[118900] in Cypherpunks
Re: Unplugged! The biggest hack in history (fwd)
daemon@ATHENA.MIT.EDU (Dave Emery)
Sat Oct 9 20:53:39 1999
Date: Sat, 9 Oct 1999 20:38:57 -0400
From: Dave Emery <die@die.com>
To: Jim Choate <ravage@einstein.ssz.com>
Cc: cypherpunks@einstein.ssz.com
Message-ID: <19991009203857.A7439@die.com>
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cypherpunks@einstein.ssz.com
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In-Reply-To: <199910090138.UAA07313@einstein.ssz.com>; from Jim Choate on Fri, Oct 08, 1999 at 08:38:58PM -0500
Reply-To: Dave Emery <die@die.com>
On Fri, Oct 08, 1999 at 08:38:58PM -0500, Jim Choate wrote:
>
>
>> by the right AT command. Surely if you are connecting to Boston from
>> SF, you don't expect the modem round trip delay to be 2 ms... Also the
>
> And what AT command might that be?
For US Robotics modem firmware the ATI6 and ATI11 commands
return this information. Much of it probably seems like mumbo jumbo to
someone who does not understand modem internals, but included in that
mumbo jumbo is the modem FIR filter echo canceller estimate of the
delay to the far end modem. This is derived from echoes of the near
end signal that bounce back to the sending modem and represents the
audio circuit delay round trip for the analog modem signal. This is
distinctly different from the digital delay in sending characters to the
far end. That is always going to be a longer value due to the
processing delay in packetizing and modulating the data and sending on
the wire and reversing that process at the other end.
> ----- End of forwarded message from Dave Emery -----
>
>
> Baloney. The modem itself won't care about anything other than the
> connection from itself to the telephone switch. The noise cancellation isn't
> from modem to modem but rather modem to switch (it aggregates the modem in
> the other end at the same time). Once it hits the switch it's digitized
> anyway.
Echo (or perhaps more properly near end transmit signal)
cancellation is done for the entire talking path both to the far end and
back from the far end. Every component of this path could potentially
generate echos of the modems own transmit signal, all of which have to
be calculated from the known transmit data and subtracted out to yeild
the signal from the other side. While it is true that the purely 4
wire digital portions (virtually all US telco toll trunking these days)
of the connection do not in general generate echoes, it is not true that
echoes of the near end signal are only generated on the local loop
between the modem and the CO. They can also be generated in the local
loop between the far end CO and the far end modem and by any two wire to
4 wire bridges that exist anywhere in the talking path such as in PBX
systems or certain antique central office and toll switches.
And because of this, the modem needs to measure and model the
echo characteristics of the entire talking path, to the far end and
back from the far end so it can cancel out the transmitted signal
in order to hear the signal from the modem at the far end reliably.
And as I point out, one byproduct of this measurement is that the
modem knows the electrical audio circuit delay to the far end. You
will find it also knows how much of its signal bounces off the far end
too... These numbers are useful as an indicator of whether you are
really talking to a remote modem some distance away or to a laptop
in the wiring closet down in the basement.
There's not a damn thing that would keep somebody from doing a
> Fourier on the digitized (or analog signal for that matter if it was a
> source tap) signal (ie a 'tee') on the MITM laptop as described. It wouldn't
> be real-time but it'd be close.
>
Not clear what a FFT would do for you here. An estimate of
the complex power spectrum wouldn't tell you much, though it is somewhat
related to the data contents of the audio signal...
> My average ISDN ping time is 2.7mS from my house to my provider who is less
> than 10 miles away. ISDN is a tad faster than a modem. I've seen ping times
> in the 20 - 100mS range for modems quite often (I have two seperate dial-in
> lines on my system, US Robotics Sportster 56k & Sportster 33k).
>
If you have USR modems, by all means try sending them a ATI6 and
ATI11 command and look at what comes back. Modems add quite a significant
amount of delay to the characters going through them because of the
complexity of the layered signalling protocol they use and the actual
time it takes them to process data. ISDN uses a much simpler protocol
for sending data on the line with significantly lower latency and that
will show up in ping times. Latency through V.34 modems or V.90s seems
to run around 50 ms each way (100 ms for the ping to come back).
> ____________________________________________________________________
>
> The best lack all conviction, while the worst are full
> of passionate intensity.
>
> W.B. Yeats
>
Fitting epitaph for cypherpunks...
--
Dave Emery N1PRE, die@die.com DIE Consulting, Weston, Mass.
PGP fingerprint = 2047/4D7B08D1 DE 6E E1 CC 1F 1D 96 E2 5D 27 BD B0 24 88 C3 18
