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Conference 7.286::fddi

Title:	FDDI - The Next Generation

Moderator:	NETCAD::STEFANI

Created:	Thu Apr 27 1989
Last Modified:	Thu Jun 05 1997
Last Successful Update:	Fri Jun 06 1997
Number of topics:	2259
Total number of notes:	8590

637.0. "Full Duplex from FDDI Newsgroup - any comment" by VIVIAN::MILTON (Invisible person it seems!) Wed Jul 08 1992 06:18

Article 432 of comp.dcom.lans.fddi:
Newsgroups: comp.dcom.lans.fddi
Path: uvo.dec.com!e2big.mko.dec.com!pa.dec.com!decwrl!uunet!sun-barr!ames!sgi!rhyolite!vjs
From: [email protected] (Vernon Schryver)
Subject: Re: Point-to-Point Full-Duplex FDDI Link
Message-ID: <[email protected]>
Organization: Silicon Graphics, Inc.  Mountain View, CA
References: <[email protected]>
Date: Tue, 7 Jul 1992 18:41:45 GMT
Lines: 23

In article <[email protected]>, [email protected] (Che-hoo Cheng) writes:
> Hi all,
> 
> I've heard that Digital supports Point-to-Point Full-Duplex FDDI mode on its 
> NIS brouter.  That means if you link up only two such nodes together 
> through point-to-point link, you can get 100Mbps throughput both ways.  My 
> questions are:  Is it a standard?  Do other vendors' products support such 
> mode?

Are you sure about that "100Mbps throughput"?  I hadn't heard the DEC was
close to 100Mb/s in one direction.
Note that I'm not talking about the raw speed of the photons, but the
number of user data bits moved from one machine to another.

Of course, there is no such thing as literally "Point-to-Point
Full-Duplex FDDI mode", because FDDI is a dual ring, not a star of FDX
links.  "Full-Duplex FDDI" is as impossible as "Full-Duplex ethernet."
It might be called something like "Point-to-Point Full-Duplex using
FDDI PMD and framing".

Vernon Schryver,   [email protected]

T.R	Title	User	Personal Name	Date	Lines
637.1	Full Duplex FDDI. Also, 100Mbits/sec. user data	MSBCS::REGE		`Wed Jul 08 1992 12:41`	21
	Some internal performance studies have shown that in the TCP/Ip enviornment user data is transmitted at 89 Mbits/sec. on a VAX 9000 machine with DEMFA. I suppose that is close to 100mbits/sec. Users can never get 100Mbits/sec., because FDDI wire runs at 100 Mbits/sec. and the user data has at least a minimal packet header. Infact the bandwidth available to a user is function of the packet size and all the headers with their sizes that are required to transmit the user packet. As you FDDI by definition cannot be full duplex. Full Duplex FDDI is a loosely coined term that allows a configuration of only two stations on a ring to use both the fibers, thus having the possibility of achieving 200 Mbits/sec. Note the packet format and other aspects of communication are same as FDDI and therefore the configuration is loosely termed Full Duplex FDDI.
637.2		KONING::KONING	Paul Koning, A-13683	`Wed Jul 08 1992 13:35`	14
	Apart from his snide remark about how slow we are, Vern's comment is technically correct. "Full duplex FDDI" is a term we've used to describe what would more accurately be called "Full duplex point to point datalink using FDDI frame formats". The reason why it exists is that it gives you twice the available bandwidth with very little change in the FDDI chips. Of course, the bigger issue is what the increased bandwidth does to the adapter and the system. I'm not sure that any of our adapters would handle the full duplex bandwidth. The other question is how many of our protocol implementations can actually consume more than 100 Mb/s. Because of these two points, full duplex is mostly a placeholder for future capabilities. It may be useable today in some limited cases, though. paul
637.3		JUMP4::JOY	Happy at last	`Fri Jul 10 1992 04:12`	7
	Much of the GIGAswitch public info and PID discuss use of FDX FDDI for communicating between the switch and a host, or between switches. I imagine that when GIGAswitch ships, we'll be seeing a more wide speread use of this capability. Debbie
637.4	where is the racetrack ?	MSBCS::KALKUNTE	Ram Kalkunte 293-5139	`Fri Jul 10 1992 13:44`	12
	re .1 The VAX 9000/DEMFA system can do 89Mbps for UDP, not TCP. I agree that FDX FDDI has no meaning. It is really FDX using optical cables, but we use the capability of already existent FDDI hardware. I believe sgi has seen throughput "close to 100Mbps" with UDP run without checksums, but Alex Conta's numbers for UDP include the checksums. Checksums cause a 20-30% hit in performance, so you know who is faster :-) Ram