[HECnet] FDDI advice

[Kari Uusim ki]

Thanks Paul for the detailed clarification!


On 11.10.2013 17:23, Paul_Koning at Dell.com wrote:

On Oct 11, 2013, at 2:28 AM, Kari Uusim  ki <uusimaki at exdecfinland.org> wrote:

...
Yes, that would be the one when converting between FDDI and 100BaseT Ethernet.

The best effort it can produce is about what 100BaseT can, because it is the lowest common nominator. FDDI performs better due to the larger packet size (which is usable only between FDDI nodes) and the lower overhead.

Larger packet size, yes.   Lower overhead, no.   Token systems of any kind are guaranteed to be less efficient than Ethernet, because you have to wait for the token.   At best, they will be slightly better under very high load than half duplex Ethernet, but full duplex Ethernet will outperform any token network under all conditions.

It's very clear that FDDI will outperform half duplex Ethernet, because there will not occur any collisions in FDDI.
But if we think about a FDDI-to-100BaseT (Full duplex) topology and the traffic flows with as large packets as possible, doesn't FDDI be able to carry about three times more payload in each packet than Ethernet? In that case one FDDI packet payload will need to be split into three Ethernet packets and three Ethernet packet payloads will fit into one FDDI packet.

Btw. Many of the FDDI principles was used when developing FibreChannel.

Not really.   The two were done at the same time (in different subgroups of ANSI X3T9) but with no significant connection between them.   FDDI was 100 Mb/s, Fibre Channel was initially 800 Mb/s (not 1 Gb/s -- FC has always committed false advertising by reporting the baud rate of the encoded signal rather than the payload rate -- if Ethernet had done things in FC terms then 10 Mb/s Ethernet would have been called 20 Mb/s Ethernet).   And even the PHY is completely different: 4b/5b coding for FDDI, 8b/10b for F
C.

I see, my memory didn't serve me right then. I thought FC was developed later than FDDI and that FC had adopted the similarities from FDDI.
Sorry about the confusion. I should dust off my bookshelf and reread the books about FDDI and FC to refresh my memory.

As I mentioned before, FDDI was based on 802.4 (token bus) MAC layer algorithms ("timed token protocol"), with vast quantities of complexity added on top.   (Partly that was done by some of the participants to delay the standard by a couple of years.)   Fast Ethernet adopted the 4b/5b encoding from FDDI, but apart from that it was a technological dead end.

	paul

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Kari