Unshielded Twisted Pair and Shielded Twisted Pair Cabling and Connectors

The Telecommunications Information Associatation (TIA) defines standards for LAN cabling. For copper-wire LAN cabling, two main branches have been defined:

■ Unshielded twisted pair (UTP)

■ Shielded twisted pair (STP)

Figure 11-5 shows a conceptual diagram of each type of cable. The figure shows a side view of each cable and a straight-on view of a UTP cable. All the parts of the figure show the cable cut open so that you can see the internal components of the cables.

Figure 11-5 UTP and STP Cable Components

UTP Cable

STP Cable

' Outer Part of Cable ' 4-Wire/2-Pair Cable Shown

' Outer Part of Cable ' 4-Wire/2-Pair Cable Shown

Twisted pair

Each "Wire" Is Copper with Colored Plastic Insulation

Outer Part of Cable

■ Shielding All Pairs

Shielding Per Pair

Outer Part of Cable

■ Shielding All Pairs

UTP Cable, End View

• Plastic Insulation Color is that You See in an RJ-45 Connector

• Plastic Insulation Color is that You See in an RJ-45 Connector

The figure shows most of the pertinent details of both types of cables. Working from the outside in, the UTP cable has an outer jacket—its main purpose is to hold all the interior cabling together. Inside, you have some number of twisted pairs of cables. From the lowest part of the figure, looking straight at the end of the cable, you can see that each wire does not simply sit inside the outer jacket—instead, each wire has some colored plastic insulation attached to it. Copper, when spun to such a small diameter, would break very easily without some support. So, the thin plastic insulation provides some strength for each individual wire.

Each wire's thin plastic insulation also is colored differently, either a solid color or a stripe against a white background. The colors help when making individual cables by cutting a length of cable off a large cable spool and adding connectors, such as an RJ-45 connector, on the end of a cable. Each wire can be identified by the color of the plastic insulation at each end of the cable. Also, each twisted pair uses the same color—one wire with the solid color and one striped.

The STP cable diagram on the right of Figure 11-5 just shows the additional components of an STP cable as compared with an UTP cable. Each pair is covered with insulating material, with another insulator covering all pairs combined. The extra materials cause the relative lack of flexibility in the cable and, of course, add the benefit of less interference.

UTP Standards

The TIA defines several standards for UTP cabling. The UTP cable types are defined in different categories—but no one would really use the term UTP category in normal speech— instead, you would say something like "Are you using CAT5 cables?" Table 11-3 lists the characteristics of the different categories of UTP cable as defined by the TIA.

Table 11-3 UTP Cable Categories/Characterisics


Max Speed Rating



Used for telephones but not for data.


4 Mbps

Originally intended to support Token Ring over UTP.


10 Mbps

Can be used for telephones as well. Popular option for Ethernet in years past, if CAT3 cabling for phones was already in place.


16 Mbps

Intended for the fast Token Ring speed option.


1 Gbps

Very popular for cabling to the desktop.


1 Gbps

Lower emissions, more expensive than CAT5, but better for Gigabit Ethernet.


1 Gbps+

Intended as a replacement for CAT5e, with capabilities to support multigigabit speeds when standards are created.

UTP Connectors

UTP cables use Regulated Jack 45 (RJ-45) connectors. Some cables need only two twisted pairs, typically using pairs 2 and 3, as specified by the TIA. Figure 11-6 shows a picture of an RJ-45 connector, with some details of the eight pins on the connector. Figure 11-7 shows the pinouts on a typical four-pair UTP cable using an RJ-45 connector, according to the TIA specifications.

Figure 11-6 RJ-45 Connector

Figure 11-7 Four-Pair UTP Cable: Pinouts Using RJ-45

The wiring diagram shows four-pair cabling that uses all eight pins on an RJ-45 connector. Some Ethernet standards require only two pairs and typically use the pair on pins 1 and 2 and the pair on pins 3 and 6.

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