The key to network
distance is Optical Power Budget: the amount of light available to make
a fiber optic connection. This paper will explain how to determine the
maximum fiber optic distances attainable using media converters in
various network environments. A simple calculation is used to determine
how much fiber optic light, measured in "dBms", is available.
The first step in
calculating the Optical Power Budget is determining how much light is
available for the electronic devices themselves. Two measurements are
needed from the manufacturer of the equipment. Minimum transmit power
represents the worst case transmit power for a device - the device is
guaranteed to provide at least that much power. Please note that some
vendors will list an "average minimum transmit power". Average minimum
transmit power does not guarantee that a product will perform at that
products' minimum transmit power. Use caution when purchasing products
based on the averages.
The second piece of
information required is the minimum receive sensitivity. This figure
represents the minimum amount of light required by the receiver to
operate correctly. Again, the actual minimum should be used, not an
average of minimums.
With minimum
transmit power and minimum receive sensitivity data, we can now
calculate the available light. To accomplish this, simply subtract the
minimum receive sensitivity from the minimum transmit power (available
light = minimum transmit power - minimum receive sensitivity). Note
that the minimum receive sensitivity is usually a negative number such
as -33dBm. Subtracting a negative number is the same as adding its
absolute value. For example, if a device has a minimum transmit power
of -10dBm and a minimum receive sensitivity of -33dBm, the available
power will be -10dB - (-)33dB = 23dB or -10B + 33dB = 23db.
|
TIA standard for connector loss
|
.75 dB
| |
Typical cable attenuation at 1310nm
|
.4 dB
| |
Typical cable attenuation at 1550nm
|
.3 dB
| |
Typical splice attenuation
|
.1 dB
| |
Typical distance between splices
|
6 km
| |
Typical safety margin
|
3 dB
|
Also, when connecting devices from different vendors,or different
models of products from the same vendor, the available power
calculation needs to be computed in both directions with the smaller of
the two used for the rest of the calculations. For example, assume you
are connecting two devices labeled "Device" 1 and Device 2. Device 1
has a minimum transmit power of -3dBm and a minimum receive sensitivity
of -32dBm and Device 2 has a minimum transmit power of -1dBm and a
receive sensitivity of -31dBm. The available power going from Device 1
to Device 2 would be calculated by -3dBm - (-)31dBm or 28dB. The
available power going from Device 2 to Device 1 would be calculated by
-1dBm - (-)32dBm or 29dB. There is less light available in the Device 1
to Device 2 direction so we will use that figure for our calculations,
if that half of the link works then so will the other.
From available power
we must subtract out all of the losses. These losses include cable
attenuation, connector loss, and splice loss. Cable attenuation is
often the most significant loss factor. Cable attenuation is determined
by getting the exact number off of the cable you are installing, or
using the manufactures worst case number of the type of fiber you
planned to install. This number will range from .22dB to .5dB per
kilometer. Multiply this factor by the number of kilometers in the
installation. A fiber with .4db per kilometer of loss will loose 16db
over a 40km distance.
Also remember that
fiber does not come in 40km spools, therefore a 40km installation will
have several splices. Each splice will typically introduce .1db of
additional loss. The fiber installers should be able to provide a
guaranteed worst case number. Multiply this number by the number of
splices.
Connectors are
another source of light loss. A typical long haul installation will
have six connectors in the installation. The first connects the fiber
to electronics. This connector is usually on indoor, plenum rated
fiber. This fiber connects the equipment room to the building entrance
for the outdoor (buried or aerial) fiber. There is another connector on
this end of the indoor cable and one on the outdoor cable. This is
repeated at the other end of the network for a total of six connectors.
Individual networks can vary however, and the exact number must be
determined. Connector loss is provided by the connector manufacturer
and the installer. Multiply the number of connectors by the loss for
each connector to get total connector loss.
Each of these
losses, cable attenuation, connector loss, and splice loss is then
subtracted from the available power. If this number is negative, there
is no need to continue, as there is not enough power to drive the
network. If this number is positive, there are two more things to
consider before pronouncing the network fit. The first is what happens
if the fiber gets cut and I have to splice it back together? A proper
installation will count on this happening and account for it in the
power budget. An estimation of the number of anticipated repairs over
the life of the fiber needs to be made. These repairs will add splice
loss, so we must multiply the number of anticipated splices by the loss
of each splice (same number we used above), and subtract this from the
remaining power. The number should still be positive.
Finally, you must
account for temperature extremes, as well as any other unforeseen
factors. This is typically done by determining a "safety factor". This
number will be different for every organization depending on how much
risk they want to assume in their network. Typically a value around 3dB
is used. To guarantee error free operation, a value no less then 1.7dB
should be used. This safety factor is subtracted from the remaining
power from above. If the number is still positive after all of this,
you can be assured that your fiber network will deliver the required
performance over the life of the installation.
The table to the right contains some typical numbers, which can be
used to approximate optical link budget calculations. If at all
possible, real numbers from the network in question should be
used.
(back to
top)
| 
