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Posted Aug 31, 2013
10:04:31 PM
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Advantages of balanced/twisted pair

Can someone explain to me in the most technical way possible how/why twisting a balanced pair allows for higher operation?

 

I know that this keeps the conductors equidistant from a source of noise hence impeding interference on the line. But if that was all that it did the amount of twist wouldn't matter. But since CAT5, 6, 7, etc are rated for higher speed the tighter twist must do something else also. The only thing I could think would be that it keeps the conductors from going perpendicular to EMF lines more so it hinders induction. A detailed answer would be appreciated.

Can someone explain to me in the most technical way possible how/why twisting a balanced pair allows for higher operation?

 

I know that this keeps the conductors equidistant from a source of noise hence impeding interference on the line. But if that was all that it did the amount of twist wouldn't matter. But since CAT5, 6, 7, etc are rated for higher speed the tighter twist must do something else also. The only thing I could think would be that it keeps the conductors from going perpendicular to EMF lines more so it hinders induction. A detailed answer would be appreciated.

Advantages of balanced/twisted pair

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Sep 1, 2013 5:01:51 AM
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ACE - Expert
drumboy35 - How about - Higher capacity requires higher frequency - Think about how capacity increases moving from audible frequency to radio frequency to light frequency. Higher frequency equates to shorter wavelength. Think higher frequency antennas are smaller. So, a tighter twist better matches the higher bandwidth/frequency, shorter wavelength content signal & moves farther away (rejects) from matching (accepting) the lower frequency, longer wavelength EMF noise signal.
drumboy35 - How about - Higher capacity requires higher frequency - Think about how capacity increases moving from audible frequency to radio frequency to light frequency. Higher frequency equates to shorter wavelength. Think higher frequency antennas are smaller. So, a tighter twist better matches the higher bandwidth/frequency, shorter wavelength content signal & moves farther away (rejects) from matching (accepting) the lower frequency, longer wavelength EMF noise signal.
*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.

Re: Advantages of balanced/twisted pair

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Sep 1, 2013 5:08:52 AM
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This might help: http://en.wikipedia.org/wiki/ISO/IEC_11801

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*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.

Re: Advantages of balanced/twisted pair

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Sep 1, 2013 6:19:55 PM
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I think this link will help:

 

http://www.vellone.com/support/cat5extender/Understanding%20Twisted%20Pair%20Cable%20Technology.pdf

 

Note that part of the magic is the fact that different pairs are twisted in different directions and at different pitches to minimize crosstalk, in addition to the common mode noise reduction of outside interference.

 

 

I think this link will help:

 

http://www.vellone.com/support/cat5extender/Understanding%20Twisted%20Pair%20Cable%20Technology.pdf

 

Note that part of the magic is the fact that different pairs are twisted in different directions and at different pitches to minimize crosstalk, in addition to the common mode noise reduction of outside interference.

 

 

*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.

Re: Advantages of balanced/twisted pair

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Sep 3, 2013 2:42:18 PM
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Thank you for all the info.

Thank you for all the info.

Re: Advantages of balanced/twisted pair

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Sep 4, 2013 9:24:20 AM
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Edited by ScottMac on Sep 4, 2013 at 9:25:00 AM

To add to the above, I wanted to clarify the differences between Cat 5, 5e, 6, 6a ... and future cable specs.

 

It is more than the twists ... just increasing the twist rate isn't enough to prevent interference, which, in most cases comes from the same cable (i.e., crosstalk).

 

The cables are engineered very carefully; starting with the conductor diameter of each wire, then the insulation of each wire (pairs usually have the same covering, but different pair can have different coverings/insulation.

 

The type and thickness of each conductor's insulation establish a specific distance (conductor-to-conductor), and depending on the material the insulation is made from, it has a "dielectric" value that is used for electrical calculations for inductance and capacitance, as well as physical properties for breakdown voltages, heat tolerance, etc. Each pair of a bundle (which for this discussion is four pair, a standard CatX cable) has its own combination of properties.

 

The collective pairs are then (usually, but not always, it's part of the engineering) twisted at a specifc rate per distance, maintaining the pair-pair relationship (called the "lay" of the cable). It's this relationship that makes "minimum bend radius" a critical spec ... too much of a bend and you tend to change the pair-pair relationship and also cause an impedence lump that degrades the signal.

 

Each pair is often twisted at different rates from the other pair ... Green and Orange tend to be more twisted than Blue and Borwn pair ... but by using a different twist for each pair helps to reject nea-end and far-end (NeXT, FeXT) crosstalk.

 

Starting at about Cat6, a center "X" member was added to keep pair separation ... which helps to reduce NeXT and FeXT.

 

Once the pair groups are in proper lay, the cable is sheathed. The material of the sheath, the thickness of the sheath, and (like Belden "media twist" cable) may have channels for each pair to maintain the proper pair-pair relationship, even when bent or stretched too much.

 

In addition to the sheath, a cable may also be "screened" (a type of drain / shield) or "shielded" (which means it's not UTP - Unshielded Twisted Pair). UTP has a characteristic impedance of 100 Ohms, STP (Shielded Twisted Pair) has a characteristic impedance of 150 Ohms.  "Screened" Unshielded Twisted Pair has a chaacteristic Impedance of 100 Ohms (that's why it's considered "screened" not shielded.

 

There are dozens of parameters that engineering has to "hit" in the design and production of the cable ... even things like whether the signal of each pair will arrive within a certain window (the "skew") ... since different pair are / may be coated with different insulators  diaelectrics, the conduction properties are different and signal are propagated at different speeds. The skew spec says they all have to arrive within a certain time within 5 nanoseconds, IIRC). This was a big deal back when "Teflon" was banned ... that was a common insulation and sheath for plenum cable. The manufacturers changed to another material, but the skew factors changed and had to be re-certified.

 

Each manufacturer's cable has its own characteristics, based on its construction. All are tested to certify them as "Cat{whatever}" ... but they are all a litle different in that some parameters are higher or lower than some other manufacturer's cable or components.

 

When a manufacturer produces the cable, it knows the characteristics of that cable, and can compensate for weak areas with the other components (plugs, panels, keystones ...) to even out the performance. It can happen (though usually not) that mixing components from multiple manufacturers may create a "collective tolerance" issue that creates an out-of-spec condition ... because in addition to the window of permitted operation, there is also a "maximum spread" specification .... so each component can be in spec, but because of the allowed tolerances, this thing can be near the top of the spec, the other thing may be at the bottom of the spec, and the range of difference may be too great for optimal operation.


The short story is that it's much, much more than the twists that differentiate the various categories of cables on the market today.

 

To add to the above, I wanted to clarify the differences between Cat 5, 5e, 6, 6a ... and future cable specs.

 

It is more than the twists ... just increasing the twist rate isn't enough to prevent interference, which, in most cases comes from the same cable (i.e., crosstalk).

 

The cables are engineered very carefully; starting with the conductor diameter of each wire, then the insulation of each wire (pairs usually have the same covering, but different pair can have different coverings/insulation.

 

The type and thickness of each conductor's insulation establish a specific distance (conductor-to-conductor), and depending on the material the insulation is made from, it has a "dielectric" value that is used for electrical calculations for inductance and capacitance, as well as physical properties for breakdown voltages, heat tolerance, etc. Each pair of a bundle (which for this discussion is four pair, a standard CatX cable) has its own combination of properties.

 

The collective pairs are then (usually, but not always, it's part of the engineering) twisted at a specifc rate per distance, maintaining the pair-pair relationship (called the "lay" of the cable). It's this relationship that makes "minimum bend radius" a critical spec ... too much of a bend and you tend to change the pair-pair relationship and also cause an impedence lump that degrades the signal.

 

Each pair is often twisted at different rates from the other pair ... Green and Orange tend to be more twisted than Blue and Borwn pair ... but by using a different twist for each pair helps to reject nea-end and far-end (NeXT, FeXT) crosstalk.

 

Starting at about Cat6, a center "X" member was added to keep pair separation ... which helps to reduce NeXT and FeXT.

 

Once the pair groups are in proper lay, the cable is sheathed. The material of the sheath, the thickness of the sheath, and (like Belden "media twist" cable) may have channels for each pair to maintain the proper pair-pair relationship, even when bent or stretched too much.

 

In addition to the sheath, a cable may also be "screened" (a type of drain / shield) or "shielded" (which means it's not UTP - Unshielded Twisted Pair). UTP has a characteristic impedance of 100 Ohms, STP (Shielded Twisted Pair) has a characteristic impedance of 150 Ohms.  "Screened" Unshielded Twisted Pair has a chaacteristic Impedance of 100 Ohms (that's why it's considered "screened" not shielded.

 

There are dozens of parameters that engineering has to "hit" in the design and production of the cable ... even things like whether the signal of each pair will arrive within a certain window (the "skew") ... since different pair are / may be coated with different insulators  diaelectrics, the conduction properties are different and signal are propagated at different speeds. The skew spec says they all have to arrive within a certain time within 5 nanoseconds, IIRC). This was a big deal back when "Teflon" was banned ... that was a common insulation and sheath for plenum cable. The manufacturers changed to another material, but the skew factors changed and had to be re-certified.

 

Each manufacturer's cable has its own characteristics, based on its construction. All are tested to certify them as "Cat{whatever}" ... but they are all a litle different in that some parameters are higher or lower than some other manufacturer's cable or components.

 

When a manufacturer produces the cable, it knows the characteristics of that cable, and can compensate for weak areas with the other components (plugs, panels, keystones ...) to even out the performance. It can happen (though usually not) that mixing components from multiple manufacturers may create a "collective tolerance" issue that creates an out-of-spec condition ... because in addition to the window of permitted operation, there is also a "maximum spread" specification .... so each component can be in spec, but because of the allowed tolerances, this thing can be near the top of the spec, the other thing may be at the bottom of the spec, and the range of difference may be too great for optimal operation.


The short story is that it's much, much more than the twists that differentiate the various categories of cables on the market today.

 

Sent from my phone.
*I am an AT&T employee and the postings on this site are my own and don’t necessarily represent AT&T’s position, strategies or opinions.

Re: Advantages of balanced/twisted pair

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