Site
Requirements
Power
Budget
To
design
an
efficient
optical
data link
evaluate
the
power budget
The
power budget
is
the
amount
of
light
available
to
overcome
attenuation
in the
optical
link
and
to
exceed
the
minimum
power
that
the receiver
requires
to
operate
within
its
specifications
Proper
operation
of an
optical
data
link
depends
on modulated
light
reaching
the receiver
with
enough
power
to
be
correctly
demodulated
Attenuation
caused
by
the passive
media components
cables
cable
splices
and connectors
is
common
to
both
multimode and
single-mode
transmission
The
following
variables
reduce
the
power
of the
signal
light
transmitted
to
the receiver
in
multimode
transmission
Chromatic
dispersion
spreading
of the
signal
in
time
because
of the
different
speeds
of
light
wavelengths
Modal
dispersion
spreading
of the signal
in
time because
of the
different
propagation
modes
in
the fiber
Attenuation
is
significantly
lower
for
optical
fiber
than
for other
media
For multimode
transmission
chromatic
and modal
dispersion
reduce
the available
power
of the system
by
the
combined
dispersion
penalty
in
decibels
The power
lost
over
the data
link
is
the
sum
of the
component
dispersion
and modal
losses
Table
2-9
lists
the
factors
of attenuation
and
dispersion
limit
for typical
fiber-optic
cable
Table
2-9
Typical
Fiber-Optic
Link
Attenuation
and
Dispersion
Limits
Single-Mode
Multimode
Attenuation
0.5
dB
1.0
dB/km
Dispersion Limit
No
limit
500
MHz/km1
The
product
of bandwidth
and
distance
must
be
less
than
500
Mflzkm
Approximating
the
AlP
Power
Margin
The
LED
used
for
multimode
transmission
light
source
creates multiple propagation
paths
of light
each
with
different
path
length
and
time
requirement
to
cross the optical fiber causing
signal
dispersion
smear
Higher
order
mode
loss
HOL
results
from
light
from
the
LED
entering
the
fiber
and
being
radiated
into
the
fiber
cladding
worst-case
estimate
of
power
margin
PM
for
multimode
transmissions
assumes
minimum
transmitter
power
PT
maximum
link
loss
LL
and
minimum
receiver
sensitivity
PR
The
worst-case
analysis
provides
margin
of error although
not
all
of the
parts
of an
actual
system
will
operate
at
the worst-case
levels
The
power
budget
PB
is
the
maximum
possible
amount
of
power
transmitted
The
following
equation
lists
the calculation
of the
power budget
PB
PT
PR
PB
18.5
dBm
-30
dBm
PB
11.5
dB
The
power
margin
calculation
is
derived
from
the
power budget and
subtracts
the link loss
as
follows
PM
PB
LL
If
the
power
margin
is
positive
as
rule
the link
will
work
2-10
Cisco
7513
Hardware
Installation
and
Maintenance
Summary of Contents for 7513 Series
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