Modem question

On Wed, 08 Sep 2010 19:47:21 -0700, Muzaffer Kal <kal@dspia.com>
wrote:

>On Wed, 08 Sep 2010 17:39:56 GMT, eric.jacobsen@ieee.org (Eric
>Jacobsen) wrote:
>>>>>Gigabit ethernet, 1000baseT, does.
>>> 
>>>> Are they cancelling the back channel?   I.e., simultaneous Tx and Rx
>>>> on the same channel with Tx cancellation at the Rx?    I think that's
>>>> what Randy was asking about.    I wasn't aware that any wired
>>>> standards did that, if they do.
>>>
>>>Yes, simultaneous Tx and Rx.  
>>>
>>>1000baseT uses four pair Cat5e cable, with all four going both
>>>directions at 250Mb/s.  It is baseband, so no channel question.
>>
>>That part I was aware of, and the four pairs and their crosstalk are
>>essentially processed like a MIMO system.
>
>Not really, at least not the way I understand MIMO. In 1000BT, only
>near-end cross-talk (NEXT) is cancelled and FEXT is ignored. In 10GBT
>over copper, FEXT cancellation is necessary and most implementation do
>that and there are some published papesr on using off-diagonal channel
>parameters (ie Hij where i!=j) as a means to estimate what data is
>being transmitted for channel I.

In MIMO the "crosstalk" is used to effectively increase the capacity
of the shared channel.  That's not quite the same as cancelling
crosstalk in separate but interfering channels, but my understanding
is that there is some fundamental overlap between how the crosstalk is
handled in 1000BT and the sub-channels are separated in MIMO systems.
At least, that's what I gleaned from some conversations with the 802.3
guys that were working on it in the IEEE 802 meetings when the MIMO
work was being done in 802.11.   I was working in the 802.11 group at
the time on the 11n amendment.

It's entirely possible that I misunderstood, but that seemed to be the
gist of it at the time.

>>
>>I didn't know that 1000bT did simultaneous Tx/Rx.   The basic ethernet
>>MAC is TDD, and I didn't know how much that had changed.
>>
>Ethernet has been full-duplex since 100BTX days and even 10BT had full
>duplex implementations. These mainly are FD over separate channels.
>Starting with 100BT2 (which wasn't popular) and mainly with 1000BT,
>simultaneous tx/rx over a single pair has been implemented. This is
>done with a hybrid to mix the 4 wires to 2 and echo cancellation is
>needed which is done by mainly digital means. 

The most fundamental element of the MAC is the CSMA mode, which is
TDD.   In IEEE 802 the MAC defines the basis of the standard, so from
that point of view all 802.3 devices have a CSMA/TDD MAC.    In
practice it's evidently not populated, at least in circuits that get
powered-on, due to the full-duplex modes that you mentioned.   I get
stuck in the fundamental stuff sometimes, especially since the other
802 groups (that I worked in) didn't make such radical changes to the
MAC.   802.3 is a bit of an oddball that way, but for good reason.

>>>I am not so sure by now when phone line modems started overlapping,
>>>but definitely by 33.6kb/s, as half a 64kb/s channel wouldn't 
>>>do that.
>>>
>>>-- glen
>>
>>Is the Tx cancellation done digitally?   I'm surprised if this is the
>>case since one would think it would make the ADC quite a bit more
>>expensive back in those days.
>
>There are implementations which do detection in digital but partial
>cancellation in analog ie before ADC to save ADC dynamic range. One
>does bulk of the near echo cancellation in analog by an adder before
>the ADC as the near echo can be 10-15 dB above received signal coming
>from a far transmitter. Then there is a digital echo canceller to
>reduce the residual echo below needed echo suppression range.
>-- 
>Muzaffer Kal
>
>DSPIA INC.
>ASIC/FPGA Design Services
>
>http://www.dspia.com

That's what I'm seeing now in the stuff that I'm reviewing, that the
majority of the cancellation is done in the hybrid (which is what I
thought initially, so the claims that it's done in DSP were
confusing).   The 10-15dB dynamic range impact is what I was thinking
of wrt impacting the ADC.   That was probably limited by the
application in that the phone system may have been tightly controlled
enough to keep the near-far difference from getting bigger than that.
For an arbitrary-distance application (like the OP was describing) I
think that 10-15dB could turn into much, much more than that.   In RF
applications it's easily >~100dB.   Not a trivial thing to deal with.



Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.abineau.com

>That's what I'm seeing now in the stuff that I'm reviewing, that the
>majority of the cancellation is done in the hybrid (which is what I
>thought initially, so the claims that it's done in DSP were
>confusing).   The 10-15dB dynamic range impact is what I was thinking
>of wrt impacting the ADC.   That was probably limited by the
>application in that the phone system may have been tightly controlled
>enough to keep the near-far difference from getting bigger than that.
>For an arbitrary-distance application (like the OP was describing) I
>think that 10-15dB could turn into much, much more than that.   In RF
>applications it's easily >~100dB.   Not a trivial thing to deal with.

What on earth are you reviewing? In the 80s the early V.32 modems often
used a hybrid tuning scheme to keep the spillback (I wouldn't really call
it echo, when the delay is zero) to a manageable level. The hybrid is on
the line side of the isolation barrier, though, so people like to avoid
putting any more complexity there than they need to. They soon started to
use nothing more than a kludgy compromise hybrid. The tuned hybrid didn't
really reduce the need for efficient echo cancellation, though, as I
explained in previous message. V.34 is a 1664 point constellation. You
*really* need to clean up the echo from the local and distant analogue line
sections if you want to reliably decode that. Its a struggle, because the
non-linearity of the lossy G.711 compression used on the PSTN means the
instantaneous SNR is only about 30dB.

Steve