Steve Reinhardt [Re: VDSL Issues ]
Analog Devices has a good write-ups concerning the comparison between DMT & CAP for ADSL. From those articles, one would feel DMT is the way to go for ADSL.
Now, VDSL is another product area. If ADSL is using DMT, there should not be any reason why VDSL has to follow the line codes of ADSL, right?
Steve,
The Robustness of DMT becomes even more importent when using VDSL.
Amati's DMT uses forward error correction (FEC) and data interleaving in ADSL and VDSL to reduce the effects of impulse noise. The higher the frequency transmission the more these effects are felt, and need to be addressed. Thus VDSL is more susceptible than ADSL.
Impulse Noise is major impairment for digital access transmission systems running on copper twister pair.
There are a variety of sources that produce short electrical transients, known as impulse noise.
Household appliances switching on and off (e.g.) Air Conditioner, Furnace, fluorescent lights, refrigerators, etc.)
The telephone itself ( on-hook/off-hook and ringing.)
These transient disturbances can be electromagneticly coupled into the access network and may cause error bursts in digital transmission.
DMT Vs CAP
1. DMT's ability to adaptively shape the Allocation of information and transmit power across a given bandwidth enables it to better approach optimum performance than is theoretically possible with CAP/QAM
2. The complex startup procedure required for DMT could result in longer activation times than is possible with CAP/QAM
3. DMT offers greatest performance advantages over CAP/QAM at high transmission rates --- the shorter loops at higher rates have more usable bandwidth giving DMT a greater degree of freedom ( more usable sub-channels for flexible adaptation.
4. DMT is more adept than CAP/QAM at coping with multiple RF interferes
5. It is simple for DMT to meet an arbitrary transmit power mask specification to meet spectral compatibility requirements.
6. echo- cancellation in DMT is non-trivial
7. DMT has greater inherent immunity to impulsive noise than CAP/QAM
8. CAP/QAM can use a simpler forward error correction than DMT
9. DMT requires minimal equalization with slower signal processing rates than CAP/QAM DMT hardware can be more easily programmed to support a variety of upstream and downstream data rates with potential for on-line configuration.
10. DMT need be no more complex to implement than CAP (in terms of signal processing MIPS per area of VLSI real estate), although it may be more complicated to understand and design, in particular the Startup sequence.
11. DMT has the potential for more sophisticated embedded line monitoring functionally (e.g. for cable sheath management).
Primary Reference Source
The BT Technical Journal vol 13 No 4 October 1995
"Broadband multimedia delivery over copper"
by G. Young, K.T. Foster and J.W. Cook
Or: techstocks.com
CAP/QAM VDSL interfers with Short Wave and Ham Radios, which are both very popular in Europe. This is a big probelm for CAP.
I hope the above helps.
Now buried way down here, I will discuss MHO, and one theory as to why
USRX bought Awares Intellectual property which I believe was Awares DWMT.
I could be way off base
It has been stated that when it comes to RFI, DWMT is equal or better
than DMT.
This may or may not be true, it will have to be tested in the Field to
satisfy my tastes.
Again IMHO,
USRX bought DWMT for use with VDSL, Not ADSL.
Of course there will be a Battle for the VDSL standard.
But after that battle there will be another, most likely in court.
Does DWMT infringe upon Amati's DMT patent.
I have it from a fairly reputable source that if you use DMT in DSL in any form or fashion, the it infringes upon Amati patents.
Perhaps through a third party USRX will pay royalties on ADSL/DMT, but
I would guess the use of DWMT in VDSL will end up in a patent infringement suit.
USRX was late and is still behind the 8 ball on ADSL, and their first offering, likely to be a 56K modem, with a software upgrade to DSL, will be less than one megabit speeds.
Aware plans to piss both sides off, and use both DMT and CAP in their VDSL solution.
Will Rupert dig down this far into this post?
JW@KSC |
