Re: 56 kbps modems

I checked the Rockwell home page, which has a pointer to the press
release.  It isn't very technical, but gives some good clues.  It
looks like they're doing an interesting trick.  The modems aren't
designed for use like traditional modems, where the same equipment is
on each end.  Instead, there is a digital interface on one side of the
phone call (like at an Internet Service Provider).  The consumer side
modem has a traditional analog interface.

The rest of this is speculation and fantasy on my part.

So, think about it.  The analog side will generate voltages and send
them to the local central office, where they will be digitized and
sent to the destination central office digitally.  There, they will be
patched into one channel of a T1 line (out of 24) and sent digitally
to the ISP's "modem bank".  Equipment is already available (and in use
all over the uunet network) that plugs T1 into a board full of digital
signal processors, decodes each of the 24 channels (each channel
running any modem signalling protocol, or ISDN), handles PPP packet
framing, and gateways the resulting packets to/from an Ethernet.

Now for Rockwell's trick, you get the DSP's in the two modems to talk
to each other.  They can run some simple coding scheme (say ordinary
2400 baud modem for this example) to pass digital data back and forth
while they're negotiating the full blown deal.  First, the analog side
sync's up with the clock for the 8000 samples/sec that the central
office is digitizing (into 8-bit samples).  You can do that by sending
one voltage and then switching to another; the far side can tell you
whether you switched on a sample-boundary or not (was there a sample
"in between" before it settled to the new value?).

OK, then, in each sample slot, the analog side can send one of 256
different voltages.  The digital side can tell it the 8-bit values it
received.  Then fine-tune that to sending 128 different voltages,
taking particular care around the ones that got distorted the first
time.  As long as you can find 128 distinct voltage levels that the
central office will reliably digitize, you're done.  You're sending
7-bit samples at 8000 samples/sec.  Do something similar for the
analog receive side, and you can start passing user data at 56K.

If the robbed-bit stuff gets in the way of seeing 128 distinct voltage
levels in every byte, you can send solid zeroes or solid ones in each
direction and see which bits they're stealing out of which bytes.
Use most of the 8 bits available in the other bytes (you can find e.g.
200 different voltage levels that will work), and in the stolen byte,
you can find e.g. 100 voltage levels that work.  This is more bits
than using 128 voltage levels in every byte, and in fact you can
probably get closer to 64 kbits/sec than to 56 kbits/sec, depending on
the analog qualities of the wire to your central office.

A nice trick!  It won't speed up analog-modem-to-analog-modem
connections, but those will increasingly be a smaller and smaller
fraction anyway, as the digital infrastructure becomes cheap.  And
of course the 56K modems will just be DSP's with decent A/D and D/A
interfaces, so they can run all the old analog protocols too, in the
case that the phone line isn't digitized, or if they want to talk to
an old modem.

	John

Forwarded:

The reason for this, Newsbytes discovered after a chat with Bill Pechey,
technical director with Hayes' European operations, is that the 56,000
bps modem system is actually a digital variant. Instead of the modem
using an audio channel of 3,600 hertz to the exchange, the modem works
across the standard copper wiring seen on most of the world's fixed wire
phone systems.

In order to achieve the 56,000 bps transmission rate, the Rockwell
chip-powered modem will actually physically control the codec at the
telephone exchange across the copper cable. According to Pechey, if a
full 4,000 hertz were available, then an ISDN (integrated service
digital network) channel of 64,000 bps would be available.

"Since the only 3,600 hertz of the audio channel is available through
the codec, we reckon that the maximum transmission speed is around
56,000 bps. Furthermore, because of the high power levels required to
achieve this transmission speed, the back channel will only operate at
standard (analog) modem speeds," he told Newsbytes.

Pechey told Newsbytes that this back channel will support data transfers
somewhere below the 28,800 bps levels, although he noted that Rockwell
claims that 28,800 bps is achievable under ideal conditions.

"This means that the 56K system is best suited for Internet access,
where the data is being transmitted mainly in one direction. For
applications such as videoconferencing, you'd be better off looking to
ISDN for a more balanced rate," he said, noting that the main advantage
of 56k technology over ISDN is the price.

"56K modem technology is cheaper than ISDN, since you don't need an ISDN
system installed. It will work across the standard phone network using a
standard phone socket," he said.

According to Pechey, because the technology involved is closer to
conventional analog modem systems than ISDN, adding 56K transmission
technology to a standard 28,800/33,600 bps modem will not be very
expensive.

"Basically you'll have a black box that will work as a normal analog
modem at 28,800 or 33,600 bps or whatever, but when accessing the
Internet, providing the distant end of the link is a digital connection,
you'll be able to use 56K in one direction, and up to 23,800 bps in the
reverse direction," he said.


--
"Of course the US Constitution isn't perfect; but it's a lot better
than what we have now."  -- Unknown.

pub  1024/C001D00D 1996/01/22  Gary Howland <gary@systemics.com>
Key fingerprint =  0C FB 60 61 4D 3B 24 7D  1C 89 1D BE 1F EE 09 06

John Gilmore sez:
> 
> 
> The rest of this is speculation and fantasy on my part.

{....}

This goes along with what others have postulated.

But I have to wonder -- how long will the [re]train take?
One of the gripes about PEP was that retrains were slow....
This sounds slower.

Further, how stable will the outcome be -- will the 'slop' in the 
CO's AD conversion overwhelm things?

Lastly, what will the RBOC's do to stifle its use? [I take it as a
given that they will object to anything that benefits subscribers
and does not give them an added cut.. witness ISDN pricing, for
example.]

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