Twisted Pair Cabling

From a networking perspective the two main types of twisted pair cable that have been used extensively; particularly with Ethernet, are Shielded Twisted Pair (STP) and Unshielded Twisted Pair (UTP). The biggest difference between the two is the presence or absence of a shielding layer but all types use various twist periods to reduce the effect of electromagnetic interference (EMI) or noise and cross-talk interference.

Period of Twist

The two wires constituting a twisted pair are twisted around their common axis and the period of the twist; also known as twist periodicity, twist rate or pitch, being defined as so many twists per unit of length of untwisted wire.

For example: x/m where x = the number of twists and m = the unit of length. So twisted pair cables with a period of 3/inch have 3 complete (3600) twists per inch of untwisted wire length.  Similarly a twisted pair with a twist rate of 25/m would have 25 complete twists per meter.

The reason for twisting the pairs of wires is that twisting the wires decreases interference because the loop area between the wires (which determines the magnetic coupling into the signal) is reduced. Both STP and UTP use the twist principle to help reduce the interference and crosstalk from other neighboring pairs within a cable.

Twisted Pair Standards

You will find that each partner of a twisted pair will have the pair’s twist rate (periodicity) printed on it. You will also find that the individual wires comprising a twisted cable are color coded; usually in accordance with the relevant IEEE standard to make identification and connector crimping more consistently reliable.

Other information printed on twisted pair cables included its category rating (CAT 3, CAT 5, CAT 5e and CAT 6 etc.) along with the cables maximum tested data throughput rate (megabits/sec), the maximum recommended single segment cable run length, manufacturer identification (usually a contraction of the manufacturer’s name of acronym) as well as various badges pertaining to the various standards with which the cable is fully compliant.

Bunches of Twisted Pairs

Whenever multiple twisted pairs are bundled together in a cable; such as is the case with CAT cabling for Ethernet networks, the various pairs will all have different periods of twist to reduce the negative (cancelling) effect of having multiple pairs of twisted wires laying next to each other for the length of a cable run. In this way the twist effect benefits are not diminished by the pairs being in close proximity to each other. This system is very effective in cables consisting of small numbers of twisted pairs such as is to be found in STP and UTP.

In order to manufacture very large cables; comprised of enormous numbers of twisted pairs, manufacturers typically combine small numbers of twisted pair wires; with each pair having a different twist rate, into bunches of twisted pairs. These bunches of twisted pairs are then combined to form the larger cables. In this way the chances of twisted pairs; with the same rate of twist lying next to each other over long sections of the cable are greatly reduced.

When it comes to even larger, longer and higher density cables manufacturers also twist the bunches of groups of twisted pairs at different rates of twist as they are being combined to form a cable that is many times larger again. This also helps to ensure that the twist effect is maintained throughout the larger cable’s length. Other tactics such as having one pair closer to the transmitting end than another pair can also be used to help reduce EMI.

Balanced Pair Differential Mode Transmission

Using transmission pairs where each wire carries an equal but oppositely polarized wave form signal is another tactic used to decrease the effects of noise and interference. Any interference will affect both conductors in the same way but since the signals are of different polarizations; or 1800 out of phase if you prefer, although in the strictest sense this is not correct since the signal was duplicated synchronously but of opposite polarity by the transmitter.

The important thing is that when the signals carried by both members of the pair arrive at the destination end the electromagnetic differential between them will still exist and the noise/crosstalk component can be eliminated by simple addition of both signals. This is because the noise component affects both wires of the pair equally and since they are of equal but opposite polarity when added together the net effect is the same as if you had just subtracted the noise element individually from each member of the pair’s signal.

In this way you don’t even need to know the exact value of the noise component nor do you have to measure it. Thus regardless of size or frequency the noise component can be cancelled out in this way.

Unfortunately; in situations where the noise component of the signal becomes too large it will literally swamp (drown) and distort the signal to the extent that the signal is no longer trustworthy or reliable. This effect is very much the same as the effect of static degradation on radio and television transmissions.

So it is advisable to incorporate other methods of noise reduction in addition to using balanced pair differential mode transmission for this purpose. This is where shielding (STP) comes into the picture. Another advantage of using balanced pair differential mode transmission is that it also helps to reduce cable signal attenuation.

Both STP and UTP use the twisted balanced pair differential mode transmission principles to help reduce the interference or crosstalk from other neighboring pairs within a cable as well as that from external EMI sources.

Additional Information and Specifications

The specifications pertaining specifically to each of the various twisted pair cable types can be found on the website of the major standards organization that drafted the standard including: ANSI, ANSI/TIA/EIA, EIA, IEC, IEEE, ISO/IEC, TIA, and TIA/EIA. The ANSI/TIA/EIA-568-B DOT specifications; for example, cover the types of cables most predominantly used including those that were and still are used for networking.

If it is information relating to the use of various twisted pair cable types with regards to computer networking implementations then it is probably best to head on over to the IEEE’s website where you will find all the required information defined in their 802 DOT standards. The IEEE 802.3 specifications will undoubtedly answer most of your Ethernet related questions. The IEEE also defines wireless transmission standards so if you are into networking then it may pay to become familiar with what they have on offer.

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