Adsl Asymmetric Digital Subscriber Line Access Technology In The Modulation-adsl, Digital Subscriber

A rate of non-symmetric copper access network technology that uses digital coding

techniques from existing copper telephone lines for larger data rates, without interference in the same line for the conventional voice service. ADSL on copper phone lines that can work simultaneously to create the three channels, namely, high-speed downlink channel (HighSpeedDownstream), medium-speed duplex channel (MediumSpeedDuplex) and POTS channels (which used to guarantee that even if the ADSL connection fails, the voice communication can still operate normally). ADSL uses advanced digital signal processing technology and the new data compression algorithm, so a lot of information to the Internet high-speed transmission.

Telephone line in order to separate the effective bandwidth, resulting in multiple channels, ADSL modems generally use two methods to achieve, frequency division multiplexing (FDM) or echo cancellation (EchoCancellation) technology. FDM bandwidth allocation in the existing data in a frequency band as the downlink channel, while the other assigned a frequency band as the uplink data channel. Downstream channel by time division multiplexing (TDM) technology and then divided into high-speed channel and low-speed channel. Similarly, the uplink channel is also low by a multi-channel component. Return wave cancellation technology makes uplink and downlink frequency bands superimposed, to distinguish the two by means of local echo cancellation band. Of course, regardless of which of two technologies, ADSL will be isolated 4KHz band for telephone service (POTS).

ADSL asymmetric data rate performance in a two-way asymmetric, downstream rates are generally much larger than the uplink rate, it is for users surfing the Internet and remote LAN access are highly absorbent. ADSL can provide to end users 8Mbit / s of downstream speed and 1Mbit / s uplink speeds far greater than traditional Modem or ISDN speed.

2ADSL modulation techniques Now widely used internationally, there are three ADSL modulation: quadrature amplitude modulation QAM (QuadatureAmplitudeModulation), no carrier amplitude / phase modulation of CAP (CarrierlessAmplitude-plaseModulation), discrete multi-tone DMT (DiscreteMultitone).

2.1QAM modulation In QAM modulation, the transmit data in the bit / symbol in the encoder is divided into each of the original rate 1 / 2 of the two signals, each with a weight multiplied by the orthogonal modulation, summing the output. Instead the receiver to complete the process, quadrature demodulation the two opposite streams, equalizer compensate the distortion caused by the channel, the verdict for Discrimination of complex signals and mapped back to binary signals. Using QAM modulation, the channel bandwidth of at least equal to chip rate, to timing recovery, but also require additional bandwidth, generally to increase about 15%. Compared with other modulation, QAM modulation with full use of bandwidth, the characteristics of strong noise. QAM modulation technology for ADSL, but the main problem is how to adapt to different telephone lines between the large difference in performance. To obtain more desirable performance characteristics, QAM, and the receiver requires a transmitter with the same spectrum and the corresponding characteristics of the input signal for decoding, QAM receivers using adaptive equalizer to compensate for transmission distortion in the signal generator, so using QAM The ADSL system complexity comes from its adaptive equalizer.

2.2CAP modulation

CAP modulation is QAM-based modulation techniques evolved, and can say that it is a variant of QAM technology, it is the difference between QAM digital means of its realization. Input data is sent to the encoder, the encoder in, m-bit input bit is mapped to k = 2m different complex symbols, the symbols k different plural form k-CAP line code.

by: gaga

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