Twisted Pair Cable

Twisted-pair cable comes in two forms: unshielded and shielded. Unshielded Twisted-Pair (UTP) Cable

Unshielded twisted-pair (UTP) cable is the most common type of telecommunication medium in use today. Although most familiar from its use in telephone systems, its frequency range is suitable for transmitting both data and voice (see Figure 5.1-2). A twisted pair consists of two conductors (usually copper), each with its own colored plastic insulation. The plastic insulation is color-banded for identification (see Figure 5.1-3). Colors are used both to identify the specific conductors in a cable and to indicate which wires belong in pairs and how they relate to other pairs in a larger bundle.

In the past, two parallel flat wires were used for communication. However, electromagnetic interference from devices such as a motor can create noise over those wires. If the two wires are parallel, the wire closest to the source of the noise gets more interference and ends up with a higher voltage level than the wire farther away, which results in an uneven load and a damaged signal (see Figure 5.1-4).

100 Hz 5 MHz

Fig. 5.1-2 Frequency range for twisted-pair cable

100 Hz 5 MHz

Fig. 5.1-2 Frequency range for twisted-pair cable

Twisted Cable Picture
Fig. 5.1-3 Twisted-pair cable

If, however, the two wires are twisted around each other at regular intervals (between 2 and 12 twists per foot), each wire is the closer to the noise source for half the time and the farther away for the other half. With twisting, therefore, the cumulative effect of the interference is equal on both wires (examine Figure 5.1-5). Each section of wire has a "load" of 4 when it is on the top of the twist, and 3 when it is on the bottom. The total effect of the noise at the receiver is therefore

0 (14-14). Twisting does not always eliminate the impact of noise, but does significantly reduce it.

Noise source

Noise effect = 16 units


Noise effect = 12 units. Fig. 5.1-4 Effect of noise on parallel lines

Noise source

Noise source

Effect Noise Parallel Line
Fig. 5.1-5 Effect of noise on twisted-pair lines

Advantages of UTP are cost and ease of use. UTP is cheap, flexible, and easy to install. Higher grades of UTP are used in many LAN technologies including Ethernet and Token Ring. Figure 5.1-6 shows a cable containing five unshielded twisted pairs.

Fig.5.1-6 Cable with 5 unshielded twisted pairs of wires

The Electronic Industries Association (EIA) has developed standards to grade UTP cables by quality. Categories are determined by cable quality, with 1 as lowest and 5 as highest. The optimal choice for any use is the cable with the minimal quality necessary to do the desired job safely and effectively. Each EIA category is suitable for certain uses and not for others: • Categoryl. The basic twisted-pair cabling used in telephone systems. This level of quality is fine for voice but inadequate for all but low-speed data communication.

Noise effect = 16 units

• Category 2. The next higher grade, suitable for voice and for digital data transmission of up to 4 Mbit/s.

• Category 3. Required to have at least three twists per foot and can be used for data transmission of up to 10 Mbit/s. It is now the standard cable for most telephone systems.

• Category 4. Must also have at least three twists per foot as well as other conditions to bring the possible transmission rate to 16 Mbit/s.

• Category 5. Used for data transmission up to 100 Mbit/s.

UTP Connectors UTP is most commonly connected to network devices via a type of snap-in plug like that used with telephone jacks. Connectors are either male (the plug) or female the receptacle). Male connectors snap into female connectors and have a repressible tab (called a key) that locks them in place. Each wire in a cable is attached to one conductor (or pin) in the connector. The most frequently used of these plugs is an RJ45 connector with eight conductors, one for each wire of four twisted pairs (see Figure 5.1-7).

Fig. 5.9 UTP connection

Shielded Twisted-Pair (STP) Cable

Shielded twisted-pair (STP) cable has a metal foil or braided-mesh covering that encases each pair of insulated conductors (see Figure 5.10). The metal casing prevents the penetration of electromagnetic noise. It also can eliminate a phenomenon called crosstalk, which is the undesired effect of one circuit (or channel) on another circuit (or channel). It occurs when one line (acting as a kind of receiving antenna) picks up some of the signals traveling down another line (acting as a kind of sending antenna). This effect can be experienced during telephone conversations when one can hear other conversations in the background. Shielding each pair of a twisted-pair cable can eliminate most of the effects of crosstalk.

Fig. 5.1-8 Shielded twisted-pair cable

STP has the same quality considerations as UTP. STP also uses the same connectors as UTP

but the shield must be connected to a ground. Materials and manufacturing requirements make STP more expensive than UTP but less susceptible to noise.

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  • filip
    Why twisted pair cable reduces noise?
    8 years ago
    Why TwistedPair Cable Reduces Noise:?
    8 years ago
  • gerardino
    How twisted pair cable prevents crosstalk?
    7 years ago

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