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TWISTED PAIRS

Twisted pair  is a type of wiring in which two conductors (the forward and return conductors of a single circuit) are twisted together for the purposes of canceling out electromagnetic interference (EMI) from external sources, for instance, electromagnetic radiation from unshielded twisted pair (UTP) cables, and  crosstalk between neighboring pairs. It was invented by Alexander Graham Bell.

In balanced pair operation, the two wires carry equal and opposite signals and the destination detects the difference between the two. This is known as differential mode transmission. Noise sources introduce signals into the wires by coupling of electric or magnetic fields and tend to couple to both wires equally. The noise thus produces a common-mode signal which is cancelled at the receiver when the difference signal is taken. This method starts to fail when the noise source is close to the signal wires; the closer wire will couple with the noise more strongly and the common-mode rejection of the receiver will fail to eliminate it. This problem is especially apparent in telecommunication cables where pairs in the same cable lie next to each other for many miles.

One pair can induce crosstalk in another and it is additive along the length of the cable. Twisting the pairs counters this effect as on each half twist the wire nearest to the noise-source is exchanged. Providing the interfering source remains uniform, or nearly so, over the distance of a single twist, the induced noise will remain common-mode. Differential signaling also reduces electromagnetic radiation from the cable, along with the associated attenuation allowing for greater distance between exchanges.

The twist rate (also called pitch of the twist, usually defined in twists per meter) makes up part of the specification for a given type of cable. Where nearby pairs have equal twist rates, the same conductors of the 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)

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-pairs color code originally developed by AT&T. A typical subset of these 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, the cable is divided into smaller but identical bundles. Each bundle consists of twisted pairs that have different twist rates. The bundles are in turning twisted together to make up the cable. Pairs having the same twist rate within the cable can still experience some degree of crosstalk. Wire pairs are selected carefully to minimize crosstalk within a large cable.

Unshielded twisted pair cable with different twist rates

UTP cable is also the most common cable used in computer networking. Modern Ethernet, the most common data networking standard, utilizes UTP cables. Twisted pair cabling is often used in data networks for short and medium length connections because of its relatively lower costs compared to optical fiber and  coaxial cable. UTP is also finding increasing use in video applications, primarily in security cameras. Much middle to high-end cameras includes a UTP output with setscrew terminals. This is made possible by the fact that UTP cable bandwidth has improved to match the baseband f television signals.

                            

STP / FTP cable format                                               S/STP, also known as S/STP

                                             

                            

S/UTP cable format                                                    S/STP cable format

Twisted pair cables are often shielded in an 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, and is improved by grounding the drain wire along with the shield.

·            Shielded twisted pair (STP or STP-A) 

150 ohm STP shielded twisted pair cable defined by the IBM Cabling System specifications and used with token ring or FDDI network. This type of shielding protects cable from external EMI from entering or exiting the cable and also protects neighboring pairs from crosstalk.

·            Screened twisted pair (STP or F/TP) 

STP cabling offers an overall sheath shield across all of the pairs within the 100 Ohm twisted pair cable. F/TP uses foil shielding instead of a braided screen. This type of shielding protects EMI from entering or exiting the cable.

·            Screened shielded twisted pair (S/STP or S/FTP) 

S/STP (Screened Shielded Twisted Pair) or S/FTP (Screened Foiled Twisted Pair) cabling offer shielding between the pair sets and an overall sheath shield within the 100 Ohm twisted pair cable. This type of shielding protects EMI from entering or exiting the cable and also protects neighboring pairs from crosstalk.

S/STP cable 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, and also eliminates alien crosstalk.

Note that different vendors and authors use different terminology (STP has been used to denote both STP-A, S/STP, and S/UTP). See below for the ISO/IEC attempt to internationally standardize the various designations.

tugas internasional physics

A.          INTRODUCTION

The ability to communicate is a gift that human children learn at a very early age. Human communication may be sophisticated speech or a simple smile. Animals can also communicate. For example, whales and dolphins communicate using high-and low-pitched sounds that travel through water over many kilometers.

In the case of human, the ability to communicate has been developed and extended so that message and information may be sent over great distances in very short time intervals. This chapter looks briefly at some aspects of communication in the modern-day world.

B.           ITEMS

CHANNELS OF COMMUNICATION

1.         WIRE-PAIRS

Wire-pairs provide a very simple link between a transmitter of information and the receiver. In the early days of electrical communication using mores code, the transmitter and receiver were connected directly to one another by means of two copper wires (or one copper wire and an ‘earth return’). In modern communication systems, wire-pairs are used mainly for short-distance communication at relatively low frequencies, for examples linking telephones to the nearest exchange. In the simplest of applications, they are used for linking a door bell in a house to the switch outside the door.

High-frequency electrical signals lose their energy over short distance in wire-pairs the signal have a high attenuation. This high attenuation is partly due to the heating caused by the electrical resistance of the wires and partly due to the emission of radiation (radio waves), since the wires act as aerials. A signal going any distance in a wire-pair must be amplified at regular intervals along the length of the wires.

A further problem is that, since the wires acts as aerials, they pick up any electromagnetic waves and these unwanted signals, or noise, will cause deterioration of the signal. Wire-pairs close to one another will pick up each other’s signals. This effect is called cross-talk or cross-linking and means that wire-pairs give to poor security since the signals can be ‘tapped’ easily.

The bandwidth of a wire-pair is only about 500 KHZ and, as a result, wire-pairs are limited as to the amount of information that they can carry.

2.         COAXIAL CABLES

The coaxial cables consist of two wire conductors. Any signal is transmitted along a central inner conductor that is covered by an insulator. The second conductor is in the form of thin wire braid that completely surrounds the insulator. This wire braid acts as the return for the signal and is earthed. The braid is covered by a protective layer of insulation.

The earthed outer braiding shields the inner conductor from external interference. Coaxial cables are, therefore far less noisy than wire-pairs and provide better security. Attenuation is also reduced. Consequently, repeater amplifiers can be further apart on coaxial cables.

The bandwidth of a coaxial cable is about 50 MHZ. much more information can be carried along a coaxial cable than along a wire-pair. However, coaxial cable is more costly. Coaxial cable is used to connect an aerial and a television.

3.         OPTIC FIBRES

An optic fibre consists of a fine strand of very pure glass, surrounded by a protective covering. The glass fibre itself is thinner than a hair. Pulses of light or infrared radiation travel along the fibre as a result of total internal reflection. These pulses carry digital information along the fibre.

The radiation pulses are provided by laser and have very high frequencies, of the order of 108 MHZ. In theory, a single pulse need only last for 10-14 s. however, lasers cannot by controlled at such high frequencies and the duration of a single pulse, or bit, is governed by the frequency at which the laser can be switched on and off. Technology is always improving and at present the frequency is about 800 MHZ. such high frequencies mean a large bandwidth, so many different phone calls can share the same optic fibre. Fibre optic transmission is being installed in many different types of communication.

C.     CONCLUSION

·               In communicating, we can use wire-pairs, coaxial cables, optic fibres, etc.

·               Wire-pairs are used mainly for short-distance communication. Its cause high attenuation of a signal and easily pick up noise. Wire-pairs is suffer from cross-talk and low security. It’s have limited bandwidth.

·               The coaxial cable is more costly and causes less attenuation. It’s less noisy and more secure. The bandwidth of a coaxial cable is a larger.

·               The optic fibres is large bandwidth, giving rise to large transmission capacity and much lower cost than metal wires. It’s much less signal attenuation, so far regenerator amplifier are required, reducing the cost of installation. Optic fibres is very high security, negligible cross-talk and can be laid alongside exiting routes such as electric railway lines and power lines.

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