On Tuesday, Jan 20, 2004, at 20:29 America/Denver, Jake Anderson wrote:

> my understanding is that standard phone calls run at about 64kbps data
> rate.
>
> in Australia at least the carriers only guarantee the call to 9600bps
> the improvement in modem speeds has been finding better ways to
> utilise the
> line, better modulations etc. The compression and error checking runs
> on top
> of that. I've had uploads of 30k/s on a 56k modem (which only connect
> at
> 33.6k up stream btw) due to the compression the modem was doing.

Welllll... kinda.  This is going to ramble a bit, but ride along for
the fun...

Now you've also got to take into account the history of the telco side
of things.  Analog lines up through the 1970's generally used to run
all the way back to the Central Office... perhaps getting amplifed and
echo-cancelled and all sorts of fun stuff along the way.  (Can you say
Bridge Taps, boys and girls?  Sure I knew you could!  Party Line,
anyone?)

Obviously this wasn't very efficient so Bell Labs came up with some
toys in the 60's to do A/D conversions and accurately cram those into a
single large synchronously-clocked signal in timeslices...
Time-Division Multiplexing was born!  That little thing they called a
"transistor" seems to have helped in this process.  (GRIN)

Early analog to digital stuff gave a standard phone call a 64Kb/s
timeslice in a larger synchronous circuit to cover the mathematical
requirements of Nyquist's Theorem and the desired "usable bandwidth" of
an analog circuit at the time.  Those A/D conversions would be stuffed
into timeslices of a faster synchronously-clocked circuit... the T1.
You'd put 24 X 64Kb/s in... synchronous, clocked to one end or the
other, all that fun stuff.

Other physical challenges awaited the early T1 makers... Framing bits
were "stuffed" in to keep early electronic line repeaters operating,
and in cases when too many 0's were sent in a row, an algorithm called
AMI (Alternate Mark Inversion) was added... this kept the line-powered
repeaters from losing power... yep, they stole power from the "AC" of
the ones and zeros coming down the line... wheee.  And a down loop was
always forced to send "All 1's" (which was actually "alternated" by AMI
so there was always enough power present to keep the repeaters that
weren't chopped out of the loop by a back-hoe... working!)  All-1's is
also known as "Blue Alarm".  Other alarm types were created... "Red
Alarm" was simply that the whole bloody thing had come out of sync and
crashed.  Yellow alarm was more interesting... bits from the audio path
were robbed and set to patterns (All 1's in the second most significant
bit of each 64 Kb/s timesliced frame) was an indication that something
was wrong at the far end, but that the circuit was still up and
synchronized.  (Usually an indication that the CO switch had dropped
ALL calls over the trunk, usually due to an unusually high
bit-error-rate.)

As clock sources got better, less of this bit stuffing needed to happen
as line lengths could driver further and the line-powered repeaters
slowly were removed from the network.  So, the extra framing bits were
stolen to move the end-to-end alarm bits outside of the analog portions
of the signal... and this is Expanded Super-Frame, or ESF.  Another
little trick added to keep things clocked up right, B8ZS (Bit Eight
Zero-Supression) was introduced as the "normal" way to bit-stuff an ESF
T1 or "T-span"... span being leftover AT&T long-lines terminology for
exactly that... huge spans of cable on the early telephone long
distance network.

Anyway, enough about how early T1's worked... Telcos figured out
rapidly that end-subscribers couldn't really tell the difference in
audio quality between a full 64Kb/s A/D conversion and say a 16Kb/s
conversion... especially if they did some tweaking to the power levels
of the mid-range and lows... so as time went on, they figured out how
to multiplex more and more analog sampled stuff into a single T1...
thus saving on trunking costs between Central Offices.  Less cable in
the ground, more money in their pockets.  Good for everyone.

As clock sources and oscillators and everything got better, the
available real bandwidth of an analog phone line to the home from the
home end to the far end, actually went DOWN.

Well... as they say, timing affects the outcome of the raindance.

All of these technologies collided in the Age of Modems (GRIN) when
modem manufacturers who were counting on that "standard" analog line
being able to carry certain frequencies were also competing with (but
most of the time they didn't know it) the local telco who wanted to not
put more copper in the ground in the Outside Plant.

The telco side of this worked its way out to neighborhoods in the form
of the Channel Bank.  Instead of just saving money between Central
Offices, the telco now wanted to run only a small amount of copper wire
all the way from the CO to your neighborhood.  They realized they could
do this by taking the signal to the neighborhood digitally and doing
the A/D conversion closer to your house.

They came up with a device that would take a whole bunch of analog
residential and business circuits and cram them into as few T1's as
possible, thus saving the telco money on putting more copper into the
ground as a neighborhood grew.  Even beyond the Channel Bank, some
devices started implementing protocols like GR-303 where the device in
the "field" would route individual analog lines to the Central Office
ONLY after they had gone to an off-hook state or they had an incoming
call.  The CO Switch and this "smart device" would steal a single 64K
channel to communicate with a serial packet protocol and the smart box
could then know when a call was coming in for one of thousands of
analog lines, and would connect that line to the appropriate channel
number of digital trunk(s) to the CO.  [This means that if all the
people in your neighborhood picked up the phone at the same time,
anywhere from 30-60% of you would NOT get a dial-tone if you live in an
area serviced by one of these boxes - depending on how hard the telco
tweaked the line usage algorithm in the multiplexers and remote
switches.  Some regulatory agencies oversee this percentage and don't
let it get too out of control in the telco's favor.  Ever get a
fast-busy signal IMMEDIATELY upon picking up the phone on a busy
holiday?  You're serviced by one of these gadgets then.]

The modem engineers fought back... smart engineers started coming up
with ingenious plans to use all of that big beautiful original analog
pipe not knowing that what they really had was an analog pipe for three
city blocks crammed through a SLC-96 or similar early model Channel
Bank at which point all of their headroom outside of the standard voice
frequencies was stripped to get rid of problems with good old Mr.
Nyquist's theorem... they created faster and faster modems and the
general population bit.  They wanted speed.  But then they also started
complaining.

"I never get a 56K connection!".  Yep.  And depending on where you
live, that's generally where it stands today.  If you're lucky enough
to be close to a Central Office and have copper that runs from the
analog card in the C.O. switch all the way to your house, you're
probably in the minority these days.  Surprisingly, your chances of
having this are BETTER if you're rural as long as you're not serviced
by an ANALOG CO Switch... not many of those beasts left today, but they
were common in the Western U.S. even into the late 1980's.

Suburbanites in new neighborhoods?  Forget it.  You're getting crammed
through a mux somewhere.

Stuff like V.90 is so sensitive it not only requires the end-user not
be routed through a tight mux, it also really requires the head-end
(ISP) modem bank be fed directly with digital (usually a T1 or in large
deployments a multiplexed DS-3) so the fewest number of A/D conversions
take place.

(The spec calls of ONLY ONE A/D conversion in the ENTIRE path from
end-user to modem pool for maximum performance.  If the ISP uses "V.90
modems" and you take a look in their POP and it's an analog modem bank
with a bunch of RJ11's for connectivity... beware.  You'll never get
full-data-rate out of a connection to it.  Ever.  Not physically
possible.)

Take a breath for some air if you made it this far...

Next, you start to realize this entire telco network is synchronous and
has to be clocked very accurately, and you can see clearly where many
of the wonderful advances we all see today in oscillators, "elastic
buffers" and other fun stuff came from.

You can also see how Europe's E1 standard evolved slightly later than
the T1 and how it was less expensive for telcos in Europe to "just
start with" a broader pipe that was taking advantage of the better
clock sources available at the time.

[Don't even get me started on ISDN... great ideas, way ahead of its
time... died a slow and painful death because it was too expensive to
deploy into the old network.  Oh I do love seeing it relabeled as
"iDSL" these days, though when you're too far out for regular DSL
technology so they take an ISDN chipset and run it in a raw 144Kb/s
data mode with alarming and no signalling and call it "DSL"... heh.
Awesome marketing!]

Boy we're up to geek party time now!  Whoo hoo.  Hey!  Who robbed my
bits!

The "generic" claim from most telcos that they'll only "guarantee" 9600
bps is silly -- none of the technologies they've widely deployed have
ever really stolen so much audio quality from the line in muxing them
down that 9600 is the best the line will do.  But line noise and other
contributing factors made them all confer with their lawyers and come
up with the 9600 bps claims -- it's a Cover Your Ass(ets) type of
thing.  :-)

So the circle of life goes around, and now the telco folks patch in a
DSL DSLAM card into your somewhat beleaguered little analog line and
use up that "overhead" that was always there... all that beautiful
analog copper wire bandwidth that "no one" was using -- is again in
use.  ;-)

Throw in fun like the switch from D4 signalling on T1's to ESF (D1 --
alarm bits are stolen from the second and third most significant bits
in the audio timeslices, ESF the alarm bits are moved out of the audio
frames into a header and trailer frame), etc. etc. etc.  It's all very
"evolutionary", with some of the technologies in analog clashing at the
very end of the timeline of analog telco.

Imagine if you will what happens to a sensitive analog signal like a
56K modem in the early days of such modems when D4 was prevalent... you
rob bits out and set them to ones when they're supposed to be zeros and
things sound a little different on the far end... eh?  Modem doesn't
like that so much.  (Seen it in the lab... not happy at ALL.)

Luckily D4 spans are almost a thing of the past... anyone using them
for trunking anymore should be hung up by their shoelaces and given
twenty lashes with a wet noodle... unless there's some god-awful
outside plant that still has line-powered repeaters somewhere?!  EEEEEK.

Now if I could just get Qwest convinced that I don't NEED analog telco
service to have DSL services on the line, and I could switch to using
something like Vonage (http://www.vonage.com) for any analog phone line
desires I have... life would be good.  About time to fire up the pen
and scribble off a note to the local Public Utilities Commission
stating that Qwest's rule that one must have dial-tone to have DSL is
outdated and holding back proper competition in the local-dial-tone
market!  :-)

Ain't all this stuff FUN?  ;-)

Telco geek for many years turned Unix geek, but still love telco as
it's such a cool "natural" progression of technology for 30 years...

Nate Duehr, nate@natetech.com

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