OPERATIONAL EXPERIENCE WITH WDM LINKS
D. RONARC’H, C. DOURTHE, M. GUIBERT, J.P. PLOUHINEC, B. RAGUENES
INTRODUCTION
The installation of synchronous equipment in the FT trunk network was completed in 1997.
This trunk network is a mesh based on STM16 point to point transmission equipment. On many
routes of this mesh, several STM16 systems are running in parallel. On the trunk routes, the
cables are shared between the different network layers and FT is now in a phase of active
optimisation of the fibre (G.652) infrastructure due to traffic increase. Saving fibres is one of
the challenges of this optimisation.
Two options are possible to save fibres : TDM or WDM (or a combination of the two). Two
years ago, when an increase of capacity on some transmission routes was decided, it appeared
that WDM was the most mature solution on the market from a technical and economical point
of view. In addition, WDM allows the re-use of the in-place SDH terminals through the use of
transponders.
To date, FT is operating two point to point WDM links with live traffic. One is equipped with a
four and the other with an eight wavelength system.
The management of the WDM section was kept independent of the management of the SDH
layer. The management information (working parameters and alarms) is available at both ends
of the WDM links while a craft terminal can be connected at each amplifier site and can have a
view of each element of a WDM link.
There is no specific protection scheme associated to the WDM sections even if an amplifier
failure is equivalent to a cable cut. Rerouting is performed at the VC4 level through VC4 crossconnects. The spare capacity in the network and the number of ports for the VC4 crossconnects have been dimensioned to overcome a breakdown of the WDM links.
Prior to operation with real traffic, conformance testing was performed on the two WDM links.
In this paper we describe the performance of the two links in nominal operating conditions.
Afterwards, the links were submitted to degradations and the robustness of the engineering of
the links was evaluated.
LINK DESCRIPTION
The FT WDM links in operation are installed between Nantes and Penmarc’h and Paris and
Rouen.
Fig.1 and Fig.2 give the schematic of the two links respectively.
1
TxT
1535 nm
26.2dB
26dB
26dB
25.4dB
Filter
1543nm
1550nm
1557.5 nm
Filter
1535 nm
TxT
1543nm
1550nm
1557.5 nm
25.8dB
26dB
NANTES
25.6dB
26dB
PENMARC'H
323 km
Fig.1 : Schematic of Nantes-Penmarc’h Link
1535 nm
27.8dB
27.3dB
1539 nm
1543 nm
1546,2 nm
1550 nm
1553,4 nm
1557,5nm
1560,5 nm
1535 nm
1539 nm
1543 nm
1546,2 nm
1550 nm
27.6dB
27dB
1553,4 nm
1557,5nm
1560,5 nm
PARIS
166 km
ROUEN
Fig.2 : Schematic of Paris-Rouen Link
PERFORMANCE IN NOMINAL CONDITIONS
- Power levels
The optical power levels can be measured in many points of the WDM equipment using the
supervisory system. The accuracy of these optical power levels was checked and was found to
be within 0.2 dB.
2
Fig.3 gives the optical power levels at different points of the WDM link operating between
Paris and Rouen. Two operating conditions are given : 5 and 8 wavelengths in the optical
multiplex.
Paris
Rouen
TxT
-15.3
+13.3
+13
-17.3
+3.7
-16.3
-18.8
-17.6
-14.3
-4.0
-17.7
-14.3
-21.6
-22.3
-20.6
-21.5
-20.6
-17.8
+13.1
+4.1
+13.7
-3.6
-13.9
+1.0
-4.2
-3.9
-3.7
-3.9
-3.6
-0.6
Levels with
5 wavelengths
TxT
Levels with
8 wavelengths
Optical power in dBm
Fig.3 : Optical levels on the Paris-Rouen Link
- Bit Error Rate (BER) performance
Bit Error Rates were measured as a function of the total preamplifier input power for 4
wavelengths. These measurements give in fact the margin you have for degradation of the last
section of the WDM link.
Fig.4 shows BER at the SDH receivers of the wavelength 1535, 1543, 1550, 1557 nm while the
system is operating eight wavelengths. At 10-10 the worst margin for degradation of the last link
section is 11.2 dB for the wavelength 1543 nm.
3
BER
10 -4
(1535) L: 16-2
(1550) L: 16-2
(1543) L: 16-2
Sensitivity at the preamplifier
input for BER 10-10
(1557) L: 16-2
10 -6
10
-8
1535 nm
1543 nm
-25.2 dBm
-25.1 dBm
1550 nm
-25.8 dBm
1557 nm
-27.7 dBm
-10
10
-11
10
-12
10
dBm
-35
-30
-25
Fig.4 : BER curves at Paris for the wavelengths 1535, 1543, 1550 and 1557 nm
- Optical Spectra
The optical spectra can be recorded at the output of each optical amplifier through a monitor
output connector. The evolution of the optical signal to noise ratio can therefore be observed
along the amplifier chain. What is of interest too is the optical signal available for each SDH
receiver. Fig.5 displays on the same graph the spectra at the 8 ports of the demultiplexer unit in
Paris. The optical signal to noise ratio in the range of 20-25 dB which is suitable for error free
transmission.
4
Site AUBERVILLIERS
dBm
-15
-20
-25
1535 lnm
1539 nm
1543 nm
1546 nm
-30
1550 nm
1553 nm
1557 nm
1560 nm
-35
-40
-45
-50
-55
Rés : 0,2nm
-60
1530
1534,7
1538,6
1543
1545,7
1549,7
1553,1
1556,5
1559,9
1565
nm
Optical Spectra at the de-Mux ports
Fig.5 : Optical spectra at the De-Mux ports at Paris terminal
5
ROBUSTNESS OF THE INSTALLED WDM LINKS
In optical transmission using electronic repeaters you can define an optical power margin per
regeneration section. This margin is a balance between the receiver sensitivity, the laser output power
and the section loss. Each regenerator acts like a screen between its input and output transmission
section as far as the optical power is concerned. Bit errors are of course transmitted.
When optical amplifiers are used, you leave the digital world and analog behaviour is to be
considered. In this context, it is not easy to define a margin per amplified section because of noise
accumulation along the amplifier chain. If the optical amplifiers are operating in satured conditions,
an optical power screening is observed while the signal to noise ratio degrades when the signal level
at the amplifier input is lowered.
We used the following method to characterise the robustness of the link :
 We apply an attenuation (degradation) in a section of the WDM link
 We measure the BER for different wavelengths as a function of the preamplifier total input power.
 The shift in optical power (in dB) of the BER curves in nominal and in degraded conditions is
called degradation penalty.
The penalty is measured as a function of the applied degradation. This penalty is in fact the reduction
in margin of the last section of the WDM link when a degradation occurs somewhere between the
transmitter and the last line amplifier of the chain.
The fig.6 and 7 show the penalty at the preamplifier input for a single degradation at point in the
Nantes-Penmarc’h link and for a double degradation at points  and  in the Paris-Rouen link
respectively. Points  and  are the input and output of the booster amplifier respectively.
3.5
For 10 dB
attenuation,
the total
power at
preamplier
input is not
modified
1543 nm
3
1550 nm
2.5
1557.5 nm
2
1.5
1
0.5
0
0
2
4
6
8
10
12
Attenuation at point 2
Fig.6 : Penalty for a single degradation
6
At 9 dB attenuation
in and 10 dB in 2
preamplifier , 3
input power is 0.3
dB lower than that
of nominal
conditions
penalty
18
16
14
12
10
8
6
4
2
0
-2
6-8
4-6
2-4
0-2
12
-2-0
9
6
3
attenuation in
3 (dB)
attenuation in
2 (dB)
Wavelength : 1543 nm
Fig.7 : Penalty for a double degradation
The measured penalties are far less than the corresponding degradations. The optical amplifiers
working in saturated conditions generate an efficient screening of the receiver against line degradation
and this is true for all the wavelengths of the optical multiplex. We found this point interesting from
an operational point of view.
CONCLUSION
FT is operating two WDM links with live traffic in the synchronous trunk network. One of them has
been working for one year.
The strategy of optimising the use of the fibre infrastructure will be continued.
The robustness of the installed links is satisfactory with regard to additional attenuation or amplifier
degradation performance.
The availability of a meshed WDM network is an issue in case of amplifier breakdown. Rerouting at
the VC4 level through SDH cross-connects is a solution for today. Redundancy in the optical
amplifiers would improve the availability of the meshed network.
The association of WDM and SDH ring architecture is efficient from a protection point of view.
7