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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

499.0. "FDDI Single Mode Spec., Where?" by KXOVAX::LOVE () Thu Mar 12 1992 13:19

              Is there any documentation on FDDI specification on 
         line?  I have a customer that is creating a FDDI network 
         using a Concentrator Tree Sturcture instead of a physical 
         ring.  The top concentrator (level 1) will connect to 3 level 
         2 concentrators and one of these will connect to 1 level 3 
         concentrator.  One of the level 2 concentrators will be 
         linked by 27 km. of single mode fiber.  All of the other 
         links will be 2 km. or less of multimode fiber.  The total 
         fiber distance will be 4+2*4+2*27 = 66 km.  The customer has 
         been told that since the 27 km. link must be counted twice 
         that the 54 km. exceeds the standard 40 km.  I know the 
         standard give a limit on the total distance of fiber that a 
         token can pass because of timing, but is not the distance 
         between stations determined by energy loss in the fiber?  
         Note 324 indicates that the above configuration is legal, but 
         could someone point me to the verbiage in the FDDI 
         specification that makes the above legal I would appreciate 
         it.
         
         Thanks for any help you may provide me.
         
         Norm
T.RTitleUserPersonal
Name		DateLines
499.1RACER::daveAhh, but fortunately, I have the key to escape reality.Thu Mar 12 1992 21:4221
Directory FILES::NET$ARCH:[APPROVED.DEC_PROPRIETARY]

FDDI_V1_0_1.PS;1       2883/2883

Total of 1 file, 2883/2883 blocks.

Directory FILES::NET$ARCH:[PAPERS]

FDDI_DUP_ADDRESS.P|     291/291       FDDI_ERROR_CHARACT|     799/807
FDDI_PERF.PS;1          614/618       FDDI_PURGER_LCN91.|     422/423

Total of 4 files, 2126/2139 blocks.

Directory FILES::NET$ARCH:[REVIEW]

FDDI_V1_0_0.DECW$B|   10511/10512     FDDI_V1_0_0.PS;3       2691/2691
FDDI_X1_0_2.PS;1       2805/2808      NCL_DNA5_FDDI.MS;1|     104/105
PHASEIV_FDDI.PS;1       244/246       PHASEIV_FDDI_EXTER|     211/213

Total of 6 files, 16566/16575 blocks.
499.2KONING::KONINGPaul Koning, NI1DThu Mar 12 1992 21:4228
Someone is confusing you.

The "count twice" rule applies to checking the circumference -- the 200 km total
limit -- since the signal travels twice across a cable, once in each direction.

The 40 km limit (2 km for multimode) is the ONE WAY limit across a piece of
glass.

Figure it this way:

Pretend you're riding around on the token, observing where you go as it
travels around.

1. The total trip distance around the ring must not exceed 200 km no matter
   how the ring is reconfigured.

2. The total trip time around the ring must not exceed 1.617 ms (D_Max) no matter
   how the ring is reconfigured.

3. The distance travelled across any one multi-mode link from transmitter to
   receiver must not exceed 2 km.

4. The distance travelled across any one single-mode link from transmitter to
   receiver must not exceed approximately 40 km -- however the real limit in
   this case is cable loss (see data from installation guide quoted a few
   notes ago).

	paul
499.3Yes! But what does the spec. say?MSDOA::LOVEDo it with DNAFri Mar 13 1992 10:306
    Re: .2.4  That is what I thought, I need to check the spec. to see if
    they refer to lengths or losses.  Thus the 2km length could be less or
    more depending upon the cable quality?  Does the spec specify the
    length or the loss expected for multi mode cable?  Thus the 2km length
    that is quoted is just the minimum length that all current multi mode cable
    fit?
499.4Fiber does age...BONNET::LISSDANIELSMon Mar 16 1992 06:4210
 
If you are trying to use longer links than 2km plese keep in mind that
what "works" today due to current low loss might not work in the future 
due to the fiber aging becomming less translucent...

I'd say that while the link distance may become longer thatn 2KM never
exceed fiber specification since you may have to replace it sooner than
expected...

Torbjorn
499.52 km is written in.COMICS::WOODWARDSmile!Mon Mar 16 1992 12:5517
re .3. ANSI X3.166-1990, the PMD specification for multi-mode fibre specifies
length, loss and a _lot_ more :-)


Section 10.1 "The specifications in this clause are intended to assure
interoperability of FDDI conforming attachments for optical cables
lengths up to 2 km."
        =====

Also note that the _minimum_ modal bandwidth specified for the cable is 
500 MHz.km. Using a cable which just conforms to the spec, the 2 km limit
is due to dispertion rather than attenuation (the specified 11dB power budget
can take you a lot further than 2km).

Hope this helps,

Steve
499.6KONING::KONINGPaul Koning, NI1DMon Mar 16 1992 14:0512
Whoa.

Re .3, .4: read my note again.  Observe that item 4 applies ONLY to SINGLE MODE
cable.

Single mode cable distance is controlled by loss.

Multi mode cable distance is controlled BOTH by loss and by "modal bandwidth".
Don't exceed 2 km even if the losses seem to allow it; the situation is not
that simple.

	paul
499.7Thanks, no problem with 2km multimode lengthMSDOA::LOVEDo it with DNATue Mar 17 1992 13:319
    Thanks for your help.  The reason for the question on loss rather than
    length was for my own information not the customers.  Their only
    concern was if the single mode fiber could be over 20km between
    concentrators in at tree configuration.  Another vendor was trying to
    confuse him. An earlier reply explained this very clearly.
    
    Thanks again.
    
    Norm