| Quite possibly, but you need to know more about the cable than just its
diameter. In particular, you need to know the specifications (attenuation
and "modal bandwidth") at 1300 nm wavelength. If the cable is "dual window"
cable designed to support that wavelength, you're probably in good shape.
You'd need to check the various parameters to make sure the links will work,
since you get some additional losses when using 50/125 fiber. But that fiber,
IF it's the FDDI kind, usually has quite low losses, so it's usually not hard
to stay within the FDDI link specifications.
paul
|
| Back in 1989, Chris Baldwin published a memo regarding supportable
distances on alternate optical fibers. This memo has been used to
generate the supported distance tables that can be found in the
Networks Buyers Guide. Since Chris is no longer with the company, I'll
take the risk of posting the memo without his permission.
As Paul indicated in his response, you must know the attenuation
and modal bandwidth of the 50/125 micron fiber. If the fiber vendor is
able to provide information regarding the chromatic dispersion values,
then you may be able to achieve the maximum distance.
You can contact me if you need more information.
Dick Kirk
Networks Engineering
LEVERS::KIRK
DTN 226-7048
|
| From: GIGA::BALDWIN "WYSIWYG....no refunds 28-Jul-1989 1127" 28-JUL-1989 11:30:32.20
To: @DIST.
CC: BALDWIN
Subj: Supportable Transmission Distances with Alternate Fiber Sizes for FDDI
+---------------------------+
! ! ! ! ! ! ! ! INTEROFFICE MEMORANDUM
! d ! i ! g ! i ! t ! a ! l !
! ! ! ! ! ! ! !
+---------------------------+
TO:Distribution: DATE:7/28/89
FROM:Christopher Baldwin
DEPT:CSE Technology: Fiber Optics
EXT:227-3637
LOC:TAY2-2/N15
NODE:GIGA::BALDWIN
SUBJECT:Supportable Transmission Distances for FDDI Physical
Connections Using Alternate Fiber Sizes.
EXECUTIVE SUMMARY:
==================
The supported transmission distances and loss budgets for DEC's
FDDI products transmitting through any of the alternate fiber
types (50/125, 100/140 or 85/125) or un-characterized 62.5/125
cableplant is detailed in this memo.
1. 50/125 um cableplants will have only a 6 dB loss budget to
operate with. Those complying with the chromatic and modal
bandwidth characteristics in the PMD Chapter 7 can support 2
km transmission distances with this loss budget.
2. ANY 50 um cableplant of 500 meters or less with 6 dB or less
of loss can be supported provided that the fiber's modal
bandwidth at 1300 nm was controlled by the manufacturer.
3. ANY 100/140 or 85/125 um cableplant of 500 meters or less
with 11 dB or less of loss can be supported provided that the
fiber's modal bandwidth at 1300 nm was controlled.
4. Up to 1.6 km lengths of 50,62.5,85/125 or 100/140 fiber that
was not characterized for chromatic dispersion can be
supported provided that the installed cableplant has a modal
bandwidth of 200 MHz at 1300 nm over its full length.
5. A PRIORI TRANSMISSION DISTANCE GUARANTEES CANNOT BE GIVEN FOR
ANY CABLE TYPE THAT WAS NOT CHARACTERIZED FOR MODAL BANDWIDTH
AT 1300 NM OPERATION BY THE MANUFACTURER.
The experiments performed to derive these results were conducted
on both the Hewlett-Packard and Sumitomo devices that will be
used in the DEC product set. The results apply equally to either
vendor's devices.
Page 2
BACKGROUND
==========
The FDDI Physical Channel is optimized for use with 62.5/125 um
GI fiber. DEC can support the alternate fiber sizes (50/125,
100/140 & 85/125) but with decreased loss budgets and/or
transmission distances due to the power penalties and decreased
bandwidth associated with use of the alternate fiber types.
The requirements in PMD Chapter 7 describe the requirements on
the cableplant (irrespective of fiber type) to support a 2 km
transmission span. However, installed cableplants may not have
been completely characterized for use with the 1300 nm technology
employed in the FDDI Physical Layer and it is accordingly
difficult to unequivocally state whether a given cableplant will
function with an FDDI implementation. This memo combines
empirical data along with published analysis of cableplant
characteristics to arrive at rules for the use of non-standard
installed cableplants.
Analysis:
=========
An alternate fiber cable must be guaranteed to adequately control
end to end loss and bandwidth when used with a FDDI
implementation. Bandwidth is composed of three components:
electrical, modal and chromatic - the latter two of which may
depend on the fiber type being used. The loss a of cableplant
consists of the bulk loss of the fiber, its connectors/splices
and additional penalties that may be incurred by the mismatch
between the cable's core size and the station's optical
components which are optimized for use with 62.5/125 cable. Loss
and bandwidth for the alternate fiber sizes are treated
separately below.
Loss Analysis:
-------------
At the transmitter end, the use of alternate cableplants can
cause more or less power to be launched due to the size of
the alternate cable's core relative to 62.5 um cable. At the
receiver end, either the focusing optics or the PiN detector
can vignette the image of a larger than 62.5 um core causing
an effective receiver penalty.
1. 50/125 um Cableplants:
13 Hewlett-Packard Transceivers were tested for
transmitter optical launch power penalty. Sumitomo
transmitters were also tested and were found to have
slightly less degradation; according the results from the
HP devices will be used.
Both NA = 0.20 and NA = 0.22 fiber was tested. The
transmitter was found to suffer a 5.0 dB penalty maximum
when used in conjunction with NA = 0.20 fiber (worst
case). No receiver vignetting will occur (50 um core is
Page 3
smaller than the 62.5 design fiber.)
LOSS BUDGET = 11 dB - 5 dB penalty = 6.0 dB with 50/125
cableplants
2. 100/140 Cableplants:
3 HP Transceivers and Sumitomo transmitters were tested
for transmitter optical launch power increase. The NA of
the fiber was 0.29. A 1.5 dB increase was found.
The receiver was found to have a 0.5 dB vignetting
penalty when used with 100 um core fiber. The penalty
was confirmed for worst case mode scrambled conditions (
mode scramble exit conditions ensure that the light is
maximally distributed throughout the core of the fiber so
that maximum vignetting will occur.) HP explained that
the source of the vignetting occurs at the detector
because the NA and pupil aperture of the focusing optics
is more that sufficient for a 100 um core 1:1 image
relay. The PiN detector is only 90 um in diameter.
While only a 0.5 dB vignetting penalty was found, 1.5 dB
will be reserved for this penalty to account for possible
misalignment between optical relay and PiN detector which
would exacerbate the penalty. The Sumitomo devices were
found to have a 0.3 dB Rx vignetting penalty; since the
difference between the HP and Sumitomo results are minor
the 1.5 dB penalty will be used.
LOSS BUDGET = 11 dB + 1.5 dB Tx gain - 1.5 dB Rx penalty
= 11 dB
NB: The use of 100/140 um patch cords to connect
stations is NOT RECOMMENDED. There is a possibility that
the Tx could saturate a Rx when using a short length of
100/140 fiber. This is currently not a serious concern
as FDDI Connectorized 100/140 cables are a special order
item.
3. 85/125 um Cableplants:
The same analysis applied to 100/140 um cableplants
applies to 85/125 cableplants.
LOSS BUDGET = 11.0 dB.
Bandwidth Analysis:
==================
A fundamental requirement that an installed cableplant MUST
satisfy is that the manufacturer controlled the 1300 nm Modal
Bandwidth (MBW) of the fiber. If the manufacturer did not
measure the 1300 nm performance of his product then it is not
possible to predict its performance now. The bandwidth of
Page 4
the specific cableplant in question need not be known now,
but it is required that the manufacturer ascertained that his
process met a minimum bandwidth at 1300 nm. Corning, ATT and
Spectran currently all measure 1300 nm MBW of all of their
fiber types and have been doing so for some (unspecified)
time. They all have a minimum requirement of 100 MHz*km.
The bandwidth of a FDDI cable is determined by the MBW and
the chromatic characteristics of the cable; the requirements
are treated separately.
If a cable can be shown to have a MBW of 200 MHz over the
length of the fiber ( MBW distance product divided by the
length of the cable), then it has sufficient MBW for a FDDI
link. Cable manufacturers state that customers almost always
specify (thus know) the MBW of their cable. Accordingly it
is likely that an installed cableplant will have a paper
trail that can divulge its MBW. In a case were the MBW of a
particular piece of cable is not known it is still possible
to define a maximum supportable length: the minimum MBW for
all fiber types from all manufacturers is 100 MHz*km.
Clearly then a 0.5 km link will have sufficient MBW over its
length. Alternately the installed cable could be measured
for modal bandwidth in the field.
The optical system also requires that the chromatic
dispersion characteristics of the fiber fit within the curves
of figure 7-1 of the PMD. The chromatic characteristics of
MM GI fibers are very difficult to measure even in a
controlled laboratory environment and impossible to do in the
field. In two separate papers (1), ATT and Corning data show
data that demonstrates that fiber NA and its chromatic
characteristics have some correlation. In the paper they
measure the chromatic characteristics of a large number of
spools of fiber (greater than 30 each) and the data
demonstrates that the maximum slope of the dispersion curve
never exceeds 0.11 ps/nm*nm*km and that the maximum minimum
dispersion wavelength does not exceed 1365. However the
curves in the PMD do not allow this particular combination of
worst case conditions. To guarantee that such a worst case
condition will work, the maximum length of a cableplant that
has not been characterized for chromatic dispersion needs to
be derated from the 2 km maximum.
Since the slope of the dispersion curve and cable length
have the same mathematical function and weighting in the
model for chromatic bandwidth, the maximum length of the
cableplant is derated by the same percentage that the maximum
slope of the dispersion curve exceeds the PMD requirements
(0.093 ps/nm*nm*km); accordingly the maximum length for the
alternate fiber sizes that cannot demonstrate compliance to
PMD figure 7-1 is 18% less than the 2 km limit or 1.6 km.
(1) "The Importance and Application of Dispersion of
Page 5
Multimode Fiber in LAN's and Its Relation to NA." M.J.
Hackert CGW
"Determining the Zero Dispersion Wavelength and Dispersion
Slope of Fibers Used for the Fiber Distributed Data Interface
(FDDI)" J.J. Refi & H. Shang.
DISTRIBUTION:
NM%LEVERS::NEWMAN
NM%LEVERS::THOMPSON
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NM%LEVERS::GINZBURG
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NM%LEVERS::TIFFANY
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NM%DELNI::SWAN
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NM%JRDV01::KAWAMURA
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NM%Levers::S_MACARTHUR
NM%Delni::B_GEARING
NM%LEVERS::SHUDA
|
|
Well, I have a customer site which seems to have 100/140 fiber.
However, this was used for interconnecting fiber LANbridge 100s. Now
they want to use it for FDDI. According to the spec sheet from the
testing company from May 1987 it was tested at 850nm. The question is,
is it possible to test this fiber for FDDI suitability with test
devices? If so, what would those test devices be?
The length of this cable is only about 200 FEET between 2 buildings.
Also, if this cable does test OK, what kind of patch cords would I use
from the patch panel to the DECconcentrator 900MX the customer is
considering buying? A BN24D-xx or something else?
I'm going to try and ask them for the attenuation and modal specs, but
given this cable is over 8 years old, I don't think they have it. So,
given this, what would be a good approach?
|