Crossover Wiring

Many UTP star-wired LAN standards, like 10BASE-T and 100BASE-T, designate distinct pairs of wires for unidirectional transmission and reception . At the hub and at the client, these pairs occupy specific pin positions on the data connectors. In the preferred arrangement, the hub and client have complementary pin assignments so that the wiring may be accomplished straight through, from end to end, connecting pin 1 on the client to pin 1 on the hub, pin 2 to pin 2, and so forth. This is what you should do for copper . For fiber, you label the TX and RX connectors on your equipment and expect the user to cross them over for you.

The preferred wiring arrangement is illustrated in Figure 7.4. In this figure, the client has a normal pin assignment, and the hub a complementary one. The wires run straight through. Inside the hub, adjacent to the imaginary transmitter and receiver, is where you are supposed to implement the crossover. A wiring crossover is an essential function in every link. It must reside either in the client, the hub, or the wiring. [63] The preferred location for a crossover is in the hub.

[63] Technically, the requirement is for an odd number of crossovers.

Figure 7.4. Example of straight-through wiring with internal crossover in hub.

The use of straight-through wiring simplifies installation and maintenance considerably. It eliminates any concern about whether a link might have a crossover at one end, the other, or both.

Certain exception conditions exist, like the connection between two hub ports. In this case both pieces of equipment may already include an internal crossover. The installer is expected to provide an external crossover (that is, a crossover explicitly implemented in the wiring).

When necessary, an external crossover should be implemented in a short, clearly visible section of cabling and boldly labeled.

Examples of crossover connections appropriate for Fast Ethernet (100BASE-TX) appear in Table 7.3 and Table 7.4

Table 7.3. Wiring Crossover for 100- W Balanced Cables

	Pair 3 2		Pair 2 3		Pair 1 4 [2]		Pair 4 1 [2]
From pin: [1]	1	2	3	6	4	5	7	8
To pin: [1]	3	6	1	2	7	8	4	5

[2] NOTE (2) ”RJ-45 pin numbers .

[1] NOTE (1) ”Not used by 100BASE-TX.

Table 7.4. Wiring Crossover for 150- W STP-A; References Are to DB-9 Pin Numbers

	Pair 1 2		Pair 2 1		Other Pins Not Used
From pin: [1]	5	9	1	6	2,3,4,7,8
To pin: [1]	1	6	5	9

[1] NOTE (1) ”DB-9 pin numbers.

POINTS TO REMEMBER

• Multi-pair building cables should be installed straight-through with no crossing of the pairs.
• When necessary, an external crossover should be implemented in a short, clearly visible section of cabling and boldly labeled.