Unshielded Twisted Pair

• Both the patch cable and the crossover cable sold in computer stores is made of Unshielded Twisted Pair cable with "RJ45" connectors at each end. Unshielded Twisted Pair is the most widely used network cable in the world and is one of the cable types recommended under the Ethernet standards. Ethernet is the most widely implemented set of standards in the world describing the physical properties of networks, which includes cable and connector type. Unshielded Twisted Pair (UTP) contains eight wires. These are grouped into four pairs, and the two wires of each pair are twisted around each other. The twisting brings the magnetic field around each wire into contact. As the two wires are a positive and a negative wire, the opposite polarity of their magnetic fields causes them to cancel each other out. The cable does not need shielding because environmental magnetic interference merges with the magnetic fields of the wires and gets wiped out when the magnetic fields cancel out.
  
  

Patch Cable

• The purpose of a patch cable is to easily connect a computer or a printer to a piece of networking hardware -- like a hub, a switch or a router. It does not connect a computer to a computer or a computer to a printer. Although UTP contains eight wires, only four actually carry a charge. These are the positive and negative paths of a transmit circuit and a receive circuit. Computers and printers listen on the "Receive" circuit and transmit on the "Transmit" circuit. Networking hardware knows this, so it listens on the "Transmit" circuit and transmits on the "Receive" circuit.
  
  

Crossover Cable

• Patch cable does not work for connections between computers, because both computers will listen on the "Receive" circuit and transmit on the "Transmit" circuit. Crossover cable resolves this problem by switching over the wiring of the two circuits in the connector at one end of the cable. This means computer A transmits on its "Transmit" channel, but the connector at the other end connects that circuit to the "Receive" contacts of computer B. When Computer B thinks it is transmitting on the "Transmit" channel, it is actually transmitting on the "Receive" circuit. Thus both computers hear each other's transmissions.
  
  

Names

• Crossover cable is always called crossover cable. However, patch cable is sometimes referred to as "straight through cable," "Ethernet cable" or plain old "network cable."