8Mbps downlink transmission rate and 1Mbps uplink transmission rate, which is nearly 2 times faster than the traditional 28.8K analog modem. This is also unmatched by ISDN (Integrated Service Data Network) with a transmission rate of 128Kbps. Compared with cable modem, ADSL has unique advantages: it provides dedicated line service for single telephone line users, while cable modem requires many users in a system to share the same bandwidth. Although the downlink rate of cable modem is higher than ADSL, considering that more and more users will surf the Internet at the same time in the future, the performance of cable modem will be greatly reduced. In addition, the uplink rate of cable modems is usually lower than ADSL. It should not be overlooked that at present, there are nearly 75 million copper telephone line users in the world, while only 12 million families enjoy cable modem service.
ADSL has two functions: high-speed data communication and interactive video. The data communication function can be Internet access, company remote computing or special network application. Interactive video includes video on demand (VoD), movies, games and so on, which require high-speed network video communication. At present, ADSL only supports the interface with T1/E1, and it can reach the desktop in the future.
second, the standard of ADSL
ADSL has always had two standards, CAP and DMT, and CAP is made by AT&; T Paradyne was designed, while DMT was invented by Amati Communication Company. The difference lies in the way of sending data. The ANSI standard T1.413 is based on DMT, which has become an international standard, while CAP is in decline. Recently, the G.Lite standard, which has been talked about a lot, is very optimistic, but DMT and G.Lite have their own strengths and are suitable for different fields. DMT is a full-rate ADSL standard, which supports high-speed downlink/uplink rate of 8Mbps/1.5Mbps. However, DMT requires users to install POTS splitters, which is complicated. Although the rate of G.Lite standard is low, the downlink/uplink rate is 1.5Mbps/512Kbps. However, because the complicated POTS separator is omitted, users can buy CPE directly from the store just like using ordinary Modem, and then simply install it themselves. In terms of application fields, DMT may be more suitable for small or home office (SOHO); G.Lite is more suitable for ordinary home users.
1. Cap (carrier less amplification/phase modulation)
Cap is at&; T Paradyne's proprietary modulation method, data is modulated to a single carrier channel and then sent along the telephone line. The signal is compressed before transmission and recombined at the receiving end.
2. DMT(Discrete Multi-Tone)
Divide data into multiple subcarrier channels, test the quality of each channel, and then give it a certain number of bits. DMT uses discrete fast Fourier transform to create these channels.
DMT uses a familiar mechanism to create connections 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 standard and interoperability testing have promoted the ISDN market, customers and manufacturers are now eagerly waiting for the arrival of an interoperability standard for DSL equipment. This standard is called G.lite, also known as Consumer Asymmetrical DSL (Consumer ADSL), and it is being developed by --Universal ADSL Working Group, a group that includes almost all major DSL equipment manufacturers. However, this standard should not be confused with the Consumer DSL chipset based on QAM that was no longer used by Rockwell Company in the summer of 1997 or with Universal ADSL. The first edition of G.lite's working document was published at the Supercomm Trade Fair held in Atlanta in June 1998. This preliminary G.lite standard was first put to the vote by UAWG, and then forwarded to ITU as a recommendation. At that time, ITU expected to sign a formal G.lite standard before the end of 1998.
some details of the future G.lite standard have been clarified, and CPE based on this standard may appear soon. The G.lite standard (ADSL) will be based on the ANSI standard "T1.413 Issue 2 DMT Line Code", and the downlink speed of 1.5Mbps and the uplink speed of 384Kbps are set as its maximum speed. "Rate-Adaptive" which is less than those maximum speeds is also a part of the standard, so Internet service providers (ISP) can provide a symmetrical speed of 256Kbps as a G.lite connection speed. However, in order to simplify equipment and supply requirements, most equipment will be limited to those maximum speeds.
although the speed limit of p>1.5Mbps seems to be restrictive compared with the maximum downlink speed of 7Mbps generally announced by DSL, it is based on the empirical test of typical customer wiring scheme and the actual backbone bandwidth available through ISP.
DSL lines need high-quality copper rings-this means that there are no inductive coils, and the distance between the bridge taps is no more than 2,5 feet, while the distance from the central office is generally no more than 18, feet. If the speed is higher, the distance requirement becomes more critical, and the lines are more easily damaged by "disruptors"-ISDN and T1 lines in the same bundle as DSL lines.
although G.lite is being promoted as a "splitterless" standard, the engineering reality faced by the new standard means that there may still be a need for splitters, filters and even new customer premises wiring at the beginning. As the standard of G.lite matures, people will better understand these problems and better implement the manufacturer's chips, and it may be closer to becoming a truly inseparable standard.
of course, even at the speed of G.lite, UARTs (universal asynchronous receiver/transmitter) on the conventional PC serial port can't keep up. Therefore, a single user's external PC modem using serial technology will adopt a Universal Serial Bus port on the PC, and may also adopt an enhanced parallel port. Routers and bridging units use Ethernet; A newer chipset, such as the V.9/ADSL pairing chipset recently promoted by Rockwell, will combine the G.lite and V.9 standards on a modem, providing customers with a choice of connection configuration.
bandwidth is another consideration. When Bellcore first announced its DSL work in 1989, its purpose was to use DSL for video-on-demand service, rather than pure data communication.
however, few ISPs can really meet the 7Mbps Internet access requirements of 1, users. The 1.5Mbps/384Kbps limit of G.lite is a reasonable maximum speed. In any case, many users are likely to choose a slower symmetric speed. ...
4. The current standard
ANSI has put forward the ADSL standard T1.413 with a rate of 6.1Mbps. ETSI (European Technical Standard Institute) has added accessories to meet the needs of Europe, which is called T1E1.4 It will expand the standard to include the multiplexing interface, network configuration and management protocol of users and other improvements.
III. Principle
ADSL uses its unique modem hardware to connect all ends of the existing twisted-pair connection, and it creates a pipeline with three channels, as shown in the figure below.
the pipeline has a high-speed downlink channel (to the user), a medium-speed duplex channel and a POTS channel (4KHz), and 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.
in the past few years, the hardware technology of the telephone system has made great progress, but ADSL uses a very simple method to get amazing speed: compression. It uses advanced DSP and algorithm to compress as much information as possible in telephone line (twisted pair).
ADSL uses frequency division multiplexing (FDM) or Echo Cancellation to create multiple channels in telephone lines. FDM uses a downlink data pipeline and an uplink data pipeline, and divides the downlink data pipeline by time division multiplexing (TDM), and the uplink data pipeline is also divided into multiple low-speed channels. Feedback suppression overlaps the downlink pipeline and the uplink pipeline, and distinguishes them with local feedback suppression (such as V.34 specification). Although feedback suppression is more effective, it increases complexity and cost.
ADSL multiplexes the downlink channel, duplexing, dividing the channel into blocks, adding an error code to each block, and then sending the data, and the receiver corrects the error according to the error code and the block length. The test shows that the error correction of ADSL modem is enough to cope with MPEG2-2 and many other digital video schemes.
fourth, the future of ADSL
the future of ADSL may not be much different from the present. At present, there are two ways to realize ADSL: CAP and DMT, and the latter has become the standard. Although CAP is not a standard, it is made by AT&: The invention of T Paradyne has been widely used in communication. G.Lite is also very promising. It is hard to say what the future will be like, but one thing is certain: manufacturers and telephone companies have a bottleneck to overcome in order to develop their networks, and the solution must be available soon at present and will still be useful in the future.