> 56kb is a bit of hype. 33kb would connect at 33kb. I've never
> gotten a
> 56kb to hookup faster than about 44kb, and often less.
Hype is a yes/no answer. You will get it to lock on at 56k
under extremely limited conditions. 56k is considered the
"absolute limit" for POTS (plain old telephone service) because
of the 300-3000 Hertz bandwidth. 56k is possible if you are at
an ideal distance, on ideal lines and most importantly the local
telephone switch and trunks support it. Not all will, and the
limit then becomes 48k. (long story why, but rest assured that
there are important reasons why those switches limit to 48k).
Anyway, a too short distance to the telephone office also
results in lower connect speeds. You will get optimum
performance on cable length of 1-2 kilometers from the central
office or access-node.
If there are many digital data services in your cable bundle,
that also affects your connect speeds. Other people's T1s, ISDN
and ADSL will affect your voice line if you are quite a ways out
from the switch/access node.
Internal house wiring is the biggest problem, though. If you
have seven telephones around the house all sharing the same
telephone line I can assure you that your modem is going to have
to do some serious fighting just to get out of the house. The
load imbalance from the heavily "bridged" circuit wreaks havoc
with the modems. And then, if your house is like mine, you
might have this one multi-conductor telephone wire snaking
throughout the entire house being tapped from at each outlet.
NASTY! Best thing to do is isolate the cable pair going to your
modem at the point that the phone line enters the house. And
finally, DO NOT plug a telephone into the modem--make sure the
modem is "at the end of the line".
Last, but not least, if you use an ISP that does not have a
local Modem Shelf in your community, the connection is
transported over the digital trunks through multiple exchanges
(and possibly multiple phone companies and carriers) to the
location where the Modem Shelf is located. Depending on the
"Framing" used on the T1's you will get either 44k or 48k as you
maximum connect speed. It is not unusual for your modem to be
connecting to a Modem Shelf as far as 250 cable/fiber miles
away! Of course, distance doesn't matter too much since the
only analog segment is the cable-pair from your modem to the
local switch or access node in your neighborhood and everything
else is pretty much fiber-optic.
ADSL is a technology that utilizes the frequency spectrum above
4000 Hz. You install filters which are low-pass filters on your
telephones to prevent the high-pitched squeels, clicks and
squawks from killing your ears. Since ADSL uses the frequencies
above 4000 Hz and POTS uses 300-3000 Hz, the two can share the
same cable pair. The technology can provide speeds as high as
10Mbps on just a normal cable pair, but these extreme speeds are
only possible for about one mile. Usually you can get 7.5Mbps
connect speeds as far out as two miles then it drops
dramitically. 18,000 feet (6000 meters) is considered the outer
limit for DSL as the speeds really go down. At 16,000 feet my
modem is connecting at almost 6Mbps at the outside of the house,
3.5Mbps at the computer. 6000 meters is also considered the
limit because DSL requires that all line "loads" be removed from
the cable pair. These filters are placed on the cable pair to
prevent attenuation in the POTS service. Without the loads,
your telephone line is starting to get inaudible.
So, my modem connects up there in the multiple megabit range.
Does that mean that I get that kind of throughput? No. That's
where Data Network Engineers (me) come in. Typically you are
purchasing download speeds (upload speeds are 1/2) of 128 kbps,
256 kbps, 512 kbps, 768 kbps, 1024 kbps and multiples of 1024.
Since the policies are "Up to, but not sustained at), we
oversubscribe the network backbones many times over. My ATM
network is humming along quite nicely at the equivelent of 100x
oversubscription. In otherwords, I have 100 one meg subscribers
sharing every one meg of bandwidth. This is actual performance
statistics, not conjecture or marketing hype. We like to keep
things around the 15-25x area though.
Ag-Schnozz
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