In the context of RJ45 wiring, "A" and "B" refer to two different standardized wiring schemes or pinouts used for Ethernet cables. These pinouts determine how the individual wires within the cable are connected to the pins on each end of the connector. The two main standards are TIA/EIA-568-A and TIA/EIA-568-B. 

Here are the key differences between the A and B wiring schemes:

1. Wire Colors:

• TIA/EIA-568-A (A): The color-coding for the individual wires is white/green, green, white/orange, blue, white/blue, orange, white/brown, brown.

• TIA/EIA-568-B (B): The color-coding for the individual wires is white/orange, orange, white/green, blue, white/blue, green, white/brown, brown.

2. Pin Assignments:

• Pin 1: White/Orange

• Pin 2: Orange

• Pin 3: White/Green

• Pin 4: Blue

• Pin 5: White/Blue

• Pin 6: Green

• Pin 7: White/Brown

• Pin 8: Brown

• Pin 1: White/Green

• Pin 2: Green

• Pin 3: White/Orange

• Pin 4: Blue

• Pin 5: White/Blue

• Pin 6: Orange

• Pin 7: White/Brown

• Pin 8: Brown

• TIA/EIA-568-A (A):

• TIA/EIA-568-B (B):

3. Usage:

• Both A and B standards are widely used, but the key is to ensure consistency on both ends of an Ethernet cable. A cable with one end wired to the A standard and the other to the B standard won't function properly. Most installations use either A or B consistently throughout the network.

4. Crossover Cables:

• For standard Ethernet cables (straight-through cables), both ends should be wired using either the A or B standard. However, for crossover cables (used to connect similar devices like two computers without a switch), one end is typically wired to A, and the other to B.

When wiring Ethernet cables, it's essential to follow either the A or B standard consistently to ensure proper functionality. The choice between A and B often depends on regional practices or specific preferences. In most cases, it's recommended to use the TIA/EIA-568-B standard for new installations as it has become more widely adopted.