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Viewing a Network Signal; Physical level

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by tontoscout 1 view

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I have looked around, but I can't find out what exactly is on a 10baseT or coax wire for a network. What frequency is it? FM,AM? Is the signal pulsed on a wire, CW? Does it ride a DC offset? If I were to draw on a blackboard, a signal with data on it, for CAT 5 ethernet, or coaxial wire, what would it look like? For any that have viewed the wire, do you find it better to see with an oscilloscope, or a spectrum analyzer? (I only have enough budget for 1!)

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by gary sharp

Okay, lets start from the beginning. I can't belive you have a budget to look at a signal to see what shape it is. I do believe you may have a budget to maintain your network.

If you want to maintain the network you may be better with a protocolanalyzer - trust me- you are never going to analyze the data on your network with an oscilloscope!!

If, on the other hand, you are a cable guy, the a (relatively) cheap cable test tool will confirm the acceptability of your network cabling.

sowhich is it? What do you REALLY want to do?

g

by johnm

Wow! No easy answer and changing rapidly. All 10baseT and 100baseT network cards are based on IEEE Standard 802.3. Card manufacturers don't provide a lot of info but see www.3com.com. The cable manufacturers provide a bit more general info. I like www.siemon.com and use their catalog for reference. To answer your questions to your own satisfaction try www.ieee.org and start searching. I found several items under 802.3 but the really interesting stuff is the next generation standards being developed Gigabit and higher speeds. They have a lot of tutorials and the Multi-level Analog Signaling one should be interesting to you. To get into the right area try: http://grouper.ieee.org/groups/802/3/tutorial/
I don't think either a 'scopeor a spectrum analyzer will show what you seem to want. Educational institution?

by jdkline

To answer your last question first: an oscilloscope; preferably a digital storage scope if you want to capture a moment in time of data. Since the ethernet (IEEE Standard 802.3) signal lacks periodicity, anything but a digital storage oscilloscope would not allow you to view a physical signal.
Ethernet is Manchester encoded, eg high to low or low to high transition within a bit frame defines the bit's value. Therefore a 10Mbps (as in 10BaseT) could generate a 20MHz signal
The signal for 10BaseT (or twisted pair) is a differential one, which is to say no DC offset and what you would see on wire referenced signal ground would be the opposite polarity of the other wire.
Here's a good site for more information on IEEE 802.3 and view of what a Manchester Encoded signal would look like:
http://www.eee.bham.ac.uk/james-roxbyp/ee3c2/L8/index.htm

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