LAN adapter selection 10M -vs- 100Mbit/s

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I am looking to source a ethernet adapter IC for an embedded product.

I have considered a number of adapter IC's from one end of the
spectrum to the other.  My first choice was the SMSC LAN91C111 10 /
100Mbit/s adapter.  But I found that the cost is 3 times as much as a
Crystal CS8900A 10Mbit/s part.  Does that soud reasonable?

Since the 10Mbit speed is adequate for the application, my main reason
for looking at 100Mbit parts was compatibily with network loading.

Can someone tell me if it is really an issue to put a 10Mbit/s device
on a moderately loaded 100Mbit/s network.  If the 10Mbit/s device only
needs to transfer a small amount of data on the network, will it make
a significant difference to the other devices on the network?

Can anyone explain how 10 / 100 auto-dectection works?

Thanks in advance.

Johnny.













Re: LAN adapter selection 10M -vs- 100Mbit/s

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Not moderately. But if things get worse and a switch is in the mix the
10Mbit/s packets could potentially begin to clog up the switch's packet
memory as the device slowly drains off the back log.

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You could start here, it's pretty simple.

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You're welcome in arears.

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- Mark ->
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Re: LAN adapter selection 10M -vs- 100Mbit/s

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Not if the switch doing the networking is worth its salt.

If you don't actually need more than 10 Mbit/s, using 100 Mbit/s
hardware is likely to be a waste of both money and energy.

--
Hans-Bernhard Broeker ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Re: LAN adapter selection 10M -vs- 100Mbit/s

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How so? If packets destined for the 10Mbit/s device are incoming to the
switch on a Gbe link and the packet memory in the switch fills up, how
does any switch become worth its salt and continue to route packets to the
remaining 100Mbit/s ports? No, pause frames are not allowed.

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Re: LAN adapter selection 10M -vs- 100Mbit/s

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Then the OP's condition that a 10Mbit/s link is all the device needs
must have been violated.

Yes, those extra 990 Mbit/s of data *must* be dropped at some point,
and it's neither the 10Mbit/s device nor the switch that bears the
responsibility for that problem --- it's the data source at the other
end of the Gbit link.

--
Hans-Bernhard Broeker ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Re: LAN adapter selection 10M -vs- 100Mbit/s
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As the chokepoint, the switch has a hard limit and it *must* be the
device to drop this traffic.  But to your point, any properly designed
protocol is responsible for detecting this condition and throttling the
source to relieve the problem.

Re: LAN adapter selection 10M -vs- 100Mbit/s

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Now suppose the 10Mbit/s link is upgraded to 100Mbit/s and the Gbe link is
feeding this port at 50Mbit/s. The 100Mbit/s MAC can now drain its packets
out of the switch fast enough to keep the switch from back logging. The
embedded product with the upgraded 100Mbit/s MAC may need to drop the
packets but at least it won't bring the switch down.

Of course the system must be designed to ensure that the embedded product
is not overrun if it can.

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Re: LAN adapter selection 10M -vs- 100Mbit/s

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Seem I have to repeat this: I strictly refuse to accept that as an
important case to be considered.  In such a case the Gbe link is
violating the original "10Mbit/s is enough" assumption, and behaving
unreasonably.  And why should we assume it stops at a mere 50Mbit/s,
anyway?  Why not do 300 Mbit/s?

Yes, a GBit line can flood the switch if it continues sending data to
a single node faster than that node can accept them, regardless of
whether that nodes is on a 100Mbit/s or 10Mbit/s line.  Requiring that
the switch not be allowed to become saturated or block packages in
such a case is to ask the impossible.  If a Gbit upstream node decides
to behave unreasonably, nothing short of an all-Gbit network will be
able to withstand it.  So what --- should we take that to mean all
embedded devices that have an Ethernet link must make that a
1000baseT, and that all 100Mbit switches are obsolete?  I don't think
so.

Not to mention there must be some serious flaw in the transport
protocol if it allows the weaker endpoint to be flooded like that,
without, e.g. the stronger one waiting for ACK packets or some other
kind of 'clear to send' signal to come from it.

--
Hans-Bernhard Broeker ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Re: LAN adapter selection 10M -vs- 100Mbit/s
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Have you found products on the market that actually work this way?  I
can't imagine they'd last long.

The approach on well-designed switches is to limit the output queue
depth on each port.  When that queue fills up, that port experiences
packet loss.  Other ports are unaffected.

Re: LAN adapter selection 10M -vs- 100Mbit/s

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Some popular switch chips have very limited packet memory. When used in an
environment of bursty traffic to a port or two on the switch one might
oversubscribe the switch memory. Everyone wins until the bursts hit all
the ports and use up all the memory. An architectural flaw to be sure.

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- Mark ->
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Re: LAN adapter selection 10M -vs- 100Mbit/s

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No. Only an obsolete part would be that much less expensive, IMHO ;-)

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That IS a question worth asking: if the network is composed of HUBS
(not switches) then all NIC see any transaction (and must to respect
  contention).  Therefore a packet for the 10Mb unit would take 10x
the bandwidth of the same packet for the other. For one embedded
gadget at low usage, no sweat.  But if someone makes up a test lab
for testing 1000 units at a shot it could be an issue!

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Generally, no.


Does it help to say it is handled by the NIC and the NIC driver? - RM


Re: LAN adapter selection 10M -vs- 100Mbit/s

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Not really.  Because there is no such thing as a HUB that you can
connect both 10Mb and 100Mb devices to, simultaneously.  There some
devices that claim to be such "dual-speed Hubs", but in reality, those
are (simple) switches.

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Such a "someone" would deserve whatever problems they got.

--
Hans-Bernhard Broeker ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Re: LAN adapter selection 10M -vs- 100Mbit/s
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Check out the ASIX AX88796L.  It's a 10/100 NE2000-based ISA
controller.  IIRC, the cost difference in reasonable quantity was not
much higher than a Realtek 10Mb chip.


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It'll make no difference to other devices on the network.  In most
cases, it'll make no difference to your device either.

Where it can matter is if your device sees a relatively high volume of
data from the network (or even moderate levels, if you're trying to
transmit a lot of data).  This can happen for a variety of reasons, even
in a switched network, but generally shouldn't be an issue if you're
sending a low volume of data (say, <100Kbps).

At high data rates, it'd be valuable to have full-duplex operation,
where you transmit without regard to incoming data.  This is a feature
that's sometimes supported on 10Mb controllers, but will be problematic
at best to apply properly since auto-detection isn't available (and this
setting needs to vary based on how your device is connected).  With
10/100 controllers, it's configured automatically during the
auto-detection sequence so you get it with no hassle.


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There's an encoding in the link signalling, IIRC.  It's a level of
detail that isn't typically documented in the chip specs because "it
just happens" when you enable it on the controller.  If you really want
to understand the encoding, you can download the related 802.3 specs
from IEEE for free, but it's likely to be a beastly set of docs.

Re: LAN adapter selection 10M -vs- 100Mbit/s
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Oh, and full-duplex is only available when connected to a switch (not a
hub).  These days you've really got to search to find hubs, but they're
still out there.

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