Twisted pair
Twisted pair cabling is a form of wiring in which two conductors are wound together for the purposes of canceling out electromagnetic interference (EMI) from external sources, electromagnetic radiation from the UTP cable, and crosstalk between neighboring pairs. wisting wires decreases interference because the loop area between the wires (which determines the magnetic coupling into the signal) is reduced. In balanced pair operation, the two wires typically carry equal and opposite signals (differential mode) which are combined by addition at the destination. The common-mode noise from the two wires (mostly) cancel each other in this addition because the two wires have similar amounts of EMI that are 180 degrees out of phase. This results in the same effect as subtraction. Differential mode also reduces electromagnetic radiation from the cable, along with the attenuation that it causes.

The twist rate (also called pitch of the twist, usually defined in twists per metre) makes up part of the specification for a given type of cable. Where pairs are not twisted, one member of the pair may be closer to the source than the other, and thus exposed to slightly different induced EMF. here twist rates are equal, the same conductors of different pairs may repeatedly lie next to each other, partially undoing the benefits of differential mode. For this reason it is commonly specified that, at least for cables containing small numbers of pairs, the twist rates must differ.In contrast to FTP (Foiled Twisted Pair) and STP (Shielded Twisted Pair) cabling, UTP (Unshielded Twisted Pair) cable is not surrounded by any shielding. It is the primary wire type for telephone usage and is very common for computer networking, especially as patch cables or temporary network connections due to the high flexibility of the cables.

 

Unshielded twisted pair (UTP)
Twisted pair cables were first used in telephone systems by Bell in 1881 and by 1900 the entire American network was twisted pair, or else open wire with similar arrangements to guard against interference. Most of the billions of conductor feet (millions of Kilometres) of twisted pairs in the world are outdoors, owned by telephone companies, used for voice service, and only handled or even seen by telephone workers. The majority of data or Internet connections use those wires. UTP cables are not shielded. This lack of shielding results in a high degree of flexibility as well as rugged durability. UTP cables are found in many ethernet networks and telephone systems. For indoor telephone applications, UTP is often grouped into sets of 25 pairs according to a standard 25-pair color code originally developed by AT&T. A typical subset of these AD1L colors (white/blue, blue/white, white/orange, orange/white) shows up in most UTP cables.

For urban outdoor telephone cables containing hundreds or thousands of pairs, different twist rates for each pair are impractical. For this design, the cable is divided into smaller but identical bundles, with each bundle consisting of twisted pairs that have different twist rates. The bundles are in turn twisted together to make up the cable. Because they reside in different bundles, twisted pairs having the same twist rate are shielded by physical separation. Still, pairs having the same twist rate within the cable will have greater crosstalk than pairs of different twist rate. Thus to minimize crosstalk within a large cable, careful pair selection is important. Twisted pair cabling is often used in data networks for short and medium length connections because of its relatively lower costs compared to fiber and coaxial cabling.

 

Uses
Unshielded twisted pair (UTP) cabling, because of its 100-year history of use by telephone systems, both indoors and out, is also the most common cable used in computer networking. It is a variant of twisted pair cabling. UTP cables are often called ethernet cables after Ethernet, the most common data networking standard that utilizes UTP cables, although not the most reliable.A telephone line or telephone circuit (or just line or circuit within the industry) is a single-user circuit on a telephone communications system. Typically this refers to the physical wire or other signaling medium connecting the user's telephone apparatus to the telecommunications network, and usually also implies a single telephone number for billing purposes reserved for that user. In 1876 the earliest lines were single electrically conducting metal wires directly connecting one telephone to another with the Earth forming the return circuit. Later in 1878 the Bell Telephone Company ran lines (called the local loop) from each user's telephone to end offices which performed any necessary electrical switching to allow voice signals to be transmitted to more distant telephones.

These wires were typically copper, although aluminium has also been used, and were carried in pairs separated by about 25cm (10") on poles above the ground, and later as twisted pair cables. Modern lines may run underground, and may carry analog or digital signals to the exchange, or may have a device that converts the analog signal to digital for transmission on optical fiber or other carrier system.In most cases, two copper wires (tip and ring) for each telephone line run from a home or other small building to a local telephone exchange. There is a central junction box for the building where the wires that go to telephone jacks throughout the building and wires that go to the exchange meet and can be connected in different configurations depending upon the subscribed telephone service. The wires between the junction box and the exchange are known as the local loop, and the network of wires going to an exchange, the access network.Most houses in the U.S. are wired with 6 position modular jacks with four conductors wired to the house's junction box with copper wires. Those wires may be connected back to two telephone lines at the local telephone exchange, thus making those jacks RJ14 jacks. Mor exchange as one telephone line, and the others are unconnected. In that case, the jacks in the house are RJ11.

 

Historical note
Soon after the invention of the telephone, open wire lines were used for transmission. Two wires, strung on either side of cross bars on poles, share the route with electrical power lines. At first, interference from power lines limited the practical distance for telephone signals. Discovering the cause, engineers devised a method, called wire transposition, to cancel out the interference, where once every several poles, the wires crossed over each other. In this way, the two wires would receive similar EMI from power lines. Today, such open wire lines with periodic transpositions can still be found in rural areas. This represented an early implementation of twisting with a twist rate of about 4 twists per kilometre.A utility pole, telegraph pole, telephone pole, power pole, or telegraph post is a post or pole upon which telecommunication network equipment is situated. They originally prevented telegraph wires from being short circuited, and continue to protect surface traffic from being inconvenienced by cables and vice versa. They are often also used for electrical cables (with pylons being used for only the higher voltage applications) and frequently a pole will share both power and communications lines. Telegraph poles first became commonplace in the middle 19th century, carrying at first one steel wire, then in urban areas many. In Canada, the poles are commonly referred to as hydro poles, as the electric companies commonly have "Hydro" in their name.

Most utility poles are made of wood pressure-treated with some type of preservative to keep away woodpeckers, insects, fungi, and fires. Many different types of trees can be used to make utility poles, including Douglas fir, Jack Pine, Lodgepole Pine, and Pacific Silver Fir. Western Red Cedar is also popular for its natural insecticidal properties and durability, though its higher price deters many utility companies. Other common utility pole materials are steel and concrete, with composites (fibreglass) also becoming more prevalent. In some countries, for example the UK, poles have sets of brackets arranged in a standard pattern up the pole to act as hand and foot holds for those working on the equipment or connections atop the pole. In the USA such steps are usually provided only for the upper part of the pole; the worker normally uses climbing spikes to reach them.The appearance of poles has changed with technology through the 20th Century, with for example the loss of the stereotypical but now redundant crossbeam used to mount rows of insulators for open wire telephone circuits. These more traditional poles can sometimes be seen unaltered beside non-electrified railways, or where no effort has been made to remove crossbeams not in use.

 

Utility pole
A utility pole, telegraph pole, telephone pole, power pole, or telegraph post is a post or pole upon which telecommunication network equipment is situated. They originally prevented telegraph wires from being short circuited, and continue to protect surface traffic from being inconvenienced by cables and vice versa. They are often also used for electrical cables (with pylons being used for only the higher voltage applications) and frequently a pole will share both power and communications lines. Telegraph poles first became commonplace in the middle 19th century, carrying at first one steel wire, then in urban areas many. In Canada, the poles are commonly referred to as hydro poles, as the electric companies commonly have "Hydro" in their name.Today utility poles may hold much more than the uninsulated thin copper wire that they originally supported. Thicker cables holding many twisted pair lines or coaxial cable or even fibre-optics may be carried. Simple analogue repeaters or other outside plant equipment have long been mounted against poles, and often new digital equipment for multiplexing/demultiplexing or digital repeaters may now be seen. In many places, as seen in the illustration, providers of electricity, television, telephone, street lighting, traffic signals and other services share poles, either in joint ownership or by renting space to each other. Such poles provide a safe gap between power lines on top and signal wires below.Wooden utility poles and railroad ties are almost always treated with creosote to slow decomposition. This is also the most common way of preserving wood in the United States.Throwing poles similar to utility poles is a traditional Scottish sport known as the caber toss.

The appearance of poles has changed with technology through the 20th Century, with for example the loss of the stereotypical but now redundant crossbeam used to mount rows of insulators for open wire telephone circuits. These more traditional poles can sometimes be seen unaltered beside non-electrified railways, or where no effort has been made to remove crossbeams not in use.However in the countries of Eastern Europe, in Russia and in countries of the third world, there are still many utility poles carrying bare wires mounted on insulators not only along railway lines, but also along roads and sometimes even in urban areas. Errant traffic being uncommon on railways, their poles are usually less tall. In the UK, many wooden poles carry electricity from super pylons to the user at lower voltages (240 V to 11 kV depending on type, Cable shieldingand often in three phases). The conductors on these are bare metal connected to the posts by insulators.

 

Cable shielding
Twisted pair cables are often shielded in attempt to prevent electromagnetic interference. Because the shielding is made of metal, it may also serve as a ground. However, usually a shielded or a screened twisted pair cable has a special grounding wire added called a drain wire. This shielding can be applied to individual pairs, or to the collection of pairs. When shielding is applied to the collection of pairs, this is referred to as screening. The shielding must be grounded for the shielding to work.STP cabling includes metal shielding over each individual pair of copper wires. This type of shielding protects cable from external EMI (electromagnetic interferences). e.g. the 150 ohm shielded twisted pair cables defined by the IBM Cabling System specifications and used with token ring networks.S/STP cabling, also known as Screened Fully shielded Twisted Pair (S/FTP),[1] is both individually shielded (like STP cabling) and also has an outer metal shielding covering the entire group of shielded copper pairs (like S/UTP). This type of cabling offers the best protection from interference from external sources.

Electromagnetic interference (or EMI, also called radio frequency interference or RFI) is a (usually undesirable) disturbance caused in a radio receiver or other electrical circuit by electromagnetic radiation emitted from an external source. [1] The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit. The source may be any object, artificial or natural, that carries rapidly changing electrical currents, such as an electrical circuit, the Sun or the Northern Lights.EMI can be induced intentionally for radio jamming, as in some forms of electronic warfare, or unintentionally, as a result of spurious emissions and responses, intermodulation products, and the like. It frequently affects the reception of AM radio in urban areas. It can also affect cell phone, FM radio and television reception, although to a lesser extent.

 

Power line noise
Virtually all power-line noise, originating from utility company equipment, is caused by a spark or arcing across some power-line related hardware. A breakdown and ionization of air occurs, and current flows between two conductors in a gap. The gap may be caused by broken or loose hardware such as a cracked insulator. Typical culprits include insufficient and inadequate hardware spacing such as a gap between a ground wire and a staple. Once an ionized path is established in the gap, current flows at all parts of the cycle where the voltage is higher than the breakdown voltage of the gap. This typically occurs only at the positive and negative voltage peaks -- the times of highest instantaneous voltage throughout the cycle.

As an example for a 60Hz system (i.e.power-lines carrying 60 Hz AC, such as in the US), the voltage on them passes through two peaks each cycle (one positive and one negative) and pass through zero twice each cycle. This gives 120 peaks and 120 zero crossings in each second (50Hz: 100 peaks and crossings correspondingly). Power-line noise follows this pattern, generally occurring in bursts at a rate of 120 bursts per second. This gives power-line noise a characteristic sound that is often described as a harsh and raspy hum or buzz. Because the peaks occur twice per cycle, true power-line noise has a strong 120-Hz modulation on the signal (50Hz system: 100Hz).EMI or RFI may be broadly categorized into two types; narrowband and broadband.

Narrowband interference usually arises from intentional transmissions such as radio and TV stations, pager transmitters, cell phones, etc. Broadband interference usually comes from incidental radio frequency emitters. These include electric power transmission lines, electric motors, thermostats, bug zappers, etc. Anywhere electrical power is being turned off and on rapidly is a potential source. The spectra of these sources generally resembles that of synchrotron sources, stronger at low frequencies and diminishing at higher frequencies, though this noise is often modulated, or varied, by the creating device in some way. Included in this category are computers and other digital equipment as well as televisions. The rich harmonic content of these devices means that they can interfere over a very broad spectrum. Characteristic of broadband RFI is an inability to filter it effectively once it has entered the receiver chain.

 

Mitigation
On integrated circuits, the most important means of reducing EMI are: the use of bypass or "decoupling" capacitors on each active device (connected across the power supply, as close to the device as possible), risetime control of high-speed signals using series resistors, and VCC filtering. Shielding is usually a last resort after other techniques have failed because of the added expense of RF gaskets and the like.

The efficiency of the radiation depends on the height above the ground or power plane (at RF one is as good as the other) and the length of the conductor in relation to the wavelength of the signal component (fundamental, harmonic or transient (overshoot, undershoot or ringing)). At lower frequencies, such as 133 MHz, radiation is almost exclusively via I/O cables; RF noise gets onto the power planes and is coupled to the line drivers via the VCC and ground pins. The RF is then coupled to the cable through the line driver as common-mode noise. Since the noise is common-mode, shielding has very little effect, even with differential pairs. The RF energy is capacitively coupled from the signal pair to the shield and the shield itself does the radiating. One cure for this is to use a braid-breaker or choke to reduce the common-mode signal.

At higher frequencies, usually above 500 MHz, traces get electrically longer and higher above the plane. Two techniques are used at these frequencies: wave shaping with series resistors and embedding the traces between the two planes. If all these measures still leave too much EMI, shielding such as RF gaskets and copper tape can be used. Most digital equipment is designed with metal, or conductive-coated plastic, cases.Switching power supplies can be a source of EMI, but have become less of a problem as design techniques have improved.Most countries have legal requirements that mandates electromagnetic compatibility: electronic and electrical hardware must still work correctly when subjected to certain amounts of EMI, and should not emit EMI which could interfere with other equipment (such as radios). Susceptibilities of different radio technologies.

 

Susceptibilities of different radio technologies
Interference tends to be more troublesome with older radio technologies such as analogue amplitude modulation, which have no way of distinguishing unwanted in-band signals from the intended signal, and the omnidirectional dipole antennas used with broadcast systems. Newer radio systems incorporate several improvements that improve the selectivity. In digital radio systems, such as Wi-Fi, error-correction techniques can be used. Spread-spectrum and frequency-hopping techniques can be used with both analogue and digital signalling to improve resistance to interference. A highly directional receiver, such as a parabolic antenna or a diversity receiver, can be used to select one signal in space to the exclusion of others.

The most extreme example of digital spread-spectrum signalling to date is ultra-wideband (UWB), which proposes the use of large sections of the radio spectrum at low amplitudes to transmit high-bandwidth digital data. UWB, if used exclusively, would enable very efficient use of the spectrum, but users of non-UWB technology are not yet prepared to share the spectrum with the new system because of the interference it would cause to their receivers. The regulatory implications of UWB are discussed in the Ultra-wideband article.