[HECnet] FDDI advice

[Brian Hechinger]

All this talk of FDDI makes me want to go get the 4000/500s. I have a
pair of QBus FDDI cards. I suppose I would have to make the Octane a
router between FDDI and ethernet. :)

-brian

On Thu, Oct 10, 2013 at 02:26:23PM +0000, Paul_Koning at Dell.com wrote:

On Oct 10, 2013, at 8:44 AM, Brian Hechinger <wonko at 4amlunch.net> wrote:

On Thu, Oct 10, 2013 at 02:36:04PM +0200, Peter Lothberg wrote:

FDDI/CDDI is a dual ring token ring bus, with 4470 MTU byte packets,
it has 802.-- frames. DEC had a mode where you turned the token off
and used it for ptp full duplex.

I didn't know about the ptp thing. That's nifty.

For example cisco/cabletron/crecendo had ethnernet switches with a
FDDI uplink, that you could use.

DEC made one as well, it was that large modular thingie. I used to have
one. Never got it powered on as it was enormous.

But you need nothing to build a FDDI ring, its a A and a B ring, you
can just plug the cards together with fiber-patch-cables.

Unless you have one of those obnoxious single attached station cards.

-brian

Ah, time to dust off some dormant memories.   I used to work on the FDDI standard at DEC; this stuff is familiar.

"CDDI" is marketing slang; it is not standard terminology.

FDDI is different from Ethernet; the MAC layer protocol is completely unrelated.   It's quite similar to token bus (802.4), actually.   (The only thing it has in commoni with 802.5 is the words "token" and "ring" -- apart from that, the two protocols operate completely differently.)

FDDI connections have a "type", which can be "A", "B", "S", or "M".   "M" ports exist on concentrators.   NICs will have A and B ports, if there are two connectors on the NIC ("dual attached station" or DAS) or an S port, if there is one connector (single attached station or SAS).

You have a number of topology options.

If you have DAS NICs, you can wire any number of them together in a "dual ring".   That's the original FDDI topology, before DEC forced concentrators to be added into the standard.   To do that, connect the NICs in circular fashion, A to B.   Connected that way, loss of any single connection is handled transparently.

If you have SAS NICs. you can connect a pair of them (S to S).

If you have DAS NICs plus or or two SAS, wire the DAS NICs A-B in a chain (essentially a dual ring cut open).   Then connect a SAS to each end (or just to one end, if you have one SAS).   There is no redundancy in this config. 

Finally, if you have any concentrators, you can build a tree config out of those.   If so, the M connectors connect to the NIC connectors (A, B, or S), and the concentrator's A and B connectors either connect to M ports higher up in the hierarchy, or in a dual ring if you have a ring of concentrators, or nowhere if you're at the root of a tree.

FDDI fiber connectors are standardized but different from fiber connectors used by other networks.   The connector is fairly large, flat rectangular with a shroud covering the fiber ends.   Connectors are keyed to match the port type, though you can omit the keys and just wire carefully.   Standard fiber is 62.5/125 micrometer, but 50/125 also works.

You can find more in the DEC Technical Journal, Vol. 3 No. 2, spring 1991.   Or the relevant ANSI/ISO standards if you are a masochist.   The topology rules are described fairly well in the concentrator article in DTJ, but their full glory can be found in the FDDI "SMT" (station management" standard.

	paul