This report shows line engineering information for WDM links in a multilayer optical network. The impairment calculations are based on the Gaussian Noise Model developed by Politecnico di Torino and 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 fibers_XXX input parameters, each one describing the parameter for the fiber types specified in fibers_types,
in the same order and separated by spaces. The fiber can be split into spans if optical amplifers (EDFAs) and/or passive components (PCs) are placed along the fiber. These spans can be of different fiber types as long as they are described in a
link attribute called "fiberTypes". Must be separated by spaces and, in case that there were more spans than elements of the attribute, the default type given in "fiber_default_type" would be used.
- 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), EDFA gains (assumed in dB) and EDFA noise figures (in dB) are read from the "edfaPositions_km", "edfaGains_dB" and "edfaNoiseFigures_dB"
attributes of the links. The format of all 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
- There are not Dispersion compensating modules (DCMs) in the topoology, since the Gaussian Noise Model is used.
- Passive components are described by the link attributes "pcPositions_km" and "pcLosses_dB". The i-th number corresponding to the position/loss of the i-th PC. If the attributes do not exist, it is assumed that no PCs are placed
in this link. Other specifications for PC will be described in teh pc_XXX input 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, both when the link has one single active channel, or when all the "gn_spec_nCh" are active
- 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