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Technology Stocks : ADSL (G-Lite) for dummies - AWRE,PAIR,ORCT,ASND,COMS,NN -- Ignore unavailable to you. Want to Upgrade?

To: JW@KSC who wrote (53)	11/25/1998 3:30:00 AM
From: Jon K.	Respond to of 201

Jim, you are a one tough cooky. Giving us a quiz on your first visit! Thank you so much for your posting, it was filled with good stuff. Did someone point you in my direction? Actually no. I am following postings of "Scrapps" and "Pat Mudge" (Scrapps has been helping me a lot.) Those are the two people seems to know this stuff pretty well. When I saw pat praising your posting it had to be good. So copied it. Watch out, from now on YOU are on my tracking list. It would be nice if you can recommend one or two more. I see you guys had some great fun in '96... Westell and PAIR almost 10 fold in 6 month! Guess ten fold in two years is quite possible huh? Now, those ADSL dream and promises are coming to real life! Boy, am I lucky or what. Right place at the right time.

To: JW@KSC who wrote (53)	11/25/1998 3:48:00 AM
From: Jon K.	Read Replies (3) \| Respond to of 201

Oh, I forgot, here is your McDMT: I guess simple answer is Amati won the battle and ANSI set DMT(Discrete Multi-Tone modulation) as ADSL standard. Do you work at NASA? I used to live in Florida, only a few hours from the space center. And now, I live right next to JPL in California. We try to visit JPL's open house party every year. Our kids love to visit JPL. I did some extra credit homework: cs.tamu.edu Modulation While the ADSL standard specifies only one type of modulation, researchers have tested and implemented three different modulation techniques for use with ADSL: Quadrature Amplitude Modulation (QAM), Carrierless Amplitude-Phase Modulation (CAP), and Discrete Multi-Tone Modulation (DMT). Quadrature Amplitude Modulation (QAM) The binary data stream is split into two sub-streams and separately modulated onto orthogonal versions of the same carrier frequency. The two modulated signals are then added and low pass filtered before transmission to the network[1]. This is the least used modulation technique for ADSL. Carrierless Amplitude-Phase Modulation (CAP) Developed by AT&T Paradyne, CAP ADSL equipment is available as an alternative to the industry standard DMT. CAP products were available before DMT equipment, and some vendors invested in CAP technology instead of waiting for the DMT equipment to mature[4]. The bit stream is divided into two symbol streams and fed to two parallel in-phase and quadrature digital-shaping filters. The transfer functions of the two filters have the same amplitude characteristics, but the phase characteristics differ by 90 degrees[6]. The outputs are added, passed through a digital-to-analog converter and filtered before being sent to the transmission network[1]. Data encoded using CAP has the same spectral characteristics and provides the same theoretical performance as QAM, but does so without a carrier frequency. Eliminating carrier modulation (mixing) provides two key advantages. First, it facilitates a less expensive all-digital transceiver implementation by eliminating the computation-intensive multiply operations that are needed for carrier modulation and demodulation. Second, the absence of a carrier provides increased flexibility, because changing the spectrum's center frequency, symmetry and shape can be done easily by downloading a new set of filter coefficients[6]. Discrete Multi-Tone Modulation (DMT) This modulation technique was decided upon as the ADSL standard. DMT was developed by Amati Communications and Stanford University, and commercialized by Northern Telecom[4]. Each serial digital input signal is first encoded into a parallel form and then passed through a fast-Fourier-transform processor. This converts the frequency domain samples into time domain values with a sliding time-window effect. These values are then transcoded into a serial format and converted from digital to analog before transmission through the ADSL circuit[1]. Some of the most important parameters for standardized ADSL DMT are described below[5]. Pilot Carrier 64 (f=276 kHz) is reserved for a pilot. The data modulated onto the pilot subcarrier shall be constant 0,0. Use of this pilot allows resolution of sample timing in a receiver modulo-8 samples. Nyquist Frequency The carrier at the Nyquist Frequency (256) may not be used for data. Synchronization Symbol The synchronization symbol permits recovery of the frame boundary after the micro-interruptions that might otherwise force retraining. ------------------ bugs.wpi.edu:8080/EE535/hwk97/hwk3cd97/ryant/ryant.html Advantages of DMT As mentioned previously, DMT eliminates the need for most adaptive equalization based on the condition that the frequency response of the channel be nearly constant across any subchannel. While this is one advantage of DMT, there are others. By (carefully) dividing the channel into subchannels and allocating the maximum allowable number of bits to each subchannel, DMT can be shown to be an optimum code ([4]). This means that this system can perform at the theoretical limit and that no other system can exceed its performance. While the proof is beyond the scope of this paper, the reader is encouraged to see [7], [8] for a formal description and proof. Since the data rate depends on the SNR of the channel, the channel can be monitored for changes and the rate dynamically updated so as not to disturb the system performance. The rate adaptation capability of DMT makes it well suited for a wide variety of channel conditions and increases the resistance to bursty, high-power noise (since a channel can be "shut off" for certain periods). Frequency division multiplexing can be easily integrated into a DMT system by grouping subchannels. Through this implementation, multiple users can be merged onto the same channel, each with his/her own transmission rate (based on the number of subchannels assigned to that user). *****I still don't get it though <ggg>***********