JSON Input Files

GNPy uses a set of JSON files for modeling the network. Some data (such as network topology or the service requests) can be also passed via XLS files.

Equipment Library

Design and transmission parameters are defined in a dedicated json file. By default, this information is read from gnpy/example-data/eqpt_config.json. This file defines the equipment libraries that can be customized (EDFAs, fibers, and transceivers).

It also defines the simulation parameters (spans, ROADMs, and the spectral information to transmit.)

EDFA

The EDFA equipment library is a list of supported amplifiers. New amplifiers can be added and existing ones removed. Three different noise models are available:

1. 'type_def': 'variable_gain' is a simplified model simulating a 2-coil EDFA with internal, input and output VOAs. The NF vs gain response is calculated accordingly based on the input parameters: nf_min, nf_max, and gain_flatmax. It is not a simple interpolation but a 2-stage NF calculation.

2. 'type_def': 'fixed_gain' is a fixed gain model. NF == Cte == nf0 if gain_min < gain < gain_flatmax

3. 'type_def': 'openroadm' models the incremental OSNR contribution as a function of input power. It is suitable for inline amplifiers that conform to the OpenROADM specification. The input parameters are coefficients of the third-degree polynomial.

4. 'type_def': 'openroadm_preamp' and openroadm_booster approximate the preamp and booster within an OpenROADM network. No extra parameters specific to the NF model are accepted.

5. 'type_def': 'advanced_model' is an advanced model. A detailed JSON configuration file is required (by default gnpy/example-data/std_medium_gain_advanced_config.json). It uses a 3rd order polynomial where NF = f(gain), NF_ripple = f(frequency), gain_ripple = f(frequency), N-array dgt = f(frequency). Compared to the previous models, NF ripple and gain ripple are modelled.

For all amplifier models:

field	type	description
type_variety	(string)	a unique name to ID the amplifier in the JSON/Excel template topology input file
out_voa_auto	(boolean)	auto-design feature to optimize the amplifier output VOA. If true, output VOA is present and will be used to push amplifier gain to its maximum, within EOL power margins.
allowed_for_design	(boolean)	If false, the amplifier will not be picked by auto-design but it can still be used as a manual input (from JSON or Excel template topology files.)

Fiber

The fiber library currently describes SSMF and NZDF but additional fiber types can be entered by the user following the same model:

field	type	description
type_variety	(string)	a unique name to ID the fiber in the JSON or Excel template topology input file
dispersion	(number)	In \(s \times m^{-1} \times m^{-1}\).
dispersion_slope	(number)	In \(s \times m^{-1} \times m^{-1} \times m^{-1}\)
dispersion_per_frequency	(dict)	Dictionary of dispersion values evaluated at various frequencies, as follows: {"value": [], "frequency": []}. value in \(s \times m^{-1} \times m^{-1}\) and frequency in Hz.
effective_area	(number)	Effective area of the fiber (not just the MFD circle). This is the \(A_{eff}\), see e.g., the Corning whitepaper on MFD/EA. Specified in \(m^{2}\).
gamma	(number)	Coefficient \(\gamma = 2\pi\times n^2/(\lambda*A_{eff})\). If not provided, this will be derived from the effective_area \(A_{eff}\). In \(w^{-1} \times m^{-1}\). This quantity is evaluated at the reference frequency and it is scaled along frequency accordingly to the effective area scaling.
pmd_coef	(number)	Polarization mode dispersion (PMD) coefficient. In \(s\times\sqrt{m}^{-1}\).
lumped_losses	(array)	Places along the fiber length with extra losses. Specified as a loss in dB at each relevant position (in km): {"position": 10, "loss": 1.5})
raman_coefficient	(dict)	The fundamental parameter that describes the regulation of the power transfer between channels during fiber propagation is the Raman gain coefficient (see [DAmicoCL+22] for further details); \(f_{ref}\) represents the pump reference frequency used for the Raman gain coefficient profile measurement (“reference_frequency”), \(\Delta f\) is the frequency shift between the pump and the specific Stokes wave, the Raman gain coefficient in terms of optical power \(g_0\), expressed in \(1/(m\;W)\). Default values measured for a SSMF are considered when not specified.

RamanFiber

The RamanFiber can be used to simulate Raman amplification through dedicated Raman pumps. The Raman pumps must be listed in the key raman_pumps within the RamanFiber operational dictionary. The description of each Raman pump must contain the following:

field	type	description
power	(number)	Total pump power in \(W\) considering a depolarized pump
frequency	(number)	Pump central frequency in \(Hz\)
propagation_direction	(number)	The pumps can propagate in the same or opposite direction with respect the signal. Valid choices are coprop and counterprop, respectively

Beside the list of Raman pumps, the RamanFiber operational dictionary must include the temperature that affects the amplified spontaneous emission noise generated by the Raman amplification. As the loss coefficient significantly varies outside the C-band, where the Raman pumps are usually placed, it is suggested to include an estimation of the loss coefficient for the Raman pump central frequencies within a dictionary-like definition of the RamanFiber.params.loss_coef (e.g. loss_coef = {"value": [0.18, 0.18, 0.20, 0.20], "frequency": [191e12, 196e12, 200e12, 210e12]}).

Transceiver

The transceiver equipment library is a list of supported transceivers. New transceivers can be added and existing ones removed at will by the user. It is used to determine the service list path feasibility when running the gnpy-path-request script.

field	type	description
type_variety	(string)	A unique name to ID the transceiver in the JSON or Excel template topology input file
frequency	(number)	Min/max central channel frequency.
mode	(number)	A list of modes supported by the transponder. New modes can be added at will by the user. The modes are specific to each transponder type_variety. Each mode is described as below.

The modes are defined as follows:

field	type	description
format	(string)	a unique name to ID the mode
baud_rate	(number)	in Hz
OSNR	(number)	min required OSNR in 0.1nm (dB)
bit_rate	(number)	in bit/s
roll_off	(number)	Pure number between 0 and 1. TX signal roll-off shape. Used by Raman-aware simulation code.
tx_osnr	(number)	In dB. OSNR out from transponder.
equalization_offset_db	(number)	In dB. Deviation from the per channel equalization target in ROADM for this type of transceiver.
penalties	(list)	list of impairments as described in impairment table.
cost	(number)	Arbitrary unit

Penalties are linearly interpolated between given points and set to ‘inf’ outside interval. The accumulated penalties are substracted to the path GSNR before comparing with the min required OSNR. The penalties per impairment type are defined as a list of dict (impairment type - penalty values) as follows:

field	type	description
chromatic_dispersion or pdl or pmd	(number) (string)	In ps/nm/. Value of chromatic dispersion. In dB. Value of polarization dependant loss. In ps. Value of polarization mode dispersion.
penalty_value	(number)	in dB. Penalty on the transceiver min OSNR corresponding to the impairment level

for example:

"penalties": [{
        "chromatic_dispersion": 360000,
        "penalty_value": 0.5
    }, {
        "pmd": 110,
        "penalty_value": 0.5
    }
]

ROADM

The user can only modify the value of existing parameters:

field	type	description
target_pch_out_db or target_psd_out_mWperGHz or target_out_mWperSlotWidth (mutually exclusive)	(number)	Default equalization strategy for this ROADM type. Auto-design sets the ROADM egress channel power. This reflects typical control loop algorithms that adjust ROADM losses to equalize channels (e.g., coming from different ingress direction or add ports). These values are used as defaults when no overrides are set per each Roadm element in the network topology.
add_drop_osnr	(number)	OSNR contribution from the add/drop ports
pmd	(number)	Polarization mode dispersion (PMD). (s)
restrictions	(dict of strings)	If non-empty, keys preamp_variety_list and booster_variety_list represent list of type_variety amplifiers which are allowed for auto-design within ROADM’s line degrees. If no booster should be placed on a degree, insert a Fused node on the degree output.

Global parameters

The following options are still defined in eqpt_config.json for legacy reasons, but they do not correspond to tangible network devices.

Auto-design automatically creates EDFA amplifier network elements when they are missing, after a fiber, or between a ROADM and a fiber. This auto-design functionality can be manually and locally deactivated by introducing a Fused network element after a Fiber or a Roadm that doesn’t need amplification. The amplifier is chosen in the EDFA list of the equipment library based on gain, power, and NF criteria. Only the EDFA that are marked 'allowed_for_design': true are considered.

For amplifiers defined in the topology JSON input but whose gain = 0 (placeholder), auto-design will set its gain automatically: see power_mode in the Spans library to find out how the gain is calculated.

The file sim_params.json contains the tuning parameters used within both the gnpy.science_utils.RamanSolver and the gnpy.science_utils.NliSolver for the evaluation of the Raman profile and the NLI generation, respectively.

If amplifiers don’t have settings, auto-design also sets amplifiers gain, output VOA and target powers according to [J. -L. Auge, V. Curri and E. Le Rouzic, Open Design for Multi-Vendor Optical Networks, OFC 2019](https://ieeexplore.ieee.org/document/8696699), equation 4. See delta_power_range_db for more explaination.

field	type	description
raman_params.flag	(boolean)	Enable/Disable the Raman effect that produces a power transfer from higher to lower frequencies. In general, considering the Raman effect provides more accurate results. It is mandatory when Raman amplification is included in the simulation
raman_params.result_spatial_resolution	(number)	Spatial resolution of the output Raman profile along the entire fiber span. This affects the accuracy and the computational time of the NLI calculation when the GGN method is used: smaller the spatial resolution higher both the accuracy and the computational time. In C-band simulations, with input power per channel around 0 dBm, a suggested value of spatial resolution is 10e3 m
raman_params.solver_spatial_resolution	(number)	Spatial step for the iterative solution of the first order differential equation used to calculate the Raman profile along the entire fiber span. This affects the accuracy and the computational time of the evaluated Raman profile: smaller the spatial resolution higher both the accuracy and the computational time. In C-band simulations, with input power per channel around 0 dBm, a suggested value of spatial resolution is 100 m
nli_params.method	(string)	Model used for the NLI evaluation. Valid choices are gn_model_analytic (see eq. 120 from arXiv:1209.0394) and ggn_spectrally_separated (see eq. 21 from arXiv:1710.02225).
nli_params.computed_channels	(number)	The channels on which the NLI is explicitly evaluated. The NLI of the other channels is interpolated using numpy.interp. In a C-band simulation with 96 channels in a 50 GHz spacing fix-grid we recommend at one computed channel every 20 channels.

Span

Span configuration is not a list (which may change in later releases) and the user can only modify the value of existing parameters:

field	type	description
power_mode	(boolean)	If false, gain mode. In the gain mode, only gain settings are used for propagation, and delta_p is ignored. If no gain_target is set in an amplifier, auto-design computes one according to the delta_power_range optimisation range. The gain mode is recommended if all the amplifiers have already consistent gain settings in the topology input file. If true, power mode. In the power mode, only the delta_p is used for propagation, and gain_target is ignored. The power mode is recommended for auto-design and power sweep. If no delta_p is set, auto-design sets an amplifier power target according to delta_power_range_db.
delta_power_range_db	(number)	Auto-design only, power-mode only. Specifies the [min, max, step] power excursion/span. It is a relative power excursion w/r/t the power_dbm + power_range_db (power sweep if applicable) defined in the SI configuration library. This relative power excursion is = 1/3 of the span loss difference with the reference 20 dB span. The 1/3 slope is derived from the GN model equations. For example, a 23 dB span loss will be set to 1 dB more power than a 20 dB span loss. The 20 dB reference spans will always be set to power = power_dbm + power_range_db. To configure the same power in all spans, use [0, 0, 0]. All spans will be set to power = power_dbm + power_range_db. To configure the same power in all spans and 3 dB more power just for the longest spans: [0, 3, 3]. The longest spans are set to power = power_dbm + power_range_db + 3. To configure a 4 dB power range across all spans in 0.5 dB steps: [-2, 2, 0.5]. A 17 dB span is set to power = power_dbm + power_range_db - 1, a 20 dB span to power = power_dbm + power_range_db and a 23 dB span to power = power_dbm + power_range_db + 1
max_fiber_lineic_loss_for_raman	(number)	Maximum linear fiber loss for Raman amplification use.
max_length	(number)	Split fiber lengths > max_length. Interest to support high level topologies that do not specify in line amplification sites. For example the CORONET_Global_Topology.xlsx defines links > 1000km between 2 sites: it couldn’t be simulated if these links were not split in shorter span lengths.
length_unit	“m”/”km”	Unit for max_length.
max_loss	(number)	Not used in the current code implementation.
padding	(number)	In dB. Min span loss before putting an attenuator before fiber. Attenuator value Fiber.att_in = max(0, padding - span_loss). Padding can be set manually to reach a higher padding value for a given fiber by filling in the Fiber/params/att_in field in the topology json input [1] but if span_loss = length * loss_coef + att_in + con_in + con_out < padding, the specified att_in value will be completed to have span_loss = padding. Therefore it is not possible to set span_loss < padding.
EOL	(number)	All fiber span loss ageing. The value is added to the con_out (fiber output connector). So the design and the path feasibility are performed with span_loss + EOL. EOL cannot be set manually for a given fiber span (workaround is to specify higher con_out loss for this fiber).
con_in, con_out	(number)	Default values if Fiber/params/con_in/out is None in the topology input description. This default value is ignored if a Fiber/params/con_in/out value is input in the topology for a given Fiber.

{
    "uid": "fiber (A1->A2)",
    "type": "Fiber",
    "type_variety": "SSMF",
    "params":
    {
          "length": 120.0,
          "loss_coef": 0.2,
          "length_units": "km",
          "att_in": 0,
          "con_in": 0,
          "con_out": 0
    }
}

SpectralInformation

GNPy requires a description of all channels that are propagated through the network.

This block defines a reference channel (target input power in spans, nb of channels) which is used to design the network or correct the settings. It may be updated with different options –power. It also defines the channels to be propagated for the gnpy-transmission-example script unless a different definition is provided with --spectrum option.

Flexgrid channel partitioning is available since the 2.7 release via the extra --spectrum option. In the simplest case, homogeneous channel allocation can be defined via the SpectralInformation construct which defines a spectrum of N identical carriers:

field	type	description
f_min, f_max	(number)	In Hz. Define spectrum boundaries. Note that due to backward compatibility, the first channel central frequency is placed at \(f_{min} + spacing\) and the last one at \(f_{max}\).
baud_rate	(number)	In Hz. Simulated baud rate.
spacing	(number)	In Hz. Carrier spacing.
roll_off	(number)	Pure number between 0 and 1. TX signal roll-off shape. Used by Raman-aware simulation code.
tx_osnr	(number)	In dB. OSNR out from transponder.
power_dbm	(number)	In dBm. Target input power in spans to be considered for the design In gain mode (see spans/power_mode = false), if no gain is set in an amplifier, auto-design sets gain to meet this reference power. If amplifiers gain is set, power_dbm is ignored. In power mode, the power_dbm is the reference power for the delta_p settings in amplifiers. It is also the reference power for auto-design power optimisation range Spans/delta_power_range_db. For example, if delta_power_range_db = [0,0,0], the same power=power_dbm is launched in every spans. The network design is performed with the power_dbm value: even if a power sweep is defined (see after) the design is not repeated. If the --power CLI option is used, its value replaces this parameter.
power_range_db	(number)	Power sweep excursion around power_dbm. This defines a list of reference powers to run the propagation, in the range power_range_db + power_dbm. Power sweep uses the delta_p targets or, if they have not been set, the ones computed by auto-design, regardless of of preceding amplifiers’ power saturation. Power sweep is an easy way to find the optimal reference power. Power sweep excursion is ignored in case of gain mode.
sys_margins	(number)	In dB. Added margin on min required transceiver OSNR.

Arbitrary channel definition

Non-uniform channels are defined via a list of spectrum “partitions” which are defined in an extra JSON file via the --spectrum option. In this approach, each partition is internally homogeneous, but different partitions might use different channel widths, power targets, modulation rates, etc.

field	type	description
f_min, f_max	(number)	In Hz. Mandatory. Define partition \(f_{min}\) is the first carrier central frequency \(f_{max}\) is the last one. \(f_{min}\) -\(f_{max}\) partitions must not overlap. Note that the meaning of f_min and f_max is different than the one in SpectralInformation.
baud_rate	(number)	In Hz. Mandatory. Simulated baud rate.
slot_width	(number)	In Hz. Carrier spectrum occupation. Carriers of this partition are spaced at slot_width offsets.
roll_off	(number)	Pure number between 0 and 1. Mandatory TX signal roll-off shape. Used by Raman-aware simulation code.
tx_osnr	(number)	In dB. Optional. OSNR out from transponder. Default value is 40 dB.
delta_pdb	(number)	In dB. Optional. Power offset compared to the reference power used for design (SI block in equipment library) to be applied by ROADM to equalize the carriers in this partition. Default value is 0 dB.

For example this example:

{
  "SI":[
    {
      "f_min": 191.4e12,
      "f_max":193.1e12,
      "baud_rate": 32e9,
      "slot_width": 50e9,
      "roll_off": 0.15,
      "tx_osnr": 40
    },
    {
      "f_min": 193.1625e12,
      "f_max":195e12,
      "baud_rate": 64e9,
      "delta_pdb": 3,
      "slot_width": 75e9,
      "roll_off": 0.15,
      "tx_osnr": 40
    }
  ]
}

…defines a spectrum split into two parts. Carriers with central frequencies ranging from 191.4 THz to 193.1 THz will have 32 GBaud rate and will be spaced by 50 Ghz. Carriers with central frequencies ranging from 193.1625 THz to 195 THz will have 64 GBaud rate and will be spaced by 75 GHz with 3 dB power offset.

If the SI reference carrier is set to power_dbm = 0dBm, and the ROADM has target_pch_out_db set to -20 dBm, then all channels ranging from 191.4 THz to 193.1 THz will have their power equalized to -20 + 0 dBm (due to the 0 dB power offset). All channels ranging from 193.1625 THz to 195 THz will have their power equalized to -20 + 3 = -17 dBm (total power signal + noise).

Note that first carrier of the second partition has center frequency 193.1625 THz (its spectrum occupation ranges from 193.125 THz to 193.2 THz). The last carrier of the second partition has center frequency 193.1 THz and spectrum occupation ranges from 193.075 THz to 193.125 THz. There is no overlap of the occupation and both share the same boundary.

Equalization choices

ROADMs typically equalize the optical power across multiple channels using one of the available equalization strategies — either targeting a specific output power, or a specific power spectral density (PSD), or a spectfic power spectral density using slot_width as spectrum width reference (PSW). All of these strategies can be adjusted by a per-channel power offset. The equalization strategy can be defined globally per a ROADM model, or per each ROADM instance in the topology, and within a ROADM also on a per-degree basis.

Let’s consider some example for the equalization. Suppose that the types of signal to be propagated are the following:

{
     "baud_rate": 32e9,
     "f_min":191.3e12,
     "f_max":192.3e12,
     "spacing": 50e9,
     "label": 1
 },
 {
     "baud_rate": 64e9,
     "f_min":193.3e12,
     "f_max":194.3e12,
     "spacing": 75e9,
     "label": 2
 }

with the PSD equalization in a ROADM:

{
  "uid": "roadm A",
  "type": "Roadm",
  "params": {
    "target_psd_out_mWperGHz": 3.125e-4,
  }
},

This means that power out of the ROADM will be computed as 3.125e-4 * 32 = 0.01 mW ie -20 dBm for label 1 types of carriers and 3.125e4 * 64 = 0.02 mW ie -16.99 dBm for label2 channels. So a ratio of ~ 3 dB between target powers for these carriers.

With the PSW equalization:

{
  "uid": "roadm A",
  "type": "Roadm",
  "params": {
    "target_out_mWperSlotWidth": 2.0e-4,
  }
},

the power out of the ROADM will be computed as 2.0e-4 * 50 = 0.01 mW ie -20 dBm for label 1 types of carriers and 2.0e4 * 75 = 0.015 mW ie -18.24 dBm for label2 channels. So a ratio of ~ 1.76 dB between target powers for these carriers.