Telephone line no noise

A

Allen Bong

Guest
Why are good telephone lines running miles of cables don't pick up much
noise along it's way to the telephone while it is hard to send data between
two computers when they are a few hundred feet apart? What's the catch?

Allen



----------------------------
The next war will determine NOT who is right BUT what is left.
 
"Baphomet" (fandanospam@catskill.net) writes:
"Allen Bong" <sfbong@tm.net.my> wrote in message
news:3f5579d5$1_2@news.tm.net.my...
Why are good telephone lines running miles of cables don't pick up much
noise along it's way to the telephone while it is hard to send data
between
two computers when they are a few hundred feet apart? What's the catch?

Allen -

I'm a bit out of my depth but let's give it a whack anyway. Convention
copper telephone lines run 600 ohm balanced, and are voice bandwith limited.
Computer lines are unbalanced with respect to ground, and have high
bandwidth to accomodate the very fast rise and fall times of digital
signals. If I've stepped in "it" with this response, please accept my
apologies :)





And of course, voice is terribly redundant. Miss a microsecond here
Click to expand...
or there, and likely the receiver will still understand what is said.
Even a whole word can be missed (provided it's not a key word) and
the message can still be understood.

Get a bit of noise on a digital line, and it can throw off significant
bits of data. Unless extra data and extra error checking is in place,
you can't make up for messed data.

Michael
 
"Allen Bong" <sfbong@tm.net.my> wrote in message
news:3f5579d5$1_2@news.tm.net.my...
Why are good telephone lines running miles of cables don't pick up much
noise along it's way to the telephone while it is hard to send data
between
two computers when they are a few hundred feet apart? What's the catch?
Click to expand...
Allen -

I'm a bit out of my depth but let's give it a whack anyway. Convention
copper telephone lines run 600 ohm balanced, and are voice bandwith limited.
Computer lines are unbalanced with respect to ground, and have high
bandwidth to accomodate the very fast rise and fall times of digital
signals. If I've stepped in "it" with this response, please accept my
apologies :)
 
"Baphomet" <fandanospam@catskill.net> wrote in message
news:vlc35iiictp08c@corp.supernews.com...
"Allen Bong" <sfbong@tm.net.my> wrote in message
news:3f5579d5$1_2@news.tm.net.my...
Why are good telephone lines running miles of cables don't pick up much
noise along it's way to the telephone while it is hard to send data
between
two computers when they are a few hundred feet apart? What's the catch?

Allen -

I'm a bit out of my depth but let's give it a whack anyway. Convention
copper telephone lines run 600 ohm balanced, and are voice bandwith
limited.
Computer lines are unbalanced with respect to ground, and have high
bandwidth to accomodate the very fast rise and fall times of digital
signals. If I've stepped in "it" with this response, please accept my
apologies :)
Click to expand...
You don't have to apologise for anything. I think you explained it very
well. The catch is the "low bandwidth of the telephone line" and the "600
ohm balanced circuit" design and the "tolorance allowed by the receiving
end". What is the actual bandwidth allowed by a typical telephone line?

If I reduce the baud rate of the transmission and use balanced protocol like
RS422/RS485 would I be able to go a few hundred feet between 2 computers?

Allen.

 
Allen Bong wrote:
"Baphomet" <fandanospam@catskill.net> wrote in message
news:vlc35iiictp08c@corp.supernews.com...

"Allen Bong" <sfbong@tm.net.my> wrote in message
news:3f5579d5$1_2@news.tm.net.my...
Why are good telephone lines running miles of cables don't pick up much
noise along it's way to the telephone while it is hard to send data
between
two computers when they are a few hundred feet apart? What's the catch?

Allen -

I'm a bit out of my depth but let's give it a whack anyway. Convention
copper telephone lines run 600 ohm balanced, and are voice bandwith
limited.
Computer lines are unbalanced with respect to ground, and have high
bandwidth to accomodate the very fast rise and fall times of digital
signals. If I've stepped in "it" with this response, please accept my
apologies :)


You don't have to apologise for anything. I think you explained it very
well. The catch is the "low bandwidth of the telephone line" and the "600
ohm balanced circuit" design and the "tolorance allowed by the receiving
end". What is the actual bandwidth allowed by a typical telephone line?

If I reduce the baud rate of the transmission and use balanced protocol like
RS422/RS485 would I be able to go a few hundred feet between 2 computers?

Allen.
Click to expand...
Yes, RS-422 will go several thousand feet at a low Baud rate, if the
line is properly terminated. Check the National Semiconductor website
for application notes on RS-422.

http://www.national.com/ is the home page.

http://www.national.com/apnotes/ is the search page for their
application notes.

http://www.national.com/apnotes/apnotes_all_1.html lists all their
application notes.

Here is a list of serial communications related application notes
available from National Semiconductor.

AN-214
AN-409
AN-438
AN-457
AN-702
AN-759
AN-806
AN-807
AN-808
AN-914
AN-916
AN-979
AN-1031
--


Michael A. Terrell
Central Florida
 
Differential drivers and receivers are readily available and cheap that will
reduce line induced noise by a hundred , a thousand or more times.(+/- .2
volt time 100 is alot of noise!!) Your friends needs would not be hard to
accomplish. Do a google search for balanced lines, common mode rejection
etc.
bg
Allen Bong wrote in message <3f569b4b_2@news.tm.net.my>...
"Michael Black" <et472@FreeNet.Carleton.CA> wrote in message
news:bj539c$1v9$1@freenet9.carleton.ca...
"Baphomet" (fandanospam@catskill.net) writes:
"Allen Bong" <sfbong@tm.net.my> wrote in message
news:3f5579d5$1_2@news.tm.net.my...
Why are good telephone lines running miles of cables don't pick up
much
noise along it's way to the telephone while it is hard to send data
between
two computers when they are a few hundred feet apart? What's the
catch?

Allen -

I'm a bit out of my depth but let's give it a whack anyway.
Convention
copper telephone lines run 600 ohm balanced, and are voice bandwith
limited.
Computer lines are unbalanced with respect to ground, and have high
bandwidth to accomodate the very fast rise and fall times of digital
signals. If I've stepped in "it" with this response, please accept my
apologies :)





And of course, voice is terribly redundant. Miss a microsecond here
or there, and likely the receiver will still understand what is said.
Even a whole word can be missed (provided it's not a key word) and
the message can still be understood.

Get a bit of noise on a digital line, and it can throw off significant
bits of data. Unless extra data and extra error checking is in place,
you can't make up for messed data.

Michael,

My friend wants to design a communication system using voltage only to
represent 24 states (signals) using some kind of DAC. Using a difference of
1 volt to represent each state...1 volt=state A, 2 volt=state B....etc for
a
total of 24 states. The receiving end would convert the voltage back to
binary using an ADC. The distance between the Host and Slave is 50 metres
and no handshaking is available to indicate an error. The margin of the
voltage is set to +/- 0.2 volt. ie 0.8V to 1.2V for state A.

Would this type of setup work? I guess it wouldn't work but it is hard for
me to explain WHY it wouldn't work to my friend. It reminds me of the
analogue computers back in the 60s where they use Op-Amp to make computers.

Allen
Click to expand...
 
Thank you very much Michael. Now I know I have enough evidence to convince
my friend. And thanks to those who teach me the differences between a
telephone line and a digital line.

Allen.

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:3F56A429.744F2D86@earthlink.net...
Allen Bong wrote:

"Baphomet" <fandanospam@catskill.net> wrote in message
news:vlc35iiictp08c@corp.supernews.com...

"Allen Bong" <sfbong@tm.net.my> wrote in message
news:3f5579d5$1_2@news.tm.net.my...
Why are good telephone lines running miles of cables don't pick up
much
noise along it's way to the telephone while it is hard to send data
between
two computers when they are a few hundred feet apart? What's the
catch?

Allen -

I'm a bit out of my depth but let's give it a whack anyway.
Convention
copper telephone lines run 600 ohm balanced, and are voice bandwith
limited.
Computer lines are unbalanced with respect to ground, and have high
bandwidth to accomodate the very fast rise and fall times of digital
signals. If I've stepped in "it" with this response, please accept my
apologies :)


You don't have to apologise for anything. I think you explained it very
well. The catch is the "low bandwidth of the telephone line" and the
"600
ohm balanced circuit" design and the "tolorance allowed by the receiving
end". What is the actual bandwidth allowed by a typical telephone line?

If I reduce the baud rate of the transmission and use balanced protocol
like
RS422/RS485 would I be able to go a few hundred feet between 2
computers?

Allen.

Yes, RS-422 will go several thousand feet at a low Baud rate, if the
line is properly terminated. Check the National Semiconductor website
for application notes on RS-422.

http://www.national.com/ is the home page.

http://www.national.com/apnotes/ is the search page for their
application notes.

http://www.national.com/apnotes/apnotes_all_1.html lists all their
application notes.

Here is a list of serial communications related application notes
available from National Semiconductor.

AN-214
AN-409
AN-438
AN-457
AN-702
AN-759
AN-806
AN-807
AN-808
AN-914
AN-916
AN-979
AN-1031
--


Michael A. Terrell
Central Florida
Click to expand...
 
Allen Bong wrote:
Thank you very much Michael. Now I know I have enough evidence to convince
my friend. And thanks to those who teach me the differences between a
telephone line and a digital line.

Allen.
Click to expand...
You're welcome, and I hope you don't get lost reading all those
application notes! ;-)
--


Michael A. Terrell
Central Florida
 
"Allen Bong" <sfbong@tm.net.my> wrote in message news:<3f5579d5$1_2@news.tm.net.my>...
Why are good telephone lines running miles of cables don't pick up much
noise along it's way to the telephone while it is hard to send data between
two computers when they are a few hundred feet apart? What's the catch?
Click to expand...
I have a huge problem with line noise. After a while windows 98 will
just stop loading pages from the internet as if to say "this is too
difficult". I then have to re-boot my system and try again.

Verizon said they checked the lines outside, but I still have the
problem. When it rains I can hear a great amount of "static", and if I
connect to the inteernet, I'm lucky if I can get as far as one page
before I have to reboot, so I know the problem must have something to
do with moisture.

Darren Harris
Staten Island, New York.
 
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