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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

2061.0. "TTRT significance for DEFPA?" by OTOOA::JPOND () Mon Jun 10 1996 16:46

    Hi there,
    
    I've seen some reference to adjustment of the TTRT parameter in this
    notesfile, and have tried unsuccessfully to obtain the white paper
    from FILES::NET$ARCH:[PAPERS]FDDI_PERF.PS, so I thought I'd pose the
    question here.
    
    A customer with a big FDDI ring (over 20 Sables with DEFPA's running NT
    Server) is asking about the TTRT and its default time of 8ms.  What
    does Digital recommend with respect to adjusting this parameter in 
    different environments?  This customer was told by someone at Frontier,
    who resells Digital FDDI cards, that they always adjust the TTRT up to
    160ms to avoid (unspecified) problems.  The customer wants to know what
    strategy should be pursued in setting this number: should they be high,
    should the number be different for each member of a ring, should some
    be high and some be low, etc.?
    
    In addition, the customer is asking about the negotiation strategy
    for the DEFPA and the TTRT. Does the negotiation of the time go
    both upwards and downwards (toward a negotiated time)?
    
    Those are his questions. If someone can provide some inputs, I
    would appreciate it.
    
    Regards,
    Jim

T.R	Title	User	Personal Name	Date	Lines
2061.1	Have heard to keep TTRT value close to default....	NETCAD::BATTERSBY	Don't use time/words carelessly	`Mon Jun 10 1996 18:40`	13
	Well, I recall that varying the value of TTRT can affect (sometime dramatically), the ultilization and latency of a very busy FDDI ring, and that it has little affect on utilization or latency on a lightly loaded ring. I don't have any personal experience with changing this parameter from its default of 8ms to an order of magnitude in the range of 160ms, so I can't speak to whether this is useful or not. However I have heard many people more knowlegeable than I of FDDI configs, not recommend changing it too far from its default. Perhaps others can chime in here with more empirical evidence of the impact of changing it by a factor of 20. Bob
2061.2		NETCAD::STEFANI		`Mon Jun 10 1996 18:57`	23
	>> -< TTRT significance for DEFPA? >- Most of the DEFEA and DEFPA device drivers offer a switch to change the MACRequestedTTRT value whose range is 4-165ms and whose default is 8ms. Note the word "Requested". The TTRT is negotiating via a lowest bidder wins algorithm during the FDDI claim process. If one node asks for 8ms and another node asks for 4ms, the 4ms. node wins. Raj Jain wrote a paper on why the 8ms. default is a good idea and should be left alone. The reason many of our device drivers allow you to change the requested value is because end nodes are inherently difficult to manage. Some FDDI network managers that want to twiddle with the TTRT value indiscriminantly set the end node requested values to something very high (eg. 160ms). Now, for all intents and purposes the end node FDDI adapters are out of the bidding process. They can then adjust the requested value at something more manageable (eg. bridge or concentrator) and know that they'll win the negotiation. Generally, this value should be left alone. - Larry
2061.3		OTOOA::JPOND		`Tue Jun 11 1996 11:18`	6
	Larry, Thanks for the info. I looked for Raj's paper at FILES::NET$ARCH:[PAPERS]FDDI_PERF.PS, but it seems to be missing. Do you have a current pointer? Jim
2061.4		NETCAD::STEFANI		`Tue Jun 11 1996 13:31`	3
	>>missing. Do you have a current pointer? Nope.
2061.5		12368::thomas	The Code Warrior	`Tue Jun 11 1996 13:45`	1
	It's there now (FILES::NET$ARCH:[PAPERS]FDDI_PERF.PS).
2061.6		OTOOA::JPOND		`Tue Jun 11 1996 19:18`	2
	Thanks, Jim