Technology conventions - Optical WDM networks

Optical networks have been established as the enabling technology for today’s high-speed communication networks. Wavelength Division Multiplexing (WDM) enables the efficient utilization of optical fibers by dividing its tremendous bandwidth into a set of disjoint channels.

The so-called wavelength grid determines the wavelengths or wavebands used by a channel:

  • In fixed-grid networks, all the channels have the same width, typically of 100 GHz or 50 GHz. For instance, according to the 100 GHz ITU-T grid, first channel in the C-band has a central frequency of 196.1 THz, next 196.0 THz, next 195.9 THz... (until 191.2 THz, the last channel in the C-band). In this case each channel is typically called a wavelength.
  • Flexi-grid networks are a generalization of fixed-grid networks, where the width of a channel is an integer multiple of the width of a so-called frequency slot (typically 12.5 GHz). A channel can typically aggregate up to tens of slots. In general (but not always), these slots are constrained to be contiguous.

Flexi-grid networks where all the channels occupy exactly one frequency slot, and the width of the slot is e.g. 100 GHz, are equivalent to a fixed-grid network with a 100 GHz width. For this reason, after Net2Plan 0.4.1 we have put together inside WDMUtils library (see its Javadoc) the functionalities of both fixed and flexi grid WDM networks (formerly, flexi-grid networks were dealt with in FlexiGridUtils library, now eliminated).

Wavelength-routed networks

In wavelength-routed WDM networks, all-optical channels traversing several fibers, called lightpaths, can be established between pairs of nodes. A lightpath occupies one channel (a wavelength in fixed-grid networks, and a set of frequency slots in flexi-grid networks) in each traversed link, and two lightpaths routed over the same physical link cannot use the same frequency slot in that link. This is called the wavelength or frequency slot clash constraint. The Routing and Spectrum Assignment (RSA) problem and the Routing and Wavelength Assignment (RWA) problem in flexi and fixed grid WDM networks respectively are the ones deciding for each lightpath the route and frequency slots (wavelengths) to occupy in each traversed fiber. Typically, the wavelength/slots of a lightpath cannot change along the traversed fibers (since wavelength converters are not available). In this case we say that the problem has the wavelength continuity constraint.

Higher layers in the network see a lightpath as a pipe to transmit traffic, and they are not necessarily aware of the actual sequence of fiber links of the lightpath.

The line rate of the lightpath is its capacity e.g. in Gbps. Typical line rates are 10, 40 and 100 Gbps. In flexi-grid networks, it is possible to have lightpaths of different line rates, that occupy the same number of frequency slots. This is because, the transponders (the ones transmitting and receiving the optical signals) can be based on different optical modulations with different spectral efficiencies. For instance, a transponder using BPSK modulation has a spectral efficiency of 1 bps/Hz, while a more sophisticated transponder using 16-QAM has 4 bps/Hz.

Nodes in wavelength-routed networks are called Optical Add/Drop Multiplexers (OADMs). They are able to add new lightpaths initiated in the node, drop lightpaths terminating in the node, and optically switch (bypass) lightpaths that traverse the node. Usually, a lightpath occupies the same wavelength in all the traversed links. This is called the wavelength continuity constraint. However, it is possible (so far only in fixed-grid networks) to allocate optical wavelength converters at intermediate nodes of the lightpath that are able to change its wavelength. Nowadays, wavelength converters are composed of an opto-electronic receiver attached to an electro-optic transmitter. Then, the optical signal is regenerated, while its wavelength can be modified. More often, optical regenerators are used not for changing the wavelength in an intermediate node of a lightpath, but to regenerate the optical signal recovering it from its normal degradation caused by channel noise and other impairments.

In Net2Plan, the WDMUtils library is provided to ease the handling of WDM networks. WDMUtils assumptions are:

  • Network layer: A WDM network is represented by a network layer. Typically, the layer name is set to "WDM" (although this is not mandatory). The measure units for the traffic in this layer is assumed to be Gbps. In its turn, the capacity of the links is assumed to be measured in number of frequency slots.
  • Links: Each link in the WDM network layer is an optical fibre. The capacity of the link is measured as the (integer) number of frequency slots available in that fiber. For instance, a typical WDM network using the C-band in 50 GHz channels, has 80 available waveleneghts, and thus the link capacity is 80.
  • Nodes: Each node in the network with input/output links in the WDM layer, represents an Optical Add/Drop Multiplexer (OADMs) capable of routing lightpaths.
  • Demands: A demand is here an intention to carry traffic between two OADMS. The offered traffic is assumed to be measured in Gbps (e.g. typically 10 Gbps, 40 Gbps, 100 Gbps).
  • Routes: A route represents a lightpath. The carried traffic of the lightpath corresponds to the lighptath line rate. The occupied link capacity of the lightpath is the total number of slots occupied (the same total number in all the traversed links). For specifying the particular frequency slots occupied by a lightpath in a link, in Net2Plan we assume that frequency slots in any link are numbered with consecutive numbers starting from zero 0,1,2,... The lowest number typically corresponds to the lower wavelength. The set of frequency slots occupied by a lightpath in each traversed link is stored by WDMUtils in a route attribute, in an internal format. The user using the WDMUtils library does not need to bother about the details of this format. Additionally, it is possible to define the set of nodes where the optical signal goes through a regenerator of the optical signal (with or without wavelength conversion). This information is also stored as Route attributes.
  • Protection segments: Zero, one or more protection segments can be associated to the route, representing (partial or total) backup lightpaths that protect the primary route. The occupied link capacity of the protection segment reflects the total number of frequency slots reserved in the traversed links. The actual set of slots reserved in each traversed, and the places where the signal is regenerated (if any) are also stored by WDMUtils in segment attributes

Offline and online network design algorithms for fixed/flexi grid WDM networks, as well as reports for showing WDM physical layer and RSA allocation information, can be found in the Net2Plan code repository under keyword WDM.