| I'm sure this has been discussed before, in various places. But
briefly: you need to distinguish the questions (1) should I use a tree
of concentrators or hang my nodes on the dual ring, and (2) should I
use SAS or DAS to connect to concentrators?
These two are often mixed together, which doesn't help the debate at
all.
Re (1): a dual ring tolerates ONLY single faults. Two faults creates a
partitioned network. So if you hook up things that are not under
careful control -- such as workstations -- if any two are unplugged
your network is mangled. Conversely, a concentrator tree with
workstations connected to the concentrators tolerates an arbitrary
number of unplugged workstations. Furthermore, depending on
configuration it can tolerate multiple concentrator failures.
Concentrators also fit well within the usual hierarchical wiring
schemes, and they offer a place for network management observation and
control.
Re (2): assuming the above convinced you to go for concentrators...
the SAS is the lowest cost connection to a concentrator. A DAS offers
a standby connection at additional cost. It's quite rare for
workstations to require a standby connection, since you're just
protecting from concentrator failures and cable breaks, both of which
are rare. (Concentrators have very high MTBF.) Note that you can get
fault tolerance by installing two SAS adapters if you really need
that, so even where you do want redundancy it doesn't force a DAS.
paul
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| I suspect the higher than expected DAS adapters is more a reflection of
how FDDI is often being used; if you are using it to connect only a
handful of stations, then DAS is probably easier because you don't need
a concentrator, and all the machines are probably close together, so
failures are easy to fix.
When more companies have thier primary networks on FDDI, they will
probably buy more concentrators and use SAS because the reliability
becomes more critical.
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