Objective 2.4: Wiring Standards
Objective 2.4: Given a scenario, differentiate and implement appropriate wiring standards
T568A and T568B are two definitions of pin/pair assignments for eight-conductor twisted-pair cabling, such as Category 3, Category 5 and Category 6 unshielded twisted-pair (UTP) cables. These assignments define the pinout, or order of connections, for wires in RJ-45 eight-pin modular connector plugs and sockets. Cables that are terminated with differing standards on each end will not function normally.
Straight vs. crossover 
The 10BASE-T and 100BASE-TX Ethernet standards use one wire pair for transmission in each direction. This requires that the transmit pair of each device be connected to the receive pair of the device on the other end. When an end device is connected to a switch or hub, this crossover is done internally in the switch or hub. A standard straight through cable is used for this purpose where each pin of the connector on one end is connected to the corresponding pin on the other connector.
One terminal device may be connected directly to another without the use of a switch or hub, but in that case the crossover must be done externally in the cable. Since 10BASE-T and 100BASE-TX use pairs 2 and 3, these two pairs must be swapped in the cable. This is a crossover cable. A crossover cable must also be used to connect two internally crossed devices (e.g., two hubs) as the internal crossovers cancel each other out. This can also be accomplished by using a straight through cable in series with a modular crossover adapter.
Because the only difference between the 568A and 568B pin/pair assignments are that pairs 2 and 3 are swapped, a crossover cable may be envisioned as a cable with one connector following 568A and the other 568B. Such a cable will work for 10BASE-T or 100BASE-TX. 1000BASET (Gigabit crossover), which uses all four pairs, requires the other two pairs (1 and 4) to be swapped and also requires the solid/striped within each of those two pairs to be swapped.
A rollover cable (also known as Cisco console cable) is a type of null modem cable that is most commonly used to connect a computer terminal to a router's console port. This cable is typically flat (and has a light blue color) to help distinguish it from other types of network cabling. It gets the name rollover because the pinouts on one end are reversed from the other, as if the wire had been rolled over and you were viewing it from the other side.
The term loopback is generally used to describe methods or procedures of routing electronic signals, digital data streams, or other flows of items, from their originating facility quickly back to the same source entity without intentional processing or modification. This is primarily intended as a means of testing the transmission infrastructure.
All TCP/IP (Transmission Control Protocol/Internet Protocol) implementations support a loopback device, which is a virtual network interface implemented in software only and not connected to any hardware, but which is fully integrated into the computer system's internal network infrastructure. Any traffic that a computer program sends to the loopback interface is immediately received on the same interface.
A loopback interface has several uses. It may be used by network client software on a computer to communicate with server software on the same computer. For example, a computer running a web server can point a web browser to the loopback address to access that computer's own web site. This works without any actual network connection–so it is useful for testing services without exposing them to security risks from remote network access. Likewise, pinging the loopback interface is a basic test of the functionality of the IP (Internet Protocol) stack in the operating system.