This report shows line engineering information for WDM links in a multilayer optical network. The
impairment calculations are inspired in the procedures described in the 2009 ITU-T WDM manual
"Optical fibres, cabbles and systems".
The report assumes that the WDM network follows the scheme:
- In the net2plan object, nodes are OADMs, links are fiber links, and routes are lightpaths:
WDM channels optically switched at intermediate nodes.
- Nodes are connected by unidirectional fiber links. Fiber link distance is given by the link
length. Other specifications are given by fiber_XXX input parameters. The fiber can be split into
spans if optical amplifers (EDFAs) and/or dispersion compensating modules (DCMs) are placed along
the fiber.
- Optical line amplifiers (EDFAs) can be located in none, one or more positions in the fiber
link, separating them in different spans. EDFAs are supposed to operate in the automatic gain
control mode. Thus, the gain is the same, whatever the number of input WDM channels. EDFA
positions (as distance in km from the link start to the EDFA location) and EDFA gains (assumed in
dB) are read from the "edfaPositions_km" and "edfaGains_dB" attributes of the links. The format
of both attributes are the same: a string of numbers separated by spaces. The i-th number
corresponding to the position/gain of the i-th EDFA. If the attributes do not exist, it is
assumed that no EDFAs are placed in this link. EDFA specifications are given by "edfa_XXX"
parameters
- Dispersion compensating modules (DCMs) can be located in none, one or more positions in the
fiber link, separating them in different spans. If a DCM and a EDFA have the same location, it is
assumed that the DCM is placed first, to reduce the non-linear effects. DCM positions (as
distance in km from the link start to the DCM location) are read from the "dcmPositions_km"
attribute of the link, and the same format as with "edfaPositions_km" attribute is expected. If
the attribute does not exist, it is assumed that no DCMs are placed in this link. DCM
specifications are given by "dcm_XXX" parameters
- Fiber links start and end in OADM modules, that permit adding, dropping and optically switch
individual WDM channels. OADMs have a pre-amplifier (traversed by drop and express channels) and
a boost amplifier (traversed by add and express channels). They are supposed to equalize the
channel power at their outputs, to a fixed value (added and express channels will thus have the
same power in the fibers). Also, OADMs attenuate appropriately the optical signal coming from the
pre-amplifier, in the drop channels, so that they fall within the receiver sensitivity range.
OADM noise figures for add, drop and express channels are given as input parameters. PMD values
for add, drop and express channels are computed assumming that: (i) add channel traverse a
multiplexer and the booster, (ii) drop channels travese the pre-amplifier and a demultiplexer,
(iii) express channels traverse the two amplifiers. The required parameters are provided in
oadm_XXX parameters.
- Each channel ends in a receiver, with specifications given by "tp_XXX" parameters.
The basic checks performed are:
- For each link, signal power levels are within operating ranges at the oadm/edfas/dcms, both
when the link has one single active channel, or when all the "channels_maxNumChannels" are
active
- For each link, chromatic dispersion is within the limits set per link
- For each route (lightpath), chromatic dispersion is within the limits of the receiver.
- For each route (lightpath), OSNR (Optical Signal to Noise Ration) is within the operating
range at the receiver. A set of margins are considered to account to several not directly
considered impairments.
- For each route (lightpath), PMD (Polarization mode dispersion) is within the operating range
at the receiver