Coriant is continuing its strategy of providing two software-defined networking (SDN) controllers, saying carriers prefer the mix-and-match environment that it implies.

One controller is for the packet network and the other for the packet-optical transport network. The obvious difference is the "optical" piece, which means the second controller has to deal with the analog nature of optical signals.

The controllers were part of a Mobile World Congress demo where Coriant showed its SDN-laden mobile backhaul working with Nokia's self-organizing network (SON). The idea was to use SDN as a quality-of-service (QoS) replacement — either adding bandwith or rerouting around congestion, rather than trying to prioritize traffic.

Coriant's packet-based controller is an extension of the 8000 Intelligent Network Manager (INM), which has been shipping for a while. To make it into a "controller," Coriant added APIs to talk northbound with applications.

The other controller spans the optical layer ("Layer 0," as people like to call it) up to Layer 2 switching. This is the Transcend Transport Controller that Coriant announced in October.

This gives Coriant a controller story for Layers 0 to 3, but it comes in two separate products. Ciena and Ericsson, in contrast, joined up to develop a single controller spanning all those layers.

But Coriant defends its two-controller approach. The largest carriers prefer it that way, "especially the largest customers, who have a framework in mind," says Bill Kautz, Coriant's director of strategic solutions marketing. They're looking for mix-and-match environments where a given vendor can be booted out for a better (or, let's face it, cheaper) alternative. That means avoiding all-in-one, "end-to-end" products.

These carriers have already picked or developed their orchestration software and and just want to buy the piece-parts for the orchestrator to control, Kautz says. "It could be an OpenDaylight or open source-based architecture, where they want plug-ins or extensions from us."

With plenty of other controllers hitting the market, Coriant thinks it has an advantage in its proprietary path-calculation algorithms for multilayer SDN. Officials also cite the company's familiarity with the analog nature of optical networking, which isn't easy to master. Then again, it's a skill Coriant shares with the other optical players, including Ciena, Cyan, and Infinera.

MWC Disclosure: Craig Matsumoto was rejected for a Mobile World Congress press pass. He attended MWC 2015 on a pass supplied by Brocade and used the Ericsson stand as an ersatz press room.