RE: [chrony-users] Ethernet hubs are great

[Bryan Evenson <bevenson@xxxxxxxxxxxxxx>]

Miroslav,

> -----Original Message-----
> From: Miroslav Lichvar <mlichvar@xxxxxxxxxx>
> Sent: Wednesday, May 4, 2022 4:14 AM
> To: chrony-users@xxxxxxxxxxxxxxxxxxxx
> Subject: [chrony-users] 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.
> 

Agreed.  Hubs are great for any network protocol testing.  Plug all the equipment into the hub, plug your PC into any port and just fire up Wireshark to spy on all the traffic.  Daisy chain multiple hubs together to get more ports if needed.  Much simpler to setup than switches with PTP support, and you don't have to worry about the hub accidentally filtering out traffic because of some buried configuration settings.

Bryan

> --
> Miroslav Lichvar
> 
> 
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