Optical Switches in the Next-Gen Transport Network - Technology Information

Telecommunications, June, 2000 by Andrew Ware, Jonathon Lacey

The most attractive transparent node architecture is the wavelength selective cross-connect (WSXC), which can be constructed out of a single fabric or several smaller fabrics (see Figure 2). The WSXC operates by switching all the "green" wavelengths between fibers on one plane, the "blue" wavelengths on another plane, and so on A WSXC node with 25 fibers and 160 wavelengths per fiber requires 160 25x25 switches or a single 4000x4000 switch. Slightly larger switches allow dropping and adding of local traffic, allow some channels to be regenerated, and allow some channels to move between wavelength planes. If one allows "k" channels (where k represents the number of extra ports) for these purposes, either 160 [25 k] x [25 k] switches or a single (4000 k) x (4000 k) switch is required. The WSXC architecture using small building block is more reliable, more serviceable, and has a lower cost of ownership than a single, large switch fabric.

Pay as You Grow

A "pay-as-you-grow" business model using architectures of small fabrics that match capital outlays (adding new witches) with revenue streams (turning up new channels) is best at meeting shareholder expectations. Using smaller switches, new fabrics are added when new wavelengths are turned up. With large single fabrics, the entire capital cost of a node has to be paid on day one making the architecture financially unattractive. For example, compare a single $100 million investment vs. five consecutive years of $20 million investments. Using a conservative cost of capital of 10 percent, the net present value of the pay-as-you-grow approach is $83 million vs. $100 million for the large fabric purchase--a significant penalty.

The only thing that service providers dread more than not meeting investor numbers is explaining to a government body why the failure of a very large node caused a very large outage. A figure of merit for service providers is system availability, which can be derived from system downtime (derived from component failure rates), system diagnosis and isolation of a failure, and mean time to repair.

For an architecture where all traffic is switched in a single large fabric, an entire spare fabric is required and a failure disrupts all channels. But the fabric failure rate is likely to go up as the size and complexity of the single switch fabric increases. In addition, it is difficult to see the problem diagnosis and failure isolation capabilities of proposed large fabrics. Finally, managing 4000x4000 fibers for a failed, large, single fabric is likely to have a tremendous mean time to repair.

If an entire small fabric went down in the previous 160-fiber fabric example, at most 25 channels would be lost. The small fabric is easy to hot-swap with another small fabric because there are many fewer fibers. System availability would be better, because it's easy to diagnose and contain faults and easy to repair. In addition, it is possible to implement a shared protection scheme with smaller fabric building blocks. Bottom line; Trusting large nodes to a single, large fabric is a difficult proposition.