| John,
I believe the recommendation refers to total nodes on the ring,
including stations attached to CONs. Once you get above this number,
the latency incurred by this many stations can start to affect the
performance of the ring. Each station's token hold time may become very
small, depending on how busy the other stations are on the ring. Plus,
with large numbers of stations on the ring, you're increasing the
chances that one may fail and cause a ring re-config. When you start to
get above 100 nodes in the ring, the time it takes for the claiming
process to complete may begin to increase and more noticeable delays may
start to occur to the users.
I don't think 100 stations is such a severe limitation, since you're
looking at directly attached FDDI stations. Many customers still have
most of their networked systems bridged onto FDDI from Ethernet. And as
the need for more directly attached stations occurs, the networks will
easily expand into many FDDI ring/segments, just as an Ethernet-based
LAN does today. With the availability of the GIGAswitch,
interconnecting many rings at full speed transparently won't be an
issue either.
Debbie
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| You didn't read it right.
It does NOT say that larger configurations are unsupported or disallowed.
It ONLY says that they are not recommended. If your customer wants them,
you should explain that, the large the ring, the less reliable it is -- but
that it IS supported and that we WILL sell such installations!
(The right message: Digital supports any FDDI configuration allowed by the
FDDI standard.)
The reasoning is simple: packets on FDDI have to go through all the components
in the ring. (That's the nature of a ring.) So the probability of packet
loss, or of ring reinitialization due to token loss, or even of undetected
data corruption, is proportional to the ring size. Furthermore, you're sharing
the capacity of the LAN with all the other nodes. For these reasons, we
determined that a limit of roughly 100 nodes is a good one to use in planning
for a high performance, highly reliable FDDI.
Larger networks will certainly work. If the individual nodes are quite
active, the bandwidth available to each will be rather small. If they are
usually idle but occasionally demand big bursts of traffic, that will work
quite well even in large configurations. Large configurations are also
more vulnerable to network problems, since there are more components that
all have to work right, and there are more places where random bit errors can
hit a packet and cause it to be lost. So large networks demand more care
in maintenance.
Incidentally, the guideline does mean 100 stations plus 20 concentrators,
not 100 DAS plus 20 concentrators plus lots of things hanging from those.
But since it's a guideline, not a limitation, you're welcome to build the
other thing -- it's legal by the ANSI rules and therefore it's supported.
paul
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