Link Engg.ppt

© BHARAT SANCHAR NIGAM LIMITED Slide No. 1 of 16
Link Engineering

Objective of the presentation
Power Budget Definition
Power Margin
Link Loss Budget
Examples

Power budget is the difference between:
The minimum (worst case) transmitter output power
The maximum (worst case) receiver input required
Power budget value is normally taken as worst case.
In practice a higher power budget will most likely exist but it
cannot be relied upon
TRANSMITTER RECEIVER
Fibre, connectors and splices
Power Budget (dB)
Power Budget Definition

Power budget
r t
P P losses
 
Pr must be higher than the sensitivity of the receiver
Pr = power received (dBm)
Pt = power transmitted (dBm)
losses = sum of all losses (dB)
• Cable losses
• Connector losses
• Source-to-cable interface loss
• Cable-to-light interface loss
• Splicing loss
• Cable bends

dB, dBm, mW
dB = 10 log (P1/P2)
dBm Value % of 1 mW Power Application
0.0 100% 1.0 mW Typical laser Peak
Output
-13.0 5% 50.0W Typical PIN
Receiver
Sensitivity
-30.0 0.1% 1.0W Typical APD
Receiver
Sensitivity
-40.0 0.01% 100.0W Typical LED Peak
Output

Power budget calculations can produce a number of
different results depending on how they are carried out.
 To check if adequate receiver power will be available,
under all conditions
 Based on a knowledge of the receiver sensitivity to
determine the maximum loss of some component.
Use of Power Budgets

•Assume worst case transmitter output power is -10 dBm and the worst case
receiver input power needed is -25 dBm
•Power budget = - 10 dBm - ( - 25 dBm )
= 15 dB
•That is 15 dB of attenuation is possible over the link before failure occurs
•As a simple example to find the maximum fibre attenuation we eliminate
from the 15 dB budget the loss due to connectors and splices
Less Connector attenuation = 1 dB
Total splice attenuation = 1.2 dB
So, Total fibre attenuation allowed = 15 - 1 - 1.2 = 12.8 dB
Simple example to find total fibre loss allowed:
Use of Power Budgets

Power margins are included for a number of reasons:
 To allow for ageing of sources and other components.
 To cater for extra splices, when cable repair is carried out.
 To allow for extra fiber, if rerouting is needed in the future.
 To allow for upgrades in the bit rate or advances in
multiplexing.
Remember that the typical operating lifetime of a
communications transmission system may be as high as 20 to
30 years.
No fixed rules exist, but a minimum for the power margin
would be 2 dB, while values rarely exceed 8-10 dB. (depends
on system)
Power Margin

Fibre is normally only available in fixed
lengths up to 2 km long, so fusion
splices are required, to join lengths.
In buildings fibre lengths will be much
shorter
In most systems only two connectors
are used, one at the transmitter and
one the receiver terminal.
Power budget calculation including power penalty used to
calculate power margin
Transmitter o/p power (dBm)
Number of Connectors
Connector loss per connector (dB)
Total connector loss (dB)
Fibre span (km)
Fibre loss (dB/Km)
Total fibre loss (dB)
Splice interval (Km)
Number of splices
Splice loss per splice (dB)
Total splice loss (dB)
Dispersion penalty estimate (dB)
Receiver sensitivity (dBm)
Power margin (dB)
0
2
0.5
1
70
0.25
17.5
0.8
87
0.04
3.46
1.5
-30
6.54 Answer
System: 70 km span, 0.8 km between splices
Sample Power Budget
Calculation (Telecoms)

Splices within patchpanels and
other splice closures
In most systems connectors are
used at the transmitter and receiver
terminals and at patchpanels.
Power budget calculation used to calculate power margin
Transmitter o/p power (dBm)
Number of Connectors
Worst case Connector loss (dB)
Total connector loss (dB)
Fibre span (km)
Maximum Fibre loss (dB/Km)
Total fibre loss (dB)
Number of 3M Fibrlok mechanical splices
Worst case splice loss per splice (dB)
Total splice loss (dB)
Receiver sensitivity (dBm)
Power margin (dB)
-18.5 dBm min, -14.0dBm max
6
0.71
4.26
2.0
1.5 dB at 1300 nm
10
0.19
1.9
-30 dBm min
2.34 Answer
Available power budget: 11.5 dB using worst case value (>FDDI standard)
Total loss: 9.16 dB
3.0
Sample Power Budget Calculation

 An optical fibre system is to operate at 622 Mbits/sec over a
distance of 71 km without repeaters.
 Fibre with a worst case loss of 0.25 dB/km is available.
 The average distance between splices is approximately 1 km.
 There are two connectors and the worst case loss per
connector is 0.4 dB.
 The power margin is to be at least 5 dB.
 The receiver sensitivity is -28 dBm and the transmitter output
power is +1 dBm
 Determine the maximum allowable attenuation per fusion
splice
Sample Power Budget
Exercise #1

Transmitter output power +1 dBm
Worst case (lowest) optical output
power
Receiver sensitivity -28 dBm Minimum input optical power required
Power Budget 29 dB
Difference between transmitter and
receiver levels.
Less power margin 5 db Allowance for repair etc..
Less connector loss 0.8 dB Two connectors at 0.4 dB max. each.
Less fibre loss 17.75 dB 71 km at 0.25 dB/km
Calculated total maximum splice
loss
5.45 dB eg. 29 - 5 - 0.8 - 17.75 dB = 5.45 dB
Total number of splices 70
There are approximately 71 lengths of
fibre in the link so there are
approximately 70 splices
Answer: Maximum splice loss 0.076 dB
Solution to Exercise #1

Link Power/Loss Analysis
Margin
System
]
[
]
/
[
]
[
2
]
[
]
[
]
[






km
L
km
dB
dB
l
P
dBm
P
dBm
P
dB
P
f
c
T
R
s
T

Total Power Loss

Link Loss Budget
Example
A system has the following characteristics:
LED power (PL) = 2 mW
LED to fiber loss (Lsf) = 3 dB
Fiber loss per km (FL) = 0.5 dB/km
Fiber length (L) = 40 km
Connector loss (Lconn) = 1 dB (one connector between two
20-m fiber lengths)
Fiber to detector loss (Lfd) = 3 dB
Receiver sensitivity (Ps) = –36 dBm
Find the loss margin?

Solution
LED power (PL) = 2 mW=3dBm

PTx = -15 dBm
500 m
Using 850nm
PSEN = -25 dBm
Attenuation Coefficient,  = 4.5 dB/km
Dispersion Coefficient, D = 18 ps/nm-km
Number of Splice = 0
Splice Loss = 0 dB
PMargin = 2 dB
Connector Loss = 0.5 dB
Server A Server B
Example:
Power Budget Measurement for LAN
IS THIS SYSTEM
GOOD?

Link Engg.ppt

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