Wireshark: How to find signal issues in network cables

In an increasingly connected world, the quality of your network can be the difference between productivity and frustration. But what happens when the internet slows down, downloads stall, and video conferences freeze for no apparent reason? Often, the culprit is not the provider or the router, but something as trivial as a network cable.

Network cables with problems — whether due to poor crimping, wear and tear, or electromagnetic interference (EMI) — can significantly degrade performance, causing high retransmissions and latency. These seemingly complex symptoms can be diagnosed with a powerful and free tool: Wireshark. It allows you to peek into what's happening on the network at a level of detail few other tools can. Let's dive into how to use it to unravel these cabling mysteries.

Why Faulty Network Cables Cause Headaches

Imagine a network cable as a road where data travels. If this road is full of potholes or dangerous curves, car traffic (i.e., data) will be slow and prone to accidents. In the networking world, these "accidents" manifest as corrupted or lost data packets, which need to be re-sent. This constant re-sending, called retransmission, consumes bandwidth and increases network response time.

Latency, in turn, is the time it takes for a packet to travel from one point to another and back. A faulty cable introduces significant delays because the electrical signal degrades, requiring more processing to be interpreted or even re-sent. This is particularly critical in delay-sensitive applications such as online gaming, Voice over IP (VoIP) calls, and video conferences, where fluidity is essential. A seemingly harmless cable can be the Achilles' heel of your entire network infrastructure, directly affecting user experience.

Wireshark and the Data Link Layer: Where Failures Emerge

Wireshark is a packet analyzer, meaning it captures and displays data traveling through your network interface. It primarily operates on the lower layers of the OSI model (Layers 1 and 2), specifically the physical and data link layers. It's at this level that cabling problems become visible. Wireshark can show us raw Ethernet packets, allowing us to see if frames are being transmitted correctly or if they are being dropped and retransmitted. To better understand its relation to network diagnostic tools, it's worth checking how to use tcpdump to troubleshoot network latency and errors.

Identifying Packet Retransmissions with Wireshark

The most direct way to identify cable issues in Wireshark is to look for retransmissions. When a packet is sent, the sender expects an acknowledgment (ACK) from the receiver. If this acknowledgment does not arrive in a timely manner, the sender assumes the packet was lost or corrupted and re-sends it. Wireshark can filter these events, which are a clear sign that something is wrong with the physical transmission.

To view retransmissions, start a Wireshark capture on the network interface connected to the suspect cable. After collecting some data, use one of the filters below:

tcp.analysis.retransmission

Or, for a more detailed analysis of lost packets and retransmissions:

tcp.analysis.retransmissions or tcp.analysis.fast_retransmissions

This filter will highlight all packets identified by Wireshark as retransmissions or fast retransmissions. A large number of these packets, especially to the same destination, suggests a physical or data link layer problem. It's like seeing the same car trying to get through a pothole repeatedly until it succeeds. This tool is vital for understanding communication quality, similar to what is done when diagnosing network packet loss using the ping command.

Analyzing Latency and RTT in Connections

In addition to retransmissions, latency is another crucial indicator of cabling problems. Round-Trip Time (RTT) is the time it takes for a packet to go from point A to point B and receive a response. High and inconsistent RTT can point to signal issues. Wireshark can calculate the RTT for TCP sessions.

To see the RTT, right-click on any TCP packet in your capture, go to "Follow," and select "TCP Stream." This will open a new window with the communication flow. In the "Statistics" menu, you can find "TCP Stream Graphs" and select "Round Trip Time Graph." This graph will visually show RTT fluctuations. Values that jump abruptly or remain consistently high are strong indications of problems in the physical medium.

Correlating Latency with Cabling Issues

Consistently high latency, or frequent spikes, especially for devices that are physically close and connected by a specific cable, can be the proof you need that the cable is compromised. In high-performance networks, where stability is crucial, any unjustified increase in RTT should be investigated. Compare the RTT between different network segments or with other devices using different cables to isolate the problem. If a server with a suspect cable shows a much higher RTT than a neighbor with a new cable, the clue is clear.

Poor Crimping and Electromagnetic Interference (EMI)

Two of the most common culprits behind problematic network cables are inadequate crimping and electromagnetic interference (EMI).

• Poor Crimping: Refers to how the internal wires of the cable are connected to the RJ45 connectors. If the wires are not in the correct order (T568A or T568B standards), are not properly inserted, or the connector is not securely fastened, the electrical connection will be faulty. This causes signal loss, short circuits between pairs, and consequently, data transmission errors that Wireshark will identify as retransmissions.
• Electromagnetic Interference (EMI): Is caused by the proximity of network cables to sources of electrical noise, such as high-voltage power cables, electric motors, fluorescent lights, or transformers. This interference can corrupt the signal traveling through the network cable, introducing noise and making packets unreadable. Shielded cables (STP) are designed to mitigate EMI, but unshielded cables (UTP) are much more susceptible. EMI can also be intermittent, making diagnosis more challenging as symptoms appear and disappear.

Visible and Hidden Signs of Cable Problems

Beyond Wireshark, there are other signs. Visibly, poor crimping can be noted by exposed wires in the RJ45 connector, or the connector not fitting snugly into the port. A severely bent or pinched cable can also damage the internal wires. Hiddenly, Wireshark is your best ally. But don't forget to check the activity LEDs on the network card or switch; an intermittently blinking link or one operating at a slower speed (e.g., 100 Mbps instead of 1 Gbps) can indicate a physical problem, as auto-negotiation tries to find a stable speed.

Step-by-Step: Using Wireshark to Diagnose Cables

Now that we understand the concepts, let's look at the practical steps for using Wireshark to diagnose cabling problems:

1. Isolate the problem: Connect the computer running Wireshark directly to the cable you suspect is problematic. If possible, eliminate switches or other intermediate devices to focus the analysis on the specific cable. If you are investigating communication between two devices, try capturing on both sides of the cable.

2. Start the capture: Open Wireshark, select the correct network interface (usually Ethernet or Wi-Fi), and start packet capture. Let it run for a few minutes, especially while some network activity (file transfer, streaming) is happening, to generate relevant traffic.

   # Example tcpdump command for similar capture, if no GUI
   sudo tcpdump -i <interface> -w /tmp/capture.pcap

3. Filter retransmissions: After capturing, apply the filter tcp.analysis.retransmission in the Wireshark filter bar and press Enter. Observe the number of packets that appear. A large volume is a red flag.

4. Analyze RTT: For a specific TCP connection that is experiencing slowness, right-click on one of the packets, choose "Follow" > "TCP Stream." Then, go to "Statistics" > "TCP Stream Graphs" > "Round Trip Time Graph." Look for spikes or consistently high RTT.

5. Investigate the data link protocol: Besides TCP, pay attention to data link layer error messages, such as CRC (Cyclic Redundancy Check) or FCS (Frame Check Sequence) errors, although Wireshark may not always directly display these errors if the network card discards them beforehand. A high rate of TCP retransmissions usually already points to these underlying issues. Tools like iperf3 can complement this investigation, helping to measure bandwidth and identify network bottlenecks in a more controlled manner.

6. Test a different cable: If Wireshark points to cable problems, the best proof is to replace it with a known good cable. Repeat the tests and compare the retransmission and RTT results. If the problems disappear, you've found the culprit.

Conclusion: Stable Network Signal is Fundamental

Diagnosing network cable problems may seem like a simple task, but the reality is that physical and data link layer failures are often the most underestimated causes of slowdowns and instability. Wireshark, with its ability to scrutinize network traffic, becomes an indispensable tool for identifying retransmissions and elevated latency, pointing directly to poor crimping or electromagnetic interference.

By understanding how to interpret retransmission patterns and RTT graphs, you can turn network slowness frustration into an accurate diagnosis and an effective solution. Remember, investing in good quality cables, ensuring proper crimping, and planning cable routing to avoid EMI sources are crucial steps to maintain a robust and reliable network infrastructure where data flows unimpeded.