Straight Through and Cross over Cable

Crossover and straight-through are two types of twisted-pair cables. Both types use the same cable, but different cable pin placements on the connectors. This tutorial explains both types, their pin placement, and supported devices.

STP and UTP

A twisted-pair cable is available in two variants: STP (Shielded Twisted-Pair) and UTP (Unshielded Twisted-Pair). You can use any variant to build a crossover or straight-through cable. The main difference between the two variants is that STP adds an extra layer of plastic coating to each pair. The additional coating layer reduces EMI (Electromagnetic Interference).

How does the twisted-pair cable work?

A twisted-pair cable transmits and receives data as electronic signals. It connects two nodes. In data transmission, one node sends data to another node. The NIC of the sender node converts the data stream into electronic signals and places them over the twisted-pair cable's copper wire. The receiver node's NIC reads those signals from the wire and converts them back into the data stream.

Electronic signals or electric currents flow in a circuit. An electric circuit needs wires. The first wire carries the electrons or current from the source to the load. The second wire completes the circuit between the load and the source. When electrons or current pass through the load, the load performs its functions.

Let's take an example. Suppose you have an LED bulb, two wires, and a battery. To light this bulb, you connect it to the battery using the cables. You connect the positive and negative terminals of the battery to the bulb separately. The following image shows this example.

A twisted-pair cable uses the exact mechanism. Two wires in a twisted-pair cable form an electrical circuit between nodes. In this circuit:-

• The NIC of the node that sends the data works as the source.
• The NIC of the node that receives the data works as the load.
• The first wire carries the current from the sender node to the receiver node.
• The second wire completes the electric circuit.

The following image shows the electrical circuit between the sender and receiver nodes.

Once the electrical circuit is up, both the sender and receiver nodes can use it to transfer the data. End devices, such as PCs or servers, store and process data in digital or binary form. To transfer binary data over an electrical circuit, the NICs of both the sender and receiver nodes use an encoding scheme.

An encoding scheme is a language that both NICs understand. In the encoding scheme, the sender node modulates the electrical signal over time, while the receiver node interprets the resulting changes as binary data. For example, to transfer a binary digit 0, the NIC of the sender node drops the voltage to the lower voltage during the middle of a 1/10,000,000th-of-a-second interval. NIC of the receiver node detects this change and interprets it as a binary digit 0. Just like this, to transmit a binary 1, the sender node's NIC raises the voltage.

Current in an electric circuit always flows in one direction, from the source to the load. For this reason, only the sender node (source) can send its data to the receiver node (load). If the receiver node wants to send its data, it must create its own circuit. The following image shows how both nodes create and use their circuits to transfer the data.

Thus, a two-way data transmission requires two electrical circuits. Two electrical circuits need four wires. A twisted-pair cable consists of eight wires arranged in four pairs. Each pair consists of two twisted wires. The first wire has a single color-coded plastic coating, while the other wire has that color plus white with a striped plastic coating. For example, in the brown wire pair, one wire's coating is all brown, while the other is brown-and-white striped. The following image shows a twisted-pair cable.

Why are the wires twisted?

When an electrical current passes through a copper wire, it generates electromagnetic interference (EMI). EMI interferes with the electrical signals in nearby wires, including those in the same cable. It is known as the crosstalk. Twisting pairs of wires eliminates crosstalk.

RJ-45 (UTP cable) connector

Both the NIC and the switch port have an eight-pin slot. An RJ-45 connector connects these pins to the wires of a twisted-pair cable. An RJ-45 connector has eight physical locations, known as pin positions or simply pins, into which you can insert the eight wires of a UTP cable. These pins provide a place for the ends of the copper wires to contact the NIC or switch port pins. The following image shows an RJ-45 connector.

Making a twisted-pair cable

A NIC uses pins 1 and 2 to transmit the data. To receive data, it uses pins 3 and 6. A switch does the opposite. It receives data on pins 1 and 2 and transmits data from pins 3 and 6.

Based on the devices you want to connect, you can adjust the wire positions in the RJ-45 connectors at both ends in two ways: straight-through or crossover.

The first type of cable connects two different types of end devices, such as a PC to a switch. The second type of cable connects two devices of the same kind, such as a PC to another PC or a switch to another switch.

Ethernet straight-through cable

This cable places wires in the same position at both ends. The wire at pin one on one end of the cable connects to pin one on the other end. The wire at pin two connects to pin two on the other end of the cable, and so on.

The following table lists the wire positions of a straight-through cable on both sides.

The following image shows the straight-through cable.

A straight-through cable connects the following devices.

• PC to Switch
• PC to Hub
• Router to Switch
• Switch to Server
• Hub to Server

Ethernet crossover

This cable connects the transmitting pins on one side to the receiving pins on the other. The wire at pin one on one end of the cable connects to pin three on the other end. The wire at pin two connects to pin six on the other end of the cable. The remaining wires connect in the same positions at both ends.

The following table lists the wire positions for a crossover cable on both sides.

The following image shows the color coding of the crossover cable.

A cross cable connects the following devices.

• PC to PC
• Hub to hub
• A hub to a switch
• A cable modem to a router
• Two router interfaces

Conclusion

Learning the differences between crossover and straight-through twisted-pair cables is crucial for effective network connectivity. Both types utilize the same fundamental twisted-pair wiring but differ in pin configuration to serve specific purposes. Crossover cables connect similar devices directly, such as PC-to-PC or switch-to-switch, while straight-through cables connect dissimilar devices, such as a PC to a switch. By understanding these concepts, you can confidently set up and troubleshoot your network, improving connectivity and performance.