The optical cross-connect (OXC) was a huge topic in optics at the turn of the millennium. Startups such as Xros and Calient were building 1,000-port OXCs (or photonic switches) and being bought for billions of dollars in some cases. But the large-scale OXC trend fizzled when modest port-count 1x2 and 1x4 ROADMs proved to be the right solution to the wavelength routing challenges of that time.

Evolution of CDC ROADMs
Over the past decade, ROADMs have evolved to incorporate more and more of the functionality of an OXC, including greater levels of automation, flexibility and scale. With the latest iteration of ROADM technologies, the "OXC" term is coming back in vogue. By adding greater automation and agility to the photonic layer, the commercial introduction of colorless, directionless and contentionless (CDC) ROADMs set this trend in motion.

Conventional ROADMs enable optical express routing (or bypass) at intermediate nodes along a route but are limited by manual wiring and connecting at the endpoints. Colorless node architectures automate the assignment of add/drop wavelength functionality such that any wavelength (or color) can be matched to any port at an add/drop site. Directionless nodes allow any wavelength to be routed to any direction served by a node without physical rewiring. Twin 1x20 wavelength-selective switches (WSSes) were key enabling technologies for colorless and directionless ROADM nodes. They replaced multiplexers and passive splitters in the design and shifted from optical loss-heavy broadcast-and-select to more efficient route-and-select transmission.

The third component of the CDC ROADM is contentionless functionality. Even with colorless and directionless functions, a ROADM network may still have limitations that require manual intervention when two wavelengths of the same color converge at the same WSS drop structure at the same time, causing wavelength-blocking. Adding an MxN switch on the drop side of the ROADM node addresses the challenge. But the current-generation multicast switches are made from a combination of power splitters and modest port-count 1xN space switches. They are bulky, power-hungry, heavy on optical loss and costly relative to colorless/directionless alone. As a result, uptake of full CDC functionality by operators has been far more limited than the CD-only combination. Without a true contentionless architecture, full automation with no restriction is not possible.

The good news for the industry is that a new generation of MxN switches has come to market for the drop-side structures that replace the early multicast version with a route-and-select architecture based on WSSes. Moving from broadcast-and-select to route-and-select add/drop eliminates the need for drop-side erbium-doped fiber amplifier (EDFA) arrays — a major culprit increasing space, power consumption and costs.

More challenges…
The CDC innovations described above — including the route-and-select MxN WSSes — are produced by ROADM manufacturers and will be used by systems suppliers to build large-scale optical nodes capable of routing hundreds of terabits of optical traffic. But beyond the CDC functionality, there is another challenge to be addressed. In large hub sites, the proliferation of ROADM nodes leads to fiber management challenges. In these large sites, thousands of interconnected ROADM fibers must be manually managed and physically moved, but this process is manual, time-consuming and prone to human error. The connectivity also consumes significant footprint in the central office.

As optical switching nodes scale upward, there is an increasing need for optical backplane connectivity. Moving fiber connectivity to the optical backplane eliminates the potentially thousands of fiber connections that can be made between large-scale optical elements. An optical backplane that eliminates external fiber connections saves on space, eliminates human error in connecting fibers, and — combined with the new generation MxN WSS components — enables fully automated routing of wavelengths at massive scale.

Early traction and future commercial activity
There is already some early traction for this new generation of OXC node. Given its immense urban populations, China is an obvious early market. As a region, China is one of the driving forces behind the OXC resurgence. China Telecom Sichuan, for example, has announced commercial OXC deployments. But there is also some early commercial activity outside China, including South Africa's Rain, United Arab Emirates' Etisalat, and additional deployments in Latin America and other parts of Asia. Heavy Reading expects commercial activity to accelerate as networks continue to grow, the need for automation increases, and the enabling technologies for photonic layer automation mature.

This blog is sponsored by Huawei.

— Sterling Perrin, Principal Analyst, Heavy Reading