UTP Connectors

One outstanding feature of UTP cabling is the connector system. The UTP connector system is based on the pinching action of a single stamped-metal fitting that strips the wire, holds it in place, and establishes a gas-tight electrical connection all at the same time. The rather high insertion force required to make this connection is supplied by a handheld, mechanically activated punch-down tool (shown in Figure 8.25) or by the crushing action of the two halves of an RJ-45 connector shell as that connector is crimped onto the end of a UTP cable (Figure 8.26). A good technician with a punch-down tool can cut and terminate 24 pairs of UTP in about the time it takes to open the bag and sort out the parts associated with one coaxial connector.

Figure 8.25. One common variation of punch-down connection technology.

Figure 8.26. RJ-45 UTP data connector.

These connectors are cheap, and the performance is outstanding. Category 5 UTP connectors based on punch-down technology are suitable for data transmission rates up to 125 Mbaud. A punch-down connector is also called an insulation displacement connector (IDC).

The RJ-45 connector goes by many names (see Table 8.8). In standard practice, work area cables use the same RJ-45 connector at the equipment and also at the wall outlet.

Table 8.9 lists the pin assignments used by Ethernet for two-pair UTP connections. This table provides two different pin assignment listings for devices with and without an internal crossover. As explained in Section 7.6, "Crossover Wiring," the customary LAN wiring arrangement presupposes a hub port equipped with internal crossover and a client port without. In this case the wire is expected to connect the pins straight through, from pin 1 to pin 1, 2 to 2, and so on.

Table 8.8. Different Names for the RJ-45 Connector

LAN ports equipped with an internal crossover should always be marked with an "X" symbol.

If the connectors at both ends of a link are marked with an "X", or neither is marked, then an external crossover may be required.

The customary correspondence between connector pins and the wiring color code, for North America, also appears in Table 8.9. On an RJ-45 connector, the physical wiring pairs are assigned to pins [4,5] and [3,6], and [1,2] and [7,8], in that order. Single-pair telephone systems most often use pair 1.

One of the most common installation errors involves accidentally swapping the two wires of a pair, for example, BLU/WHT and WHT/BLU. This mistake is called a polarity reversal . Some systems are designed to tolerate polarity reversal. Systems that tolerate polarity reversal greatly simplify installation.

As pointed out in TIA/EIA 568-B, a good cable, coupled with good connectors, is not enough to ensure good overall link performance. Other factors, such as the quality of the jumper cables, the total number of connections in a link, and the care with which the connecting components were installed, may affect link performance. Nevertheless, so you'll have some understanding of the magnitude of connector effects, Table 8.10 and Table 8.11 list the attenuation and noise performance of TIA/EIA 568-B connecting hardware. Specialized connectors are available for use with 150- W STP-A cabling whose performance extends well beyond the basic frequency ranges indicated in the TIA/EIA 568-B standard.

Table 8.9. 10BASE-T North American Contact Assignments for RJ-45

In an ordinary, uncompensated connector the parasitic effects, which often involve a preponderance of either inductive or capacitive coupling, generate NEXT and FEXT noise with roughly the same magnitude. The asymmetry evident in the specifications below gives you a hint that the connectors are highly compensated for the purpose of reducing NEXT. This may be done by controlling the precise shapes of the metal electrodes and the proximity with which they approach each other within the shell of the connector.

Unfortunately, compensating elements that reduce crosstalk within one specified frequency band sometimes dramatically exaggerate it at other frequencies outside the band . Never extrapolate the crosstalk performance of a connector beyond its specified limits.

POINTS TO REMEMBER

• UTP connectors are cheap, and the performance is outstanding.
• Systems that tolerate polarity reversal greatly simplify installation.

Table 8.10. Insertion Loss of Connecting Hardware

Table 8.11. Noise Budget Items for Connecting Hardware