Wavelength Switching
What does it really mean?
In the last couple of years, there has been a renewed interest in manipulating large data streams optically instead of electrically. This comes primarily from the realization that in increasing parts of networks, the wavelength is the unit of commerce. In these regions, it is not necessary to disaggregate the traffic for grooming or other electronic processing. The dramatic improvements in economics and flexibility from dealing with wavelengths at the optical layer have resulted in a surge of creative optical techniques and jargon. We see increased use of terms such as: ROADM, TOADM, Multi-hubbed ROADM, Wavelength Selective Switch (WSS), and Photonic Cross-Connect.
These new terms reflect a broad spectrum of wavelength switching capabilities which were first catagorized over a decade ago. To put the new terms in perspective, let’s review the old ones and how they apply to modern networks.
OADM – Optical Add/Drop Multiplexer. This network element adds and/or drops wavelengths from a WDM fiber. Replacing the old electrical ADMs, they are used to build an optical layer version of SONET/SDH rings. Many of the original OADMs used fixed optical filters to extract the wavelengths and, as a result, were inflexible and hard to upgrade. They have been superseded by ROADMs.
ROADM – Reconfigurable Optical Add/Drop Multiplexer. The major improvement offered by ROADMs is the use of a configurable wavelength selective element to pick the wavelength to be dropped. As a result, these are the first true wavelength switches. They permit a network operator to dynamicly reconfigure the wavelengths in a WDM fiber ring. Competition among the many vendors of ROADMs has resulted in a number of added features including optical layer ring protection switching and a limited cross-connect capability for interconnecting rings (see WISX below). The limited cross-connect capability is frequently implemented using a bank of small (e.g. 1 x 9) switches called a Wavelength Selective Switch. This has led to confusion with the true cross-connects below.
OXC – Optical Cross-connect. Also once known as Fiber Cross-connect (FXC), an OXC connects one fiber to another fiber without concern for what traffic is being carried. As a result, OXCs could be used to switch wavelengths if the WDM Mux/Demux were handled externally or they could be used for dynamic fiber management independent of WDM. The term “Optical Cross-Connect” now frequently refers to an electrical switch with optical inputs and outputs and has been replaced by the term Photonic Cross-Connect (PXC) for true optical switches.
WSXC – Wavelength Selective Cross-connect. Unlike a PXC, a WSXC handles the WDM Mux/Demux internally and selectively directs individual wavelengths from one fiber to another. Thus wavelength 1 of input fiber 1 can be switched to output fiber 2 while wavelength 2 of input fiber 1 is switched to output fiber 5. This capability is useful for building WDM mesh networks or interconnecting multiple WDM rings. As a result, a limited version of WSXC has been incorporated into some ROADMs, frequently known as “hubbed ROADMs”. However, note that WSXCs do not convert wavelengths. The same wavelength must be available along the entire mesh optical path or in both rings. In essence, a WSXC incorporates the WDM Mux/Demux with a stack of small PXCs, one for each wavelength (typically 40 or 80). As network congestion increases the odds that an end-to-end wavelength is available significantly decreases. In congested network regions, precisely those areas where wavelength switching flexibility is most needed, the benefits of WSXCs wane. For full network flexibility, a different network element is required, a WIXC.
WIXC – Wavelength Interchange Cross-connect. A WIXC incorporates the functionality of a WSXC with the added capability to change the wavelength as it passes through the node. Any input wavelength in any input fiber can be directed to any output wavelength in any output fiber, a true non-blocking optical cross-connect. There are a number of ways of adding the wavelength conversion capability. A decade ago, it was widely viewed that, like the switching, the wavelength conversion would be done optically. All-optical wavelength conversion has proven more difficult than then thought and most today view the wavelength conversion as an OEO operation. This has the added benefit of permitting (although not requiring) a full 3R regeneration at this point if the network architecture requires. Note, however, if this is carried too far, a WIXC begins to have all of the extreme cost, heat, power and size problems of all-electronic switches.
Fortunately, while wavelength conversion will be required in a congested network, it will not be required everywhere. As a result, instead of providing wavelength conversion for every port, a more cost-effective architecture is appearing. In this revised view, wavelength conversion, 3R regeneration, traffic monitoring and other capabilities are shared resources which are available in the WIXC node and switched in when and where required. This significantly reduces the node cost, but does require a sufficiently large cross-connect to include ports not only for the traffic bearing fibers, but also for these added shared network resources.
