sbrodsky@hawk.CS.ULowell.Edu (Tabu) (10/23/87)
Can somebody please clarify something for me? I have been told for the past 2 years that the max speed possible on regular phone lines was 9600 baud. Now I am being told that 19K is possible, and something about USRobotics now has an upgrade for their 9600 HST to upgrade it to 17K. What is the highest speed available?????? Thanks.... Scott A. Brodsky, University of Lowell CS dept. sbrodsky@hawk.ulowell.edu ----------------------------------------------------------
dmw3@ur-tut.UUCP (David Walsh) (10/24/87)
In article <1886@ulowell.cs.ulowell.edu> sbrodsky@hawk.ulowell.edu (Tabu-Fiero builder) writes: > > Can somebody please clarify something for me? > [lines deleted] > > What is the highest speed available?????? > Thanks.... > >Scott A. Brodsky, University of Lowell CS dept. >sbrodsky@hawk.ulowell.edu Well, I haven't heard anything about it lately but I do remember a high speed modem that was capable of 50kbaud over good phone lines. All I can remember is that it us multiple frequencies at the same time and it decided how many bits it could send on each frequency, something like 1, 2, 4, or 8 bits (I'm not sure about all of these) at the time of original connection; there was something about them that they couldn't "talk" with other modems because of the way they sent all of that data. If someone else has ever heard of these I'd love to find out who makes them, or where I could get in contact with someone who knows. Sorry that I can't remember the name of the manufaturer. Dave Walsh - hiding @ rochester!ur-tut!dmw3 DISCLAIMER: No sane individual condones my acts, therefore my employers can't be held responsible for me or my actions (or can they???)
WANCHO@SIMTEL20.ARPA ("Frank J. Wancho") (10/27/87)
The highest "speed" possible on conventional dialup lines is 1200 baud. That figure is due to the limited bandwidth of the phone lines when non-metallic circuits are involved. However, the highest bit rate possible is a function of the bit-encoding technique used to squeeze more bits per baud using analog technology. With digital technology and short haul point-to-point metallic circuits, you can expect 10MBps (twisted-pair Ethernet). Someday soon (in this century I hope) the phone system will be completely digital and you can expect to run at least one 56Kbps circuit, plus voice... --Frank
ron@topaz.rutgers.edu.UUCP (10/29/87)
Foo. Note that twisted pair Ethernet is a kludge. This high-speed (Radio Frequency) unshielded twisted pair stuff is going to be put to an end as this stuff is not going to comply with FCC regulations on emissions. -Ron
larry@kitty.UUCP (10/29/87)
In article <16064@topaz.rutgers.edu>, ron@topaz.rutgers.edu (Ron Natalie) writes: > Foo. Note that twisted pair Ethernet is a kludge. This high-speed > (Radio Frequency) unshielded twisted pair stuff is going to be put > to an end as this stuff is not going to comply with FCC regulations > on emissions. Actually, I don't believe that the FCC can do much about it. The Ethernet transceivers certainly meet FCC emission requirements when used in a proper manner with coaxial cable (as intended). The problem comes to pass when people buy these $ 25.00 balun transformers which match coaxial cable to balanced line, and run them over unshieled telephone pairs. The balun transformers themselves are just passive devices which need meet no FCC requirements of any kind; I can't envision their sale being "outlawed" or otherwise controlled. Wanna see something neat? Try running two separate Ethernet circuits any significant distance in multi-pair telephone cable using these balun transformers. <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"
larry@pdn.UUCP (Larry Swift) (10/29/87)
In article <309@ur-tut.UUCP> dmw3@tut.cc.rochester.edu.UUCP (David Walsh) writes: >speed modem that was capable of 50kbaud over good phone lines. All I can ^^^^ I suspect that you mean "bits". >remember is that it us multiple frequencies at the same time and it decided >how many bits it could send on each frequency, something like 1, 2, 4, or 8 ^^^^^^^^^ I think you mean "baud". >bits (I'm not sure about all of these) at the time of original connection; >there was something about them that they couldn't "talk" with other modems >because of the way they sent all of that data. This is true of many higher speed modems today, although some are compatible with other manufacturers, and some are not. I haven't any idea who might be making (attempting to make?) a 50kbps modem for public switched anlog circuits (ie, phone lines). (If it's my employer, forget you read this.) Have you tried asking various manufacturers reps? Larry Swift UUCP: {gatech,codas,ucf-cs}!usfvax2!pdn!larry Paradyne Corp., LF-207 Phone: (813) 530-8605 P. O. Box 2826 Largo, FL, 34649-9981
kaufman@Shasta.STANFORD.EDU (Marc Kaufman) (10/29/87)
In article <2174@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes: .In article <16064@topaz.rutgers.edu>, ron@topaz.rutgers.edu (Ron Natalie) writes: .> Foo. Note that twisted pair Ethernet is a kludge. This high-speed .> (Radio Frequency) unshielded twisted pair stuff is going to be put .> to an end as this stuff is not going to comply with FCC regulations .> on emissions. . . Actually, I don't believe that the FCC can do much about it. The .Ethernet transceivers certainly meet FCC emission requirements when used .in a proper manner with coaxial cable (as intended). If you can demonstrate interference to some other service, the FCC will write a letter to the interferer requiring them to take action to stop the interference. This may include turning the interfering source off. (I had a neighbor who refused to turn off a 'Decimate' bug repeller that wiped out the local public service radio band). The letter is sufficiently threatening that it will probably work. Marc Kaufman (kaufman@Shasta.stanford.edu)
brian@casemo.UUCP (Brian Cuthie ) (10/29/87)
In article <16064@topaz.rutgers.edu>, ron@topaz.rutgers.edu (Ron Natalie) writes: > Foo. Note that twisted pair Ethernet is a kludge. This high-speed > (Radio Frequency) unshielded twisted pair stuff is going to be put > to an end as this stuff is not going to comply with FCC regulations > on emissions. > > -Ron I doubt seriously that DEC would announce a major developement such as this and start selling it before they were sure that it would pass FCC part 15. Besides, you would be suprised at what *doesn't* get radiated from a balanced transmission over twisted pair. Cheers, Brian ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Brian Cuthie Naturally, these are my opinions and no one CASE Communications else's. Columbia, MD 21046 (301) 290 - 7443
ron@topaz.rutgers.edu.UUCP (10/29/87)
If you set up equipment such that it radiates you are liable regardless of whether the equipment you use has or needed to have complied with FCC regulations. All this RF jumbled in with ordinary telephone wire is a problem just waiting to happen. -Ron
miw@uqcspe.OZ (Mark Williams) (10/30/87)
In article <WANCHO.12345854244.BABYL@SIMTEL20.ARPA> WANCHO@SIMTEL20.ARPA ("Frank J. Wancho") writes: >The highest "speed" possible on conventional dialup lines is 1200 >baud. ... > However, the highest bit >rate possible is a function of the bit-encoding technique used to >squeeze more bits per baud using analog technology. Almost, but not quite. The V.32 standard for 9600 bps modems actually specifies 2400 baud. The highest bit rate possible is purely a function of bandwidth and signal to noise ratio. The bandwidth of a normal voice circuit is 3 khz. This is ensured by anti-aliasing filters on the local loop. These are needed to allow the analog signal on your local loop to be converted to a 64kbps digital stream. Now to work out the best possible signal/noise ratio. In practice, you just take this from the guaranteed figure as given by your friendly telco. For example, if your S/N is 40 dB, the maximum possible bit rate is given by: Bandwidth*(Log base 2(1+S/N)) = 3000* (Log base 2(1+100)) (S/N in power ratio, not dB) = 19931 bps You will, of course, never achieve this, since it assumes that the S/N is constant and that you use the full dynamic range, and that you use the best possible modulation technique. Mark Williams -- The views expressed above are not necessarily those of my employer. In a couple of hours they may not even be my own. Small boys throw stones in fun, but the frogs die in ernest. -- Mark Twain.
pavlov@hscfvax.UUCP (840033@G.Pavlov) (10/30/87)
In article <309@ur-tut.UUCP> dmw3@tut.cc.rochester.edu.UUCP (David Walsh) writes: >speed modem that was capable of 50kbaud over good phone lines. All I can >remember is that it us multiple frequencies at the same time and it decided >how many bits it could send on each frequency, something like 1, 2, 4, or 8 >bits (I'm not sure about all of these) at the time of original connection; >there was something about them that they couldn't "talk" with other modems >because of the way they sent all of that data. - this sounds like a somewhat garbled (no insult intended !) description of the Telebit Trailblazer modem. It has been discussed here before, so I won't repeat the specs, other than to say that it's capability is apx. 18 kbuad max. greg pavlov, fstrf, amherst, ny
peter@julian.UUCP (10/30/87)
In article <16064@topaz.rutgers.edu> ron@topaz.rutgers.edu (Ron Natalie) writes: >Foo. Note that twisted pair Ethernet is a kludge. This high-speed >(Radio Frequency) unshielded twisted pair stuff is going to be put >to an end as this stuff is not going to comply with FCC regulations >on emissions. Synoptics' Lattisnet has been approved by the FCC. They told us that they worked hard in order to comply. It also works and despite its initial expense it looks as if it would be much more easily maintainable than "real" ethernet. Also would Digital and 3-Com and HP really announce a product that they could not sell in the USA? -- Peter Marshall, Data Comm. Manager CCS, U. of Western Ontario, London, Canada N6A 5B7 (519)661-2151x6032 pm@uwovax.BITNET; pm@uwovax.uwo.cdn; peter@julian.uucp; ...!watmath!julian!peter
smv@necis.UUCP (Steve Valentine) (10/31/87)
In article <2174@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:
+ Wanna see something neat? Try running two separate Ethernet
+circuits any significant distance in multi-pair telephone cable using
+these balun transformers.
+
+<> Larry Lippman @ Recognition Research Corp., Clarence, New York
The first implementation of an etherealnet gateway!
--
Steve Valentine, NEC Information Systems 289 Great Rd., Acton, MA 01720
smv@necis.nec.com
Life is like a movie, but you have to be Shirley Maclaine to see the sequel.lyndon@ncc.UUCP (Lyndon Nerenberg) (11/02/87)
In article <2174@kitty.UUCP>, larry@kitty.UUCP (Larry Lippman) writes: > > Actually, I don't believe that the FCC can do much about it. The > Ethernet transceivers certainly meet FCC emission requirements when used > in a proper manner with coaxial cable (as intended). > The problem comes to pass when people buy these $ 25.00 balun > transformers which match coaxial cable to balanced line, and run them over > unshieled telephone pairs. The balun transformers themselves are just > passive devices which need meet no FCC requirements of any kind; I can't > envision their sale being "outlawed" or otherwise controlled. Owning the parts is not illegal. Using them to build an radio transmitter might be. Part 97 of the FCC regs pertains to permissable field strength for unlicensed transmitters. It's not too generous (how far does your kids walkie-talkie transmit). To be legal, you would have to ensure that the field strength falls within the regs. I believe Part 97 also specifies the ranges of frequencies you're allowed to use. You could probably get away with it, but if someone complains about interference you're SOL as far as the FCC is concerned. > Wanna see something neat? Try running two separate Ethernet > circuits any significant distance in multi-pair telephone cable using > these balun transformers. Wanna see something *really* neat? Turn on an electric pencil sharpener within about 10 feet of this wiring and watch your "Ethernet" die... --lyndon VE6BBM {alberta,pyramid,uwvax}!ncc!lyndon
eshop@saturn.ucsc.edu (Jim Warner) (11/03/87)
In article <138@ncc.UUCP> lyndon@ncc.UUCP (Lyndon Nerenberg) writes: >In article <2174@kitty.UUCP>, larry@kitty.UUCP (Larry Lippman) writes: >> >> The problem comes to pass when people buy these $ 25.00 balun >> transformers which match coaxial cable to balanced line, and run them over >> unshieled telephone pairs. The balun transformers themselves are just >> passive devices which need meet no FCC requirements of any kind; I can't >> envision their sale being "outlawed" or otherwise controlled. Questions of legality not withstanding, could someone explain how these things work? I thought Ethernet required low frequency response in order for collision detection to work. A collision is declared when the average voltage on the cable gets too negative. How do you get DC through a balun? On the other hand, if these are AUI (transciever cable) extensions, why would a balun be necessary at all? The impedance of phone wires isn't that far from the impedance of shielded twisted pairs, so what's to match?
bitar@ji.Berkeley.EDU (Philip Bitar) (11/03/87)
In article <1641@uqcspe.OZ> miw@uqcspe.oz (Mark Williams) writes: > ... The highest bit rate possible is purely a function of bandwidth and >signal to noise ratio. ... Then what is a `baud' and why should anyone ever bother referring to it? Also, for bit rates x > 2400, how is compatibility among different manufacturers acheived?
W8SDZ@SIMTEL20.ARPA (Keith Petersen) (11/03/87)
[From SIMTEL20 file PD:<MISC.MODEMS>WHATBAUD.DOC]
What is Baud?
Anyone reading technical echomail may have noticed a large
number of messages about baud rates, bit rates, band widths, and
modulation techniques involved with modems. There is a fair
amount of confusion relating to baud rate verses bit rate and how
they are limited by the telephone line band width. This is
nothing new, texts on the subject generally avoid the term "baud"
except within the narrow context where it is germane. This
article will define some of the various terms used in data
communications, and discuss limitations of phone lines to
communication speed.
Serial transmission of data is the most common method of
moving data over distance, and the most common way of interfacing
serial devices to each other is RS232. The essence of RS232 is
signal levels which represent ON or SPACE levels, and OFF or MARK
levels. ON is any voltage between +3 volts and +15 volts while
OFF is -3 to -15 volts. The signal shape is really a square wave
centered at zero volts and is a baseband signal. A baseband
signal is one whose spectrum extends down to zero hertz, or near
zero. The signal is polled at regular intervals to determine its
voltage, and therefore the data it contains. RS232 has only the
two voltage levels defined so it is a binary coded signal.
Besides signal shape and levels, RS232 defines a number of
parallel signals such as "clear to send" and "data terminal
ready". Some of these signals are status flags such as "carrier
detect", while others are meant as flow control, such as "data
terminal ready" or "request to send". Since a typical modem
provides only the transmit and receive functions, the flow
control must be done within the data stream. Some modems however
send data as packets with error control (akin to XMODEM) that can
recreate all of the hardware signals so as to make a distant
terminal appear to be hard wired.
Regular phone lines were designed for voice communications,
but due to their wide use and therefore low cost, they have been
widely used for data communications. A baseband signal such as is
found in RS232 doesn't lend itself to phone lines since they
don't have frequency response down to D.C. or zero hertz. When
voice grade phone lines were designed the band of frequencies
they had to pass was determined by the nature of human speech.
Very low frequencies (below 300 hz) and higher ones (above
3000hz) were found to be unnecessary for voice recognition at the
receiving end, as a result the total "BAND WIDTH" available to a
phone user is only around 2700 hz. BAND WIDTH when used to
describe frequency response is the difference in hertz between
the high and low, half power frequencies. In electrical systems,
power dissipated across a load (or resistance) in terms of
voltage is: V * V / R. Where "V" in this case will be the
amplitude of the sign wave being applied to the circuit. In
between the high and low half power frequencies there will be a
frequency where the amplitude of the response wave will be
maximized, call that voltage Vm. The half power points will be
reached when V=Vm/sqrt(2). The output power at that point would
be Vm * Vm / (2 * R) which is one half the mid band power. The
cause for the fall off of power at different frequencies is due
to capacitive and/or inductive elements in the circuit. In phone
lines capacitance comes naturally in parallel, that is it tend to
shunt the signal to ground. In this configuration the higher the
frequency the lower the "resistance" will be. Inductance is added
on purpose by the phone companies in the form of loading coils
which are added to decrease signal attenuation in the mid
frequencies. In any case the band width of a voice grade line is
strictly limited so that many calls can be stacked on top of each
other, in order to use transmission lines more effectively. Data
equipment must strive to make the best use of this narrow band
width. There is a hard limit to the amount of data that can be
sent through a telephone line as will be seen later.
The signal type of choice to carry data through phone lines
is the sine wave. A sine wave has but one frequency associated
with it. This means that if its frequency is within th band width
of the line carrying it, then the received sine wave will not
have its shape altered due to clipping of the high frequency
components. This can't be said for the square or triangular
waves which requires an infinite band width to fully describe
them. The sine wave is simply defined:
v= A * sin( w*t + p)
A is the amplitude (in units of volts for this exercise)
w is the frequency which must be in radians per second
t is time in seconds
p is the phase angle in radians
Information can be encoded by the sine wave in three
different ways. By altering "A" which is amplitude modulation
(AM), by altering "w" which is frequency modulation (FM), or by
altering "p" which is phase modulation (PM). There are mixed
modes which come into use in the more exotic modem schemes, used
to get the really high bit rates.
FSK or frequency shift keying was the most widely used
method for data transfer through modems. This is a FM process
whereby a RS232 MARK would be represented by the presence of one
frequency while a SPACE would be indicated by another frequency.
Now is the time to introduce the term BAUD. A "BAUD" is the time
interval in which data is carried, that is the minimum time in
which a signal holds a single state which the receiver is to
recognize and convert into data. The baud rate is how many BAUD
times occur in a second. The baud rate is NOT the same as the bit
rate as will be seen later in multiple state modulation. In FSK
type modems (Bell 103) the baud time happens to be the same as
the bit rate since each signal state encodes a single bit. Many
would think that the baud rate is limited to highest frequency
available to be transmitted (3000 hz) but this is not necessarily
so. Nyquist showed in 1928 that the maximum signal change rate
(baud rate) for a band width "W" would be 2*W baud. This is
called the Nyquist rate and is an upper limit that assumes no
inter-symbol interference. This could be visualized by
considering a sine wave. Each cycle has a positive and negative
part. The amplitude in each part could be independently altered
while still having a sine wave, therefore a 1200 hz signal could
be changed 2400 times a second which is 2400 baud. This type of
change (AM) is not very useful in phone lines since it is the
type of change most commonly caused by natural phenomena. In Bell
103 the baud rate is commonly 300 which is over 3.5 cycles at the
lowest carrier frequency. The practical considerations of
detecting frequency changes requires about 1.5 cycles so 300 baud
is some what conservative.
PSK or phase shift keying gets by the frequency barrier that
keeps FSK from producing the higher bit rates. In PSK (a PM
method) the only parameter in the sine wave changed each BAUD is
the phase angle "p". The Bell 212A modem specification uses a
four level phase modulation technique. The term four level means
that four different phases are used and detected by the receiver
during each BAUD. The number of bits an M level state can
represent is n=log2(M) or log(M)/log(2). A four level state can
represent two bits during each baud. In the 212A specification
+90 degrees would be 00, 0 degrees 01, +270 degrees 11, and +180
degrees 10. The 212A baud rate is 600 which means the bit rate
would be:
bit rate=600 BAUD/second * 2 bits/BAUD= 1200 bits/second
To increase the bit rate to the next state would require 3
bits/baud or 8 phases. To get 2400 bps using such a scheme would
require 16 different phases (22.5 degrees apart). The problem
here is that one class of noise present in phone lines called
"phase jitter" can cause phase errors up to and sometimes over 30
degrees. To avoid the bulk of this type of noise the phase angle
difference should be kept above this amount. The 2400 baud modems
common today use a form of QAM (quadrature amplitude modulated).
This method uses a combination of two waves with different
amplitudes to get the required 16 levels. A QAM signal can be
expressed in equation as follows:
s(t)=a(t)cos(wt+p)+b(t)cos(wt+90+p)
a(t) is the in-phase modulating wave form while b(t) is the
quadrature modulating wave form. During each baud a(t) and b(t)
are constants so the equation simplifies to:
s(tn)=c*cos(wt+theta+p) {p is a single arbitrary phase angle tn
stands for a specific baud time
c=sqrt(a*a+b*b) theta=atan(b/a)}
The new pure sine (cosine) wave has amplitude and phase
differences which can multiply the number of states possible. If
there are 4 values for amplitude and 8 values for phase then the
signal could have 32 different states or five bits. The random
variations in phase "p" has less effect the outcome since it
effects both waves the same. The phase difference in the
resultant wave is due to the interaction of the two wave forms.
QAM is the method used to get up to 9600 baud out of a phone
line. Other types of noise are present in a phone system, and are
mostly due to switching and cross over with other wires. The
error rate at the higher bit rates would be unacceptable if there
weren't some error recovery used. This is now quite easy to do
from a hardware standpoint, since the memory and processing power
needed to do it takes a small amount of space and cost little
enough to make the increase in bit rate worth it. The ability
for high speed modems to run will increase due to another reason.
More and more fiber optic phone line will replace conventional
ones. These are immune from many of the noise sources that effect
copper wires, sources such as RF and magnetic fields.
The ultimate bit rate that could be "pumped" through a phone line
is fixed by the band width AND the signal to noise ratio.
Shannon's law relates random bit transmissions/second to band
width and signal to noise ratio. It is derived from the concept
of entropy. Entropy is a measure of randomness in a system. It is
really a thermodynamic property but has applications in
information theory. The maximum bit rate for a channel with
signal power S and noise power N is given by:
C=BW log2(1+S/N) where BW is the band width, S is the signal
power and N is the noise power.
With a typical band width 2600 hz and a typical signal to noise
ratio of 30db (or 1000/1) the bit rate would be:
bit rate=2600 * log2( 1001 ) or 25,914bps
The noise factor is assumed white or Gaussian. This kind of noise
is unavoidable in electrical systems. In fact "N" can be
calculated by:
N=kTW where k=1.37e-23 joule/degree
T is absolute temperature (Kelvin)
W is the band width in HZ
This product gives "N" in terms of joules per second or watts.
At room temperatures noise would be on the order of 1e-17 watts
over a 2600 hz band width. If a phone signal were just 0.1 watts
the signal to noise ratio could be as high as 160db. In practice
it is about 30db so it can be expected that the signal to noise
ratio will increase in the future so that the top bit rates will
increase, and this increase will happen without an increase of
the band width available.
One last problem to consider with the high speed modems is
compatibility. To get 9600 bps from a 600 baud signal would
require 65,536 levels in a state. There is no obvious way in
which to assign a level to a 16 bit pattern so the manufacturer
must invent an "ALPHABET" for that conversion. Until an alphabet
is standardized as well as error recovery techniques there is
simply no way the modems will talk to each other.
GLOSSARY:
Alphabet: A table to convert signal states into
characters they represent
Amplitude modulation: Where information is encoded by changes in
amplitude only.
Band width: Range of frequencies within the half power
limits. That is the difference between the
two -3db frequencies.
Baseband: The signal at its original frequency and
shape.
Baud: The minimum time where all signal
parameters are held constant.
Baud rate: The number of times the basic signal can be
changed per second.
Bit rate: The number if bits per second passing
through a channel. In a modem it is the
baud rate times the number if bits per
baud.
Decibel (db): 10 * log10(p2/p1) where p2 is referenced to
p1. Both p1 and p2 represents power. For
voltage db is calculated 20 * log10(v2/v1)
Frequency modulation: Where information is encoded by changes in
the carriers frequency.
FSK: A way of represents data by a discrete
change in frequency of the carrier.
Hertz: Number of events per second.
Modulation: The process of varying a signal according
some aspect of another signal.
Phase: The angular displacement of a cyclic signal
In a sine wave Y=A sin(wt+p) p is the
phase.
Phase modulation: Technique of changing phase of signal to
represent changes of the baseband signal.
References:
1.*Clark, A.P., 'Principles if Digital Data Transmission' 2nd ed.
(1983)
2. Martin, James, 'Telecommunications and the Computer', (1979)
3. 'IBM PC Technical Reference', (July 1982)
4. 'Hayes Smartmodem 1200 manual'
*Reference (1) gives the most technical detail and over 500 other
specific references.
-----------------------------------------------------------------larry@kitty.UUCP (Larry Lippman) (11/03/87)
In article <1070@saturn.ucsc.edu>, eshop@saturn.ucsc.edu (Jim Warner) writes: > Questions of legality not withstanding, could someone explain how these > things work? I thought Ethernet required low frequency response in > order for collision detection to work. A collision is declared when > the average voltage on the cable gets too negative. How do you get > DC through a balun? The balun products on the market which appear to work for Ethernet employ transformers with split primary and secondary windings about a toroid core (i.e., a total of four windings). Add a bit of resistor and capacitor madness, and you have something which not only attempts to match single-ended coaxial cable to balanced pair, but which also passes DC. In the case of Ethernet, these devices are extremely prone to ground-loop current problems. I never implied these baluns were a marvel of electrical engineering; they are a Kluge with a capital K. But they do work, more or less, over short distances. *I* would never use one of these gadgets, but one of our more "enterprising" clients took it upon themselves to use 'em because their plant engineer was too lazy to fish coaxial cable through floor duct; since there was several hundred feet involved (which is no longer a "short" distance for this balun business), the results were, as expected, disastrous. If my memory is correct, these baluns first started out for short length 327X applications, and then people became bolder and tried Ethernet. <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"
lyndon@ncc.UUCP (Lyndon Nerenberg) (11/05/87)
As several people were kind enough to point out, unlicensed transmitter operation is covered by Part 15 of the FCC regs. Part 97 addresses Amateur Radio. (What, ME biased? :-) --lyndon VE6BBM
pozar@hoptoad.UUCP (11/06/87)
In article <138@ncc.UUCP> lyndon@ncc.UUCP (Lyndon Nerenberg) writes: > >Owning the parts is not illegal. Using them to build an radio transmitter >might be. Part 97 of the FCC regs pertains to permissable field strength >for unlicensed transmitters. It's not too generous (how far does your >kids walkie-talkie transmit). To be legal, you would have to ensure that >the field strength falls within the regs. I believe Part 97 also specifies >the ranges of frequencies you're allowed to use. > Almost. Part 15 describes "Radio Frequency Devices". This Part covers unlicensed devices that emit RF. This covers every- thing from your kid's Handie Talkies, to Television tuners, to Radio Recivers, to Field Disturbance Sensors,to Computers, to Radio Thermy machines. Part 97 covers Amateur Radio. If you are trying to design devices that have the potential to radiate Radio Energy (computers and the like) I would HIGHLY suggest getting part 15. You can purchase it from your local Federal Building Bookstore. In San Francisco (for those up this neck of the woods) the building is on Golden Gate and Polk. >You could probably get away with it, but if someone complains about >interference you're SOL as far as the FCC is concerned. > >> Wanna see something neat? Try running two separate Ethernet >> circuits any significant distance in multi-pair telephone cable using >> these balun transformers. > >Wanna see something *really* neat? Turn on an electric pencil sharpener >within about 10 feet of this wiring and watch your "Ethernet" die... > >--lyndon VE6BBM {alberta,pyramid,uwvax}!ncc!lyndon -- ======================================================================= | ...sun!hoptoad!\ Tim Pozar | | >fidogate!pozar Fido: 1:125/406 | | ...lll-winken!/ PaBell: (415) 788-3904 | | USNail: KKSF 77 Maiden Lane San Francisco CA 94108 | =======================================================================
bitar@ji.Berkeley.EDU (Philip Bitar) (11/07/87)
Thanks to Keith Petersen for a detailed explanation of baud rate. I would like to ask several more questions. 1. Baud rate From his msg: >... The 2400 baud modems >common today use a form of QAM (quadrature amplitude modulated). >... >QAM is the method used to get up to 9600 baud out of a phone >line. ... It is clear from the context that 2400 bps and 9600 bps are meant, where both bit rates are obtained using a baud rate of 600. Are baud rates other than 600 used? If so, this would introduce another dimension of incompatibility, along with the alphabet or representation problem mentioned in the msg. 2. Frequency, amplitude Under QAM, frequency and amplitude could also be incompatible (unless there is automatic detection of them), so am I correct in assuming that these are standardized at 600 baud? -- And that the phase jitter of 30 degrees refers to signals at the respective frequency and amplitudes?
brian@casemo.UUCP (Brian Cuthie ) (11/13/87)
woops !! I meant to say, in my previous posting (use 'p' key) that the baud rate is also constrained by the bandwidth of the transmission medium (obviously). So, please, no flames :-) -Brian ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Brian Cuthie CASE Communications Columbia, MD 21046 (301) 290 - 7443