5.2 Given a scenario, troubleshoot common cabling and physical interface issues
Cable Issues
📘CompTIA Network+ (N10-009)
Definition:
- TX stands for Transmit – this is the port or wire that sends data.
- RX stands for Receive – this is the port or wire that receives data.
- TX/RX reversed happens when the transmit wires on one device are connected to the transmit wires on the other device, and the receive wires are connected to the receive wires. In simple terms, both devices are trying to send data on the same wires and listen on the same wires, so communication fails.
Why it happens
- Using the wrong type of cable:
- Straight-through cable: Designed to connect different types of devices (e.g., switch → PC, router → switch).
- Crossover cable: Designed to connect similar devices (e.g., switch → switch, PC → PC).
- If you connect two similar devices with a straight-through cable, you may end up with TX/RX reversed because the send/receive pairs are not matched properly.
- Miswiring a custom cable:
- When building a cable manually, if you accidentally swap the wires for TX and RX, the devices won’t communicate.
Symptoms of TX/RX reversed
- Devices show link light, but no data flows.
- Ping or data transfer fails.
- Only one-way communication works in rare cases (half-duplex scenario).
How to Fix TX/RX reversed
- Use the correct cable type:
- Straight-through: For connecting different devices (PC → switch, router → switch).
- Crossover: For connecting similar devices (switch → switch, PC → PC).
- Modern devices often have Auto-MDI/MDIX, which automatically corrects TX/RX, but not all older devices support this.
- Check cabling with a cable tester:
- A cable tester can show if TX and RX pairs are correctly wired.
- Replace or rewire the cable:
- If wiring is manual, make sure the TX pair on one end goes to the RX pair on the other end.
IT Environment Examples
- Switch-to-switch connection: If two switches are connected with a straight-through cable and TX/RX is reversed, devices on each switch cannot communicate.
- PC-to-PC direct connection: Using a straight-through cable may cause TX/RX reversed. Using a crossover cable or enabling Auto-MDI/MDIX fixes it.
- Network troubleshooting: A technician notices a server cannot reach a storage device. Testing the cable shows TX/RX reversed. Replacing with the correct cable restores communication.
Key Exam Points
- TX/RX reversed = transmit wires are connected to transmit wires, receive to receive.
- Causes: wrong cable type or miswiring.
- Fix: use correct cable type, test cable, or rewire.
- Auto-MDI/MDIX can automatically fix this on modern devices.
- Common scenario: switch ↔ switch, PC ↔ PC, server ↔ server.
This topic is small but important for the exam because it tests your understanding of cabling and device connectivity. Remember: TX must connect to RX; if not, communication fails.
