Amazing ADSL MODEM technology!
Since Internet came to people, the speed became a heart disease of all online users. Satisfied from the speed of the Internet, people always say: "Fast, fast, just a little more ..."; may never be achieved in the world to meet people's desires. The development of various access technologies is in order to cater to this strong requirement, here, we will make an overview and comparison of ADSL, cable modem (Cable Modem), 56KMODEM, DSVD technology several times. In order to make readers have a comprehensive understanding of today's access technology. ADSL technology revealed
ASYMMTRIC DIGITAL SUBSCRIBER LINE, Chinese name is a non-symmetrical digital user line, which is a new name for most people, but this is not important. The key is to place ADSL MODEM at both ends around you. At this telephone line, three information channels have been generated: a high-speed downlink channel with a rate of 1.5Mbps-9Mbps, used for user download information; a speed of 16kbps-1Mbps; a normal duplex channel; one ordinary Vintage phone service channels; and these three channels can work at the same time. (Of course, the specific communication rate is also dependent on the quality and length of the loop.) What does this mean? You can watch the big pieces you play on the Internet while downloading the file and comment on the big piece by phone and your friends. Note that the most attractive thing is that all this is on a telephone line, while doing. What is ADSL to achieve this? Speaking, the interior of ADSL is very complicated. It uses advanced digital signal processing technology and new algorithm compressed data to enable a lot of information to be transmitted on the Internet. We know that data is transmitted on the existing longer copper twisted pair (ordinary telephone line), which is very serious, and ADSL achieves a large dynamic range in such a harsh environment. Channel, and maintain low noise interference, it is difficult to know that it is no wonder that some people say that ADSL is a miracle of modulation technology.
In order to separate the effective bandwidth on the telephone line, the multi-channel channel is generated, and the ADSL modem is generally implemented, frequency division multiplex (FDM) or echo cancellation technology. FDM assigns a frequency band as a data downlink channel in an existing bandwidth, and allocates another frequency band as a data uplink channel. The downlink channel is divided into a plurality of high-speed channels and low speed channels through the time division multiplex (TDM) technology. Similarly, the uplink channel is also composed of multiple low speed channels. The echo elimination technology makes the upper frequency band and the downlink band, distinguishes between the two-band through local echo cancellation. This technique comes from a V.32 and V.34 modems, which is very effective in use of limited bandwidth, but from complexity and prices, it is large. Of course, from which one of the two technologies is used, ADSL will separate 4kHz frequency band for Vintage Channels (POTS).
Current ADSL modems use three line coding technologies, called suppression carrier amplitude and phase, CAP, discrete multi-tone reuse, DMT, and discrete small wave Discrete Wavelet MultiTONE, DWMT. Where the CAP is based on orthogonal amplitude modulation (QAM), in CAP, data is modulated to a single carrier; and in the DMT, the data is modulated over the plurality of carriers, and the data on each carrier uses QAM. modulation. DMT is used to do digital signal processing for the fast Fourier transform algorithm that is well known, and in DWMT, the emerging wavelet transform algorithm in recent years is used instead of fast Fourier transformation. An ADSL modem intersects the data stream in the multi-down channel, in the dual-work channel, and the maintenance channel, and adds an error correction code in each data block, and the receiving end passes this error correction code. Correct the error generated during the transfer process. Experiments show that this error correction coding technology can achieve the requirements of MPEG-2 and other digital image compression methods. The main difference between non-digital user line (ADSL) and past modem techniques is that the upper and downstream rates are asymmetrical, that is, the upper and downstream rates are not equal, and the high row rate and relative relative relatively slowed speed rate of ADSL technology are very suitable. Do Internet browsing.
ADSL's tempting
