It is a registered jack.The telephone RJ45 specifies the physical male and female connectors as well as the pin assignments of the wires in a telephone cable.In simple words , it is a connector having 8 pins.
A hard loopback test in telco speak is a simple device that connects the transmit tip side of a circuit to the receive tip side, and the transmit ring side to the receive ring side to complete a loop where the circuit is actually sending and receiving to itself. The device is usually an RJ45x plug that a tech will modify and place into a D-mark jack. By doing this, say in a T-1 circuit, you end up with a monitoring situation where the remote end tester can send and receive patterns to check whether a circuit is taking any errors. This can also be achieved by sending a loop pattern to a piece of equipment causing it to go into a loop for testing. But the hard loop is a physical piece placed into the circuit usually where the customer plugs his equipment. Mike Q Special Equipt Installer --GTE/Verizon 30 years
A hard loopback test in telco speak is a simple device that connects the transmit tip side of a circuit to the receive tip side, and the transmit ring side to the receive ring side to complete a loop where the circuit is actually sending and receiving to itself. The device is usually an RJ45x plug that a tech will modify and place into a D-mark jack. By doing this, say in a T-1 circuit, you end up with a monitoring situation where the remote end tester can send and receive patterns to check whether a circuit is taking any errors. This can also be achieved by sending a loop pattern to a piece of equipment causing it to go into a loop for testing. But the hard loop is a physical piece placed into the circuit usually where the customer plugs his equipment. Mike Q Special Equipt Installer --GTE/Verizon 30 years