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Optical Transport Network (OTN) simulation

ENP is able to simulate the full Optical Transport Network (OTN) stack according to ITU-T G.709, from the Optical Data Units (ODUs), and its routing over the Optical Transport Units (OTUs), down to the Optical Channels (OChs), WDM links and fiber spans (Optical Transmission Section, OTS).

Advanced aspects like a combination of multiple recovery techniques at ODU, OTU, OCh, and WDM levels can coexist in arbitrary arrangements, reflecting the heterogeneous settings found in optical networks.

ITU-T G.872 / G.709 OTN hierarchy

The ITU-T G.872 OTN and ITU-T G.709 OTN documents define the hierarchy of network elements inside the Optical Transport Network, also called the Optical Transport Hierarchy (OTH). OTH combines both optical and electrical multiplexing of signals in a common framework. The electrical domain is structured in a hierarchical order based on electrical frames with hierarchical encapsulations. The optical domain is targeted to use Wavelength Division Multiplexing (WDM) technology, with standardized methods e.g. for Operations, Administration, and Maintenance (OAM) aspects. A keystone element in the hierarchy is the definition of the Optical Channel (OCh), the optical signal to be multiplexed with its standardized overheads.

The figure below illustrates the key elements of the OTN hierarchy, as enumerated below, and described in the next sections in this page

  • Optical Data Unit (ODU / ODUk). The electrical client signals to be transported in the OTN are introduced into Optical Payload Units (OPUs), with an overhead to support any rate or format adaptation process required. Then, the OPU is mapped into the Optical Data Unit (ODU), which includes extra overhead for OAM functions. Then one or more ODUs are multiplexed into an OTU to be transported via the OTN. Different rates are available for the OPU/ODU signals, e.g. ODU0 (1.25 Gbps), ODU1(2.5 Gbps), ODU2 (10 Gbps), ODU4 (100 Gbps), and the ODUflex configuration, that basically can carry any configured rate.
  • Optical Transport Unit (OTU / OTUk). An OTU is a placeholder for carrying in a multiplexed form a number of ODUs. OTUs add a Forward-Error-Correction (FEC) and other overhead to the signal for OAM. Different rates are available for the OTU signals, e.g. OTU1(2.5 Gbps), OTU2 (10 Gbps), OTU4 (100 Gbps), and others.
  • Optical Channel (OCh). OChs represent an end-to-end optical connection (with no conversion to electronic domain in the middle), transmitted at a particular wavelength, with a particular optical modulation and optical signal characteristics. An OCh carries encapsulated in it the client data of one OTU. The OCh payload has an electrical standardized structure, where the OTU transported in it is at the highest multiplexing level. Actually, an OTU is realized via a sequence of one or more OChs, of the same rate, with Optical-Electrical-Optical signal regeneration at intermediate nodes (where OChs end).
  • Optical Multiplex Section (OMS). Multiple OChs can be transported in a Wavelength Division Multiplexing (WDM) aggrupation, as long as its occupied optical spectrum is not overlapping. The section of the optical network between two WDM multiplexers/demultiplexers is called an Optical Multiplexing Section (OMS), or more informally called a WDM link.
  • Optical Transmission Section (OTS). Corresponds to passive fiber sections between any two network elements in the OTN, including optical amplifiers. More informally called a fiber span. An OMS is typically composed of a sequence of OTS sections and optical line amplifiers in the middle (potentially with other optical signal compensation elements, like dispersion compensators). Additionally, a single OCh may be directly transmitted between two elements without WDM multiplexing (and thus without the OMS section).

OTN includes and standardizes a number of OAM functions, also for the interoperation between different network providers. At OPU, ODU, and OTU layers, this involves the definition of the overheads in the different electrical frames. At OCh, OMS, and OTS levels, supervision signals are sent as optical signals outside the ITU grid, in an out-of-band common Optical Supervisory Channel (OSC).

ENP modeling of the ODUs

ENP models the ODUs using three elements:

  • Unidirectional ODU demands (or requests). These elements represent the request to create an ODU between two particular end nodes, of a particular rate, and with an intended ODU recovery scheme.
  • Unidirectional ODUs. An unidirectional ODU satisfies a particular ODU request, ending in particular ODU-enabled ports of the end nodes.
  • Unidirectional ODU paths. A sequence of OTUs between ODU end ports. These elements are represented in the model with the UnidiSubpathOdu objects. Each ODU can be realized via one or two ODU paths, in the latter case following a 1+1 configuration.

Unidirectional ODUs can be arranged in bidirectional pairs.

ODUs can be tagged to be recoverable under failures in different forms:

The ODU has one ODU path. If the ODU path fails, the ODU is considered as down, and thus not carrying traffic.

The ODU has one ODU path. If the ODU path fails, a customizable algorithm is run in order to find a new path for the failed ODU.

The ODU has two ODU paths of the same rate, that reserve capacity in typically OTU-disjoint routes. If both ODU paths are failed, the ODU is failed.

ENP modeling of the OTUs

ENP models the OTUs using two elements:

  • Unidirectional OTUs. These elements are represented in the model with the UnidiOtu objects, that define an OTU between two particular OTU-enabled ports of the end nodes.
  • Unidirectional OTU paths. A sequence of OChs between OTU end ports, all of a line rate equal to the OTU line rate. These elements are represented in the model with the UnidiSubpathOtu objects. Each OTU can be realized via one or two OTU paths, in the latter case following a 1+1 configuration.

Unidirectional OTUs can be arranged in bidirectional pairs.

OTUs are mainly characterized by their end nodes, line rate, and recovery type under failures. As for the recovery type, the following alternatives are eligible:

The OTU has one OTU path. If the OTU path fails, the OTU is considered down, all its ODU paths are down, and thus not carrying traffic.

The OTU has one OTU path. If the OTU path fails, a customizable algorithm is run in order to find a new path for the failed OTU.

The OTU has two OTU paths of the same rate, that reserve capacity in typically fiber-disjoint routes. If both OTU paths are failed, the OTU is failed.

ENP modeling of the OChs

ENP models the OChs using two elements:

  • Unidirectional OChs. These elements are represented in the model with the UnidiOch objects, that define an OCh between two particular OCh-enabled ports of the end nodes.
  • Unidirectional OCh paths. A sequence of WDM links and/or fiber spans between the OCh end ports. These elements are represented in the model with the UnidiSubpathOch objects. Each OCh can be realized via one or two OCh paths, in the latter case following a 1+1 configuration and fiber disjoint paths, given that the same optical signal would be propagated in both paths.

Unidirectional OChs can be arranged in bidirectional pairs.

OChs are mainly characterized by their OCh type, represented by an object of the type OchType. This element defines the OCh rate, optical modulation, tunability range, and other physical layer characteristics of the OCh. For the OCh to exist, both OCh end nodes must be capable of handling OChs of its type. See this section for details on how the optical signal performances at the receiver end of the OCh are estimated.

OChs can be realized with one or two OCh paths:

The OCh has one OCh path. If the OCh path fails, the OCh is considered as down, and thus its traversing OTU path is also down.

The OCh has two OCh paths of the same type, so the same optical signal is propagated through two different fiber-disjoint paths. If both OCh paths are failed, the OCh is failed.

ENP models the WDM links using two elements:

  • Unidirectional WDM links. These elements are represented in the model with the UnidiWdm objects, that define a WDM link between two particular WDM-enabled ports of the end nodes.
  • Unidirectional WDM link paths. A sequence of fiber spans (OTS) between the WDM link end ports. These elements are represented in the model with the UnidiSubpathWdm objects. Each WDM link can be realized via one or two WDM paths, in the latter case following a 1+1 configuration and fiber disjoint paths, given that the same WDM signal would be propagated in both paths.

Unidirectional WDM links can be arranged in bidirectional pairs.

WDM links can be realized with one or two OCh paths:

The WDM link has one WDM path. If the WDM link path fails, the WDM is considered as down, and thus its traversing OCh paths are also down.

The WDM has two WDM paths, and the same optical signal is propagated through two different fiber-disjoint paths. If both WDM link paths are failed, the WDM link is failed.

ENP modeling of the fiber spans (OTS)

ENP models the fiber spans (Optical Transmission Section, OTS) using the UnidiOts objects.

Unidirectional OTS links can be arranged in bidirectional pairs.

Unidirectional OTS links are characterized by their end nodes and ports, fiber length, different parameters of the physical fiber, and user-defined information on their end connectors.