Ethernet hubs are great

An ethernet hub is an old piece of technology from times when switches
were too complicated and expensive. As it doesn't need to inspect
frames and simply repeats them on all ports, it is supposed to have
a much shorter and symmetric latency than a switch. That should be
great for NTP.

Switches typically have an asymmetry and jitter in their latency of
tens or hundreds of nanoseconds. The asymmetry is different between
different ports and it impacts accuracy of clocks synchronized over
the switch. PTP requires hardware support in the switches in order to
compensate for the asymmetry and reduce the jitter, but I have not
seen any switches with hardware support for NTP yet.

I was curious to see how much better would a hub work. I found an
8-port 3Com OfficeConnect Fast Ethernet hub for testing. It is a
10/100 Mbit/s dual-speed hub, which apparently means it internally
works as two hubs connected with a two-port switch, where each
external port can be connected to one of the hubs. I wasn't sure if
that wouldn't degrade its performance to a switch, but it seems it
doesn't.

Using a computer with two I210 NICs (which support HW timestamping), I
tested NTP synchronization through different pairs of the hub's ports.
The latency of the hub seems to be only about 50 nanoseconds and I
didn't see any asymmetry within the stability of the test (about 2
nanoseconds). That's amazing.

Too bad hubs are no longer manufactured and there don't seem to be any
for faster network speeds. Also, there is a limitation on the number
of chained hubs, which limits the network size. But if you can find
some and don't mind maintaining a separate network for timekeeping,
this looks like a great option for a lab. Much cheaper than switches
with PTP support and you can still use NTP.

-- 
Miroslav Lichvar


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