Conference 7.286::fddi

Yes, but that will introduce some losses at the point of connection.
Depending on what other losses are present, it may limit your distance
to less than 2 km.  The ANSI PMD standard has some additional information
on this.

	paul

    Even more interesting, this is a real pitfall!
    
    Since FDDI signal budget margin is 6 dB @ 50/125 instead of 11 dB @ 62/125
    PATCH CABLES become increasingly important.
    
    Is there a source for 50/125 patch cables with FDDI MIC's within DEC?
    
    Could we please elaborate a little on patch coupling loss?
    
    Assume 50/125 fiber installed in patchboxes in the various
    computerrooms. If we use STD. fddi patch cables, we will incur
    an additional coupling loss when going from 62/125 patch cable
    to the permanent 50/125 fiber, due to the smaller core.
    
    When arriving at the other patchbox, what will be the coupling loss?
    The N/A of the 50/125 fiber is .20. The fiber loss @ 1300 nm of the
    50/125 fiber is 1.45 db/km, and distances are below .5 km.
    
    Loss = 10*log10(62�/50�) + .5*1.45 + loss_at_2nd_box
    
    Loss = 1.86 + .72 + (???)
    
    Loss budget = 2.58 + (???)
    
    Is there also a loss due to dissimilar N.A. at box1 ? (N.A.(62) > N.A.(50)
    Is there also a loss due to dissimilar N.A. at box2 ( N.A.(50) < N.A.(62)
    
    Gullik

It doesn't really matter what size patch cables you use.  The loss occurs
at the interface between the two fiber sizes if you mix sizes.  The equivalent
loss occurs at the transceiver if you use 50 �m fiber throughout.  (Think of
the transceiver as having 62.5 �m fiber internally...)  The effect is the
same in either case.  So by using 50 �m vs. 62.5 �m patch cords, you don't
change the loss, but only move it to a different place.

	paul

    Thank's Paul, 
    
    Well, I just wanted to verify this, would it be true to say that
    we will have the 6 dB margin ( minus 2 connector losses )?
    
    Is it true to say that a 50 -> 62 interface is 'lossless' since
    the core of the 62 fiber will catch all the light from the 50 fiber?
    
    What's the formula for calculating fiber-fiber interface losses?
    
    Is N.A. and core size all parameters needed?
    
    Gullik
    

Check the PMD spec for answers to those questions...

	paul

    Would there be any problem if, say for example, part of the ring was
    50/125 and part of it was 62.5/125? For example:-
    
    
                            50/125
                    ==========================
             ------                            ------
            |Conc |                           | Conc|
             ------                            ------
                    ==========================
                            62.5/125
    
    Would there be any problems, other than the losses at the connection
    points?
    
    Jim.
    
    

    No problem, as long as loss budget and distance specifications are met.

    
    Don't Forget Bandwidth.  Ensure that the 50um fiber has appropriate
    attenuation and bandwidth specs for the second window (1300nm).  Many
    fibers don't meet the FDDI spec (500MHz.Km, -1.5dB/Km, @1300nm).
    
    A special note, The PATCH Cables can't be the standard DEC BN24x 
    series duplex jumpers (which are 62.5um).  You must use 50um patch
    cables if your cable plant is 50um.  They are available through our
    distributors.
    
    See Page A-2 and A-3 of The DECconnect System Fiber Optic Planning and
    Configuration (EK-DECSY-FP) Guide.   
    
    JR

Huh?  I don't see what's wrong with using standard patch cables.  That is, 
if you use them at the ends of cable runs.  You're going to get the size
mismatch loss at the device itself if you use 50 �m patch cables, but if
you use 62.5 �m patch cables, you'll get the SAME loss at a DIFFERENT spot.

The only reason I can see for using 50 �m patch cords is when you have them
in the middle of a run, i.e., between two pieces of 50 �m cable.  In that
case, using standard patch cables would introduce additional size changes
and thus additional places where there is a loss.

	paul

    Size mismatch loss is not the only mechanism. The two fiber types
    differ in NA, as well as zero_dispersion_wavelength range. Also, there
    are mode-filling differences between source/fiber coupling and
    mis-matched fiber coupling.
    
    62.5 �m patch cords will often work, but potentially at the expense of
    a reduced system margin or a slightly worse BER. Use them if nothing
    else is available.
    
    When designing a system, the patch cords should match the
    characteristics of the installed cableplant.

    Is Paul's point on .9 still not relevant though? Is it not the case
    that the loss is going to be at one end of the patch cable if I use
    50um cables (at the device), and at the other if I use 62.5um cables
    (at the patch panel).
    
    Jim.

    What you're really asking is: Is the spatial and power distribution
    pattern of the FDDI transmitter equivalent to the output of a 62.5 �m
    fiber? 
    
    The answer is no.

That makes sense.  Thanks for the explanation...

	paul

    Makes sense to me too now, thanks for the info.
    
    Jim.