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Webcast-Catalyst9k
New Member

I want to ask about twisted pair cables and coaxial cables. Please Help me

My first question is about twisted pair cable. How twisting pairs can reduce the electromagnetic induction (I want technical information).

The second quesion is How the braid (the part of the coaxial cable that consist of a net of wires can help to reduce the noice or interference. I also need a technical information here.

Thank you very much.

4 REPLIES
VIP Purple

Re: I want to ask about twisted pair cables and coaxial cables.

Hello,

when talking about twisted pair cabling, you have to differentiate between UTP (Unshielded Twisted-Pair) and STP (Shielded Twised-Pair). UTP consists of two or four insulated copper wires. Depending on the particular purpose, there are UTP specifications which govern how many twists are permitted per foot of cable. Currently there are 6 categories of UTP cabling, of which I think only Cat 5 and Cat 6 cables are sufficient grade to support data transmission. Basically, UTP cabling is, as the name says, unshielded and therefore particularly susceptible to crosstalk.

STP uses a woven copper braid jacket which is a higher-quality, more protective jacket than UTP has. STP also uses a foil wrap between and around the wire pairs, and internal twisting of the pairs. This gives STP excellent insulation to protect the transmitted data from the outside interface. What this means is that STP is less susceptible to electrical interference and supports higher transmission rates over longer distances than UTP.

Coaxial cable consists of a core made of solid copper surrounded by insulation, a braided metal shielding, and an outer cover. One layer of foil insulation and one layer of braided metal shielding is referred to as dual shielded. Quad shielding is available for environments that are subject to higher interference. Quad shielding consists of two layers of foil insulation and two layers of braided metal shielding. Shielding refers to the woven or stranded metal mesh (or other material) that surrounds some types of cabling. Shielding protects transmitted data by absorbing stray electronic signals, called noise, so that they do not get onto the cable and distort the data.

HTH,

Georg

New Member

Re: I want to ask about twisted pair cables and coaxial cables.

Hi hqattous,

It looks like your question still needs an answer on why the number of twists reduces EMI. Allow me to relate in laymans terms. Imagine you are driving down the freeway and in the sun. After a few hours you get sun burnt from the "solar radation." Since your friend riding shotgun also wants some rays you switch sides and get to cool off for a few hours. In this instance the sunshine is like EMI. To keep from getting really hot (generating voltage) you switched sides (twisted the pairs). As you can also imagine the more you switch the less hot you will feel. Our CAT_V twisted pair is the same way.

The shielding on twisted pair is like tinted soundproof windows on our drivers. I keeps them from feeling the radiation as much. It also keep the noise inside the car from getting out to everyone else.

Hopefully this story helps.

R/S

Dave

New Member

Re: I want to ask about twisted pair cables and coaxial cables.

The other responses here do not seem to answer your question (and Cat3 UTP was used extensively for networking, think about it, Ethernet over UTP was in existance way before Cat5 cable was introduced).

Anyway, for some reasonably techical explanations to your questions please see :

http://bwcecom.belden.com/college/college.htm

I quickly looked at the document about the Belden 'Bonded-Pair' technology and it will provide you with an explanation, but not the equations for calculating the various parameters of specific cable geometries.

There are also techical papers on coax. BTW, I'm sure that other cable manufacturors will also have good technical information on wire stuff.

Dave

Anonymous
N/A

Re: I want to ask about twisted pair cables and coaxial cables.

Noise spikes induced into the unshielded wires of UTP network cables are detected and eliminated by noise filters in the end devices. Ethernet uses a balanced signal. That is, the data signal is sent down one wire of the pair and the exact opposite polarity signal is sent down the other wire of the pair. Any noise picked up on the way will create a spike with the same polarity on each wire. When the data arrives at the end of the cable, any signal that is matched with an exactly opposite signal on the pair is passed on as data. Any signal that matches the polarity on the other wire is discarded. The circuit that performs this filtering is called a common mode reject filter.

Twisting of paires reduces crosstalk inside the cable. Signals radiated from twisted wires will be in opposite phase from each other. When two signals of equal and opposite phase are received, they cancel each other out.

The frequency of the signal, twists per foot, and relation of where the twists occur in relation to other pairs are all critical variables. That is why kinking a UTP cable can seriously increase the crosstalk and induce errors in the data. Belden went to bonding pairs to help maintain the pair to pair geometry.

The braid of a coax cable forms a shield similar to a foil sheld in STP cable. However, there is a capacitive component between the outer shield and inner conductor with the insulation between being a dialectric. That is one of the reasons coax cable is much better than twisted pair. If you bend a coax cable too sharply, the inner conductor presses against the insulation and changes the capacitive value of the cable which is a common cause of coax cable failure.

UTP is good enough to get by, and is much cheaper to make than coax or STP. Cheap always wins over better.

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