Re: [Hampshire] 8% Packet loss due to ethernet cable

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Author: Chris Ellis via Hampshire
To: Hampshire LUG Discussion List
CC: Chris Ellis
Subject: Re: [Hampshire] 8% Packet loss due to ethernet cable
On Thu, Mar 4, 2021 at 6:39 PM Joseph Bennie via Hampshire <
hampshire@???> wrote:

> Actually the issue is more likely naturally collisions on the network.
> I noted that you said ‘hub’ not ‘switch’.

It's actually really hard to get hold of a true 'hub' these days. When
most people say hub they typically mean a switch.

> A switch has logic to balance all the traffic, like traffic lights (gross
> simplification).
> where as a hub is like all the lights are always green/ no lights at all,
> and you just take your chance crossing.
> With a switch when there are more devices on the network it manages a
> controlled degradation, like lights at a roundabout. That leads to slower
> response times but high reliablity.
> With a hub : well its pot luck. packets bouce and need to be resent futher
> amplifying the chaos. the more devices. the more chaotic.
> so sall that really happened when you changed the cable was the traffic
> settled and the previous collisions cleared.

The key difference is a hub electrically copies the incoming data to all
other ports. It relies on the collision detection of ethernet to deal with
multiple devices trying to talk at the same time.

A switch however is store and forward. A frame is received, buffered and
forwarded. With MAC address learning and CAM they can very efficiently
forward frames. It's also important
to remember that you only get packet loss when links are contended, ie at
100% utilisation. So all those fancy control features of managed switches
rarely kick in on a well designed network.

An important result of this difference is that it is impossible to have a
gigabit hub, since you cannot electrically copy the signals to all ports
like with fast ethernet.

> the other factor might just be old age - simple oxidation of the
> connections....

I've seen packet loss from an old cable which is straining the connector.
It's good advice not to reuse old cables.

The other thing to be very aware of is most Cat5e cables on Amazon are not
upto spec. The rating only means something if you can trust the source.
Cheap cables on
Amazon often are CCA (Copper Clad Aluminium) which does awful stuff to high
frequency signals. You'll find a CCA cable will work for a short run, but
will go out of spec
if longer than about 5m. It's definitely worth spending the extra 20p and
getting a good cable from a good supplier.

> So : a basic switch is probably a wise purchase. :)

If anyone genuinely has a hub, not for debugging stuff like LLDP, then I'd
echo replacing it, but I'd be skeptical that any 'hub' brought in the last
10 years is really a hub.

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