iperf indicating 940 Mbits/sec using cat 5 cable

Question

I have a box of old ethernet cables and was going to use iperf to determine if they were cat 5 or cat 5e. As a sanity check I found one that still had labels on it indicating it was cat 5 but when I connected it between 2 PCs and tested it with iperf I am getting 940~ Mbits/sec. I found a 2nd one labelled as cat 5 and got the same result.

Both PCs have gigabit ethernet ports. One is running Windows and one is running Ubuntu. They are connected to a gigabit switch with a cat 5 cable going from the switch to the Ubuntu PC and a cat 5e cable going from the switch to the Windows PC.

On Ubuntu:

iperf3 -s

On windows:

.\iperf3.exe -c myip

The result:

 ID] Interval           Transfer     Bandwidth
[  4]   0.00-1.00   sec   113 MBytes   946 Mbits/sec
[  4]   1.00-2.00   sec   112 MBytes   934 Mbits/sec
[  4]   2.00-3.00   sec   112 MBytes   944 Mbits/sec
[  4]   3.00-4.00   sec   112 MBytes   944 Mbits/sec
[  4]   4.00-5.00   sec   113 MBytes   944 Mbits/sec
[  4]   5.00-6.00   sec   112 MBytes   944 Mbits/sec
[  4]   6.00-7.00   sec   112 MBytes   943 Mbits/sec
[  4]   7.00-8.00   sec   113 MBytes   945 Mbits/sec
[  4]   8.00-9.00   sec   112 MBytes   944 Mbits/sec
[  4]   9.00-10.00  sec   113 MBytes   945 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth
[  4]   0.00-10.00  sec  1.10 GBytes   943 Mbits/sec                  sender
[  4]   0.00-10.00  sec  1.10 GBytes   943 Mbits/sec                  receiver

Am I going crazy or am I doing something wrong?

Spiff · Accepted Answer · 2019-03-25T21:47:43.403

The gigabit Ethernet standard, 1000BASE-T as specified in IEEE 802.3, is specified to work over up to 100 meters of plain old Cat 5. The idea that it requires Cat 5e is a pervasive falsehood.

The 802.3 standard is freely downloadable from the IEEE, so you can download it and verify it for yourself.

~943 megabits per second is the theoretical max throughput you can get with TCP over IPv4 over 1000BASE-T using standard 1500 byte payloads, because of mandatory inter-packet gaps and other protocol overhead. So you're seeing full speeds as expected.

iperf indicating 940 Mbits/sec using cat 5 cable

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