What happened to the explosive market for 3D-MEMS photonic cross-connects?

What happened to the explosive market for 3D-MEMS photonic cross-connects?
Whatever happened to the explosive market for the revolutionary 3D-MEMS photonic cross-connects (PXCs) promised back at OFC-2000, with their supposedly "thousand-by-thousand" port counts? A very different future for the PXC has emerged, one that is arguably even brighter than that articulated by the industry two years ago.

In looking back, one must first ask why it was thought necessary to deploy large port count PXCs in the first place. Until recently, dense wavelength-division multiplexed transport had limited reach. As a result, the optical network was architected as a constellation of point-to-point links. These links are jointed by back-to-back DWDM terminals where the optical signals are demultiplexed into individual wavelengths and converted into parallel streams of broadband electrical signals for 3R-regeneration.

The electrical signals are fed through a number of boxes that can be collectively referred to as electronic cross-connect (EXC). Here they are aggregated and groomed to individual output channels, which are then optically remultiplexed and launched into the next DWDM link.

This process is very expensive and was thought to be not scalable enough to accommodate the explosive growth of Internet traffic. One solution is to replace the EXCs with inexpensive and more scalable PXCs. Since each fiber could contain hundreds of wavelengths, it did not take much of a jump to envision PXCs with thousands of ports.

The most exciting application is the deployment of PXCs in regional central offices to manage the adding and dropping of wavelength-level local traffic either between metro rings or between the metro core and the long haul-backbone network. Unlike two years ago, the purpose of the PXC is not to replace the EXC, but to coexist with it, complementing and enhancing its functionalities.

This so-called hybrid EXC/PXC architecture comes in at least two flavors, neither of which is exclusive to a particular customer. While these two approaches differ in the implementation details, their common purpose is to eliminate as much unnecessary electrical terminations for optical signals as possible.

The first hybrid architecture is the most optimal but also the most ambitious: the complete integration of not only the EXC and PXC but also of DWDM transport. In this case, traffic coming into a node is classified into two categories — express and local add/drop. Express traffic is either passed through or rerouted to the next node by the PXC. The granularity of the all-optical switching functions performed by the PXC can be in fiber or wavelengths. The second type of traffic, for local add/drop, requires electrical termination and is directed onto the EXC for wavelength conversion, grooming — where the granularity is STS-1 — and local consumption.

With the advent of ultralong-reach transport technology, the distance that the optical signal can travel can extend to as much as 6,000 km, so that express traffic can be routed through multiple nodes without electrical termination and regeneration.

This architecture also recognizes that Internet traffic is inherently long distance, such that the sole purpose of up to 90 percent of the traffic that comes to a node is to pass through to another node. Carriers could realize a 50 percent cost saving in the construction and operation of an optical network if unnecessary back-to-back electrical termination/regeneration for transit/express traffic was eliminated.

The second hybrid architecture is far less ambitious and is not an integration of the EXC and PXC so much as a tight coordination of the two. In this architecture, the EXC and the PXC coexist synergistically, but perform different functions. The EXC is part of an existing Sonet metro ring managing voice-dominated traffic that requires STS-1 grooming. The PXC is part of a separate, overlay metro Internet Protocol network managing wavelength-level data-centric traffic.

The PXC can be thought of simply as a transponderless, network-aware automatic patch panel. Although its function can be performed by the EXC, the PXC is designed to deliver wavelength-level services at less than one-tenth the cost. The cost comparison is not between the switch fabrics of PXC and EXC, but between PXC and OC-192 transceivers/transponders, which are necessary to terminate traffic for the EXC.