New Ethernet Cable Testing

PC-to-PC testing is an excellent practical method. The best software is iPerf3, which measures TCP/UDP throughput, retransmissions and packet loss. It is free and specifically designed for network performance testing.

What you need

• Two computers with Ethernet adapters supporting the required speed
• The cable being tested
• iPerf3 installed on both computers
• Matching NIC speeds—for example, two 2.5G adapters to test 2.5 Gb/s
• Modern adapters support Auto-MDI/MDIX, so a crossover cable is normally unnecessary

Connect the cable directly between the two computers.

1. Configure IP addresses

On PC 1:

IP address: 192.168.50.1
Subnet mask: 255.255.255.0
Gateway: blank

On PC 2:

IP address: 192.168.50.2
Subnet mask: 255.255.255.0
Gateway: blank

Disable Wi-Fi temporarily to ensure the test uses Ethernet.

Confirm connectivity from PC 2:

ping 192.168.50.1

2. Confirm negotiated data rate

On each Windows computer:

Get-NetAdapter | Format-Table Name, Status, LinkSpeed

You should see the expected speed:

Ethernet    Up    1 Gbps

If a Cat6 cable repeatedly negotiates at 100 Mb/s instead of 1 Gb/s, one or more pairs are probably faulty.

3. Run a TCP throughput test

On PC 1, start the server:

iperf3.exe -s

On PC 2, run a five-minute test:

iperf3.exe -c 192.168.50.1 -t 300 -P 4

-P 4 creates four parallel streams, which can help fully load faster connections.

Typical healthy results are approximately:

Negotiated link	Expected TCP result
100 Mb/s	93–95 Mb/s
1 Gb/s	930–950 Mb/s
2.5 Gb/s	2.3–2.4 Gb/s
5 Gb/s	4.6–4.8 Gb/s
10 Gb/s	9.2–9.5 Gb/s

These are approximate because Ethernet, TCP and IP overhead reduces usable throughput.

4. Test the opposite direction

From PC 2:

iperf3.exe -c 192.168.50.1 -t 300 -P 4 -R

-R makes traffic travel from the server towards the client.

With a recent iPerf3 version, test both directions simultaneously:

iperf3.exe -c 192.168.50.1 -t 300 -P 4 --bidir

For initial testing, I prefer separate forward and reverse tests because the results are easier to interpret.

5. Look for TCP retransmissions

The result contains a Retr column:

[SUM]  0.00-300.00 sec  32.8 GBytes  939 Mbits/sec  0  sender

You want:

• Stable throughput
• Ideally zero retransmissions
• No link disconnections
• Similar performance in both directions

One or two retransmissions over a long test do not automatically prove the cable is faulty, but persistent or rapidly increasing retransmissions need investigation.

6. Run a UDP packet-loss test

For a 1 Gb/s link, start below the maximum:

iperf3.exe -c 192.168.50.1 -u -b 900M -t 300

Then increase it:

iperf3.exe -c 192.168.50.1 -u -b 950M -t 300

The receiver reports lost datagrams:

Lost/Total Datagrams
0/2000000 (0%)

Ideally, you want 0% packet loss. However, if the sending PC cannot generate packets quickly enough or the receiving PC is overloaded, UDP loss may occur even with a good cable.

7. Check Windows adapter error counters

Before testing:

Get-NetAdapterStatistics -Name "Ethernet" | Format-List *

Run the test, then issue the command again:

Get-NetAdapterStatistics -Name "Ethernet" | Format-List *

Examine:

• ReceivedPacketErrors
• OutboundPacketErrors
• ReceivedDiscardedPackets
• OutboundDiscardedPackets

Microsoft confirms that this command reports adapter errors and discards, although the exact counters available depend on the NIC and its driver.

PS C:\Users\Administrator> Get-NetAdapterStatistics -Name "Wi-Fi" |
>>     Format-List * |
>>     Out-String -Stream |
>>     Select-String "rror|iscard"

OutboundDiscardedPackets : 0
OutboundPacketErrors     : 0
ReceivedDiscardedPackets : 0
ReceivedPacketErrors     : 0