Before you start
Objectives: learn what is the difference between a straight-trough and a cross-over cable, and which tools you will need to make a desired type of cable.
Prerequisites: you should know what is a Twisted Pair network cable.
Key terms: cable, transmit, receive, wire, standard, network, pin, connector, crimper, crossover
Straight Through versus Crossover Cable
Before we can create a cable we need to know the difference between a straight through and a crossover cable. Remember, we use RJ45 standard to connect our cables to network cards and other network devices. On each RJ45 jack we have eight pins. Now, Twisted Pair Ethernet specification uses the following pins (Tx is a pin used for transmitting and Rx is a pin used for receiving):
- Pin 1: Tx+
- Pin 2: Tx-
- Pin 3: Rx+
- Pin 4: Unused in 10BaseT and 100BaseTX (used in 1000BaseT)
- Pin 5: Unused in 10BaseT and 100BaseTX (used in 1000BaseT)
- Pin 6: Rx-
- Pin 7: Unused in 10BaseT and 100BaseTX (used in 1000BaseT)
- Pin 8: Unused in 10BaseT and 100BaseTX (used in 1000BaseT)
Let’s say that we have two computers and that we want to connect them together, directly. We’re going to take a piece of UTP cabling and we’re going to plug one end into RJ45 jack on one computer, and we’re going to plug the other end into the RJ45 jack on another computer. Remember that on each host, on each RJ45 jack, we have eight pins. Two of those pins are for transmit (1 and 2) and two of them are for receive (3 and 6) on both jacks. By connecting those two computers with a straight through cable, we actually connect pin 1 to pin1, pin 2 to pin 2, pin 3 to pin 3, and so on. If we do that we connect the transmit pin on one network board to the transmit pin on the second network board. Also, we connect the receive pins on one network board to the receive pins on the other network board. This is not going to work because we’ve got transmit going to transmit and we’ve got receive going to receive. To make this two computers communicate we have to connect the transmit pin to the receive pin, and the receive pin to the transmit pin. In our case with two hosts, we can do that with a crossover cable. A crossover cable is a network cable that has the transmit and the receive wires crossed over so that transmit goes to receive and receive goes to transmit. Most crossover cables are color coded red to indicate that they are a crossed over. On the other hand, if we connect those two computers using a Hub or a Switch, we will use the straight trough cable Hub or Switch will perform the crossing over function.
Straight-trough Cable
Within Twisted pair network cable there are four pairs of wires, which is eight wires in total. We have green (G), green with white (GW), orange (O), orange with white (OW), blue (B), blue with white (BW), brown (Br) and brown with white (BrW). The way we insert these wires into RJ45 connector is important. What we need to do is use a standard way of ordering those wires inside the RJ45 connector, so that anybody knows what we did and know what type of cable it is. We should use one of two standards that have been defined for the ordering of the wires inside of an Twisted pair network cable. Those standards are T568A and T568B. It doesn’t matter which one we choose.
T568A Standard
To use T568A standard we should arrange the wires from pins 1 to 8 in each RJ45 connector in the following order:
- GW (Green White)
- G (Green)
- OW (Orange White)
- B (Blue)
- BW (Blue White)
- O (Orange)
- BrW (Brown White)
- Br (Brown)
T568B Standard
To use T568B standard, we should arrange the wires from pins 1 to 8 in each connector in the following order:
- OW (Orange White)
- O (Orange)
- GW (Green White)
- B (Blue)
- BW (Blue White)
- G (Green)
- BrW (Brown White)
- Br (Brown)
It doesn’t matter which standard we use but once we choose a standard, we should do all your cables that way to avoid confusion during troubleshooting. Remember, if we want to make a straight through cable, we use exactly the same ordering on both ends of the cable.
Image 217.1 – T568A versus T568B
Cross-over Cable
The trick which we can use to make a cross-over cable is to use T568A standard on one end and T568B standard on the other end of the cable. If we do that we are actually switching the transmit and the receive pins.
Image 217.2 – Cross-over
Remember that we have two transmits (one is positive, one is negative), and we have two receives (one is positive and one is negative). By doing one end with T568A and one end with 568B, we’re switching the transmit and receive pins, so that the transmit matches up to the receive and the receive matches up to the transmit.
Tools
To make a cable we need a piece of generic CAT5 UTP networking cable, which is really inexpensive. We also need a special tool called a Crimper, which can be really expensive compared with the UTP cable.
Image 217.3 – Crimper
It usually has one slot for RJ11, and one for RJ45, that we use to crimp little metal connectors down onto the wires, piercing the insulation and making contact with the wire inside. Some crimping tools have cutters which allow us to cut the cable sheeth without cutting the wires inside. Also it can have another cutter for cutting through the cable. If our crimper does not have cutters we can manually take off the sheet or use universal strippers.
Image 217.4 – Universal Stripping Tool
We have to make sure that we cut off just the right amount of sheath. We don’t want to cut it too short, nor do we want to cut it too long. Basically, a little bit of sheet has to come into the connector.
The hardest part of the whole process is to get the wires in the right order (T568A or T568B) when putting them into RJ45 connector. Before we stick them in RJ45 we should do a visual check and make sure that they stayed in the proper order. Inserting wires into RJ45 is the second hardest part. Inside of RJ45 connector there are separate channels for each wire. We have to straighten out our wires so that they’re in the right order and as flat as possible, and then push them all the way into the RJ45. Remember, every wire has to be in its own channel. Also, make sure that they are pushed all the way to the end.
When the wires are in, we take our crimper, stick the RJ45 in the crimper, squeeze the crimper down and then pull it up. That will do a couple of things. Each metal tab on the RJ45 connector will be forced down, which will pierce the insulation on each wire. By piercing it, it will come in contact with the core of each wire. Also, every RJ45 has a locking tab on the end. By squeezing it down it locks the end of the cable which keeps it from pulling out. For straight-trough cable we would use the same standard on both ends of cable, but for crossover we would use the opposite standard on one end of the cable. Once we get it done we can use a media tester that will test the cable to make sure that it works correctly. There are various types of testers available.
Image 217.5 – Cable Tester
Remember
We use RJ45 standard to connect UTP cables to network devices. RJ45 has 8 pins. Two of those pins are for transmit (1 and 2) and two of them are for receive (3 and 6) on both ends of the cable. We have to connect the transmit pin to the receive pin, and the receive pin to the transmit pin. A crossover cable is a network cable that has the transmit and the receive wires crossed over. The way we insert these wires into RJ45 connector is important. We should use T568A or T568B standard for ordering the wires inside of RJ45 connector. If we want to make a straight through cable, we use the same ordering on both ends of the cable. To make a cross-over cable we can use T568A standard on one end and T568B standard on the other end of the cable. To make a cable we need a Crimper which we use to crimp little metal connectors inside of RJ45 down onto the wires. Once our cable is finished we should use a media tester to make sure that our cable works correctly.