Correction to the "Ethernet problem"

[Thomas Doerfler]

Leon,
Leon Pollak schrieb:
>> Apart from this hint: maybe you can check/eliminate part of the
>> communication path, by directly connecting the PC and the RTEMS system
>> using a TP crossover cable.
> I think this was the most productive idea..:)
> To rework the cables was the problem, but I connected my PC and the box via 
> the old 10BaseT HUB and, meanwhile, I am not able to reproduce the problem - 
> nothing is lost!
When changing from 100Base-T Switch to a 10MBit Hub, you change several 
things:
- The bit rate is slower
- All connections work in half-duplex instead of full duplex
- This also means you MIGHT get collisions (which is implossible, I 
think, in full duplex)
- The Hub is a really dumb "repeater", which multiplies a bitstream 
coming in to all ports in realtime
- A switch learns and forgets:

It learns, which MAC addresses are available at which port (which source 
MAC addresses come from a certain port), and directs any packet only to 
the port which corresponds to the destination MAC address. So, before it 
can route a packet to your system, your system must have sent a packet 
with its own MAC address in the source MAC address field. Only then the 
switch knows where it is connected.

And the switch forgets: When it has not received a certain MAC address 
for a while (about 15 minutes, I guess), then it erases the proper MAC 
address from its routing table.

Can you check, wheter the packet loss only occurs after about 15 minutes 
of non-traffic with your system?

In that case it would be worth to put in a different (higher-quality) 
switch, which does the learning faster. Actually I never herad about a 
switch learning too slow, but ethernet and its protocols rely on the 
fact, that packet may get lost, so the effect would not be obvious in 
most cases.

wkr,
Thomas.

> 
> 
> 
> So, aren't these the PHYs?
> ---------------------------
> 
> Many thanks to both of you for your help.


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