We seem to be in the heart of a true avalanche of industry events, and that always creates a parallel avalanche of news. There’s always the question of whether a given news item is actually relevant, of course, and I thought it might be helpful to look at optical networking before the action gets underway. Maybe we can identify some of the points that should be addressed. After all, this is an important space.
The thing to remember about optical is that it’s all about cost. If you go back to my OSI model blog, you recall that each layer builds on the services of the layer below, and only those services. What’s deeper in the mix is invisible—abstracted away. Since few services are directly and exclusively “optical” services (dark fiber or “dark lambdas” would be about it), optical provides underlying carriage to the service layers. It has to do that at the lowest possible cost, overall.
There are three broad mechanisms that could allow optical technology to reduce cost. The most obvious is by reducing the cost of transport itself. If you cut the price of a hundred gigs of transport in half you improve service margins, reduce customer prices, or both. But here we have to remember that the cost of fiber optics isn’t just the cost of the interfaces; you have to consider the cost of the fiber itself and most importantly the cost of laying the glass in the first place. Some form of wavelength-division multiplexing has been important for the last decade or so because it allows operators to leverage a single fiber bundle by using multiple wavelengths to support multiple parallel optical paths.
We hear about the “lower-transport-cost” angle mostly in discussions about dense wavelength-division multiplexing (DWDM) because that gives us the most leverage. However, it’s obvious that 1) putting more fiber strands in the ground will also increase capacity along a path, 2) there are only so many bits you need to transport between points “A” and “B”, and 3) the total cost of the infrastructure needed to support the service determines its cost base, so as fiber cost falls it becomes less a component of total cost and it’s less useful to make it cheaper.
That brings us to the second mechanism, which is the good old “flattening layers” approach. If you think about an OSI-modeled network, you’d see a set of overlay network technologies/protocols, often supported by layers of devices. Obviously if you could create a service with a single layer the elimination of the rest would reduce the costs. The notion of flattening layers through SDN is one of the popular mantras of the optical crowd these days, and there is some sense to it.
But only some. Truth be told, we’ve been flattening layers ever since we put optical interfaces on routers or switches. Electrical devices today typically support optical connections directly, which means lower-layer optical devices are required only if we have to support parallel (and presumably different) higher-layer stacks on a common fiber path. Optical multiplexing is also needed to take full advantage of DWDM, since typically a single router couldn’t fill all the optical pipes. However, since most fiber is deployed in the metro and since the metro is an aggregation network that concentrates paths toward its core, most of the paths are probably rather thinly traveled and this approach isn’t universally helpful.
Where that leaves us is operations cost management, and this is another place where SDN is presumed to have a mission (even NFV). If we could visualize a network with agile optics that could be reconfigured easily, we can visualize a network that can restore services, redirect capacity, and do all manner of other useful things. Agile optics has always been presumed useful because it could replace at least some of the electrical steering functions of higher layers, and if you could operationalize the agility that means you harness at least parts of two of my cost-reducing mechanisms.
The challenge here is defining exactly how this would work. OSI-modeled protocols don’t expect to see path reconfigurations underneath them, and when physical paths change in most IP networks you’ll still have to have convergence at the IP level, so you have service disruptions. What would be helpful would be a redefinition of the OSI relationship between Level 3 (IP) and the lower-layer protocols that would accommodate more intelligence at those lower layers.
An alternative approach, the one I think is frankly the right one, is to assume that “SDN” creates a completely new set of services with their own rules of topology management and addressing, and that these new services are built using a single layer that rides above optics. The optical layer is then designed to deliver “path services” that support this higher layer. Some of the higher-layer service protocols can look like IP or whatever you like insofar as their user interfaces are concerned, but you redefine the internal control protocols. This is more in keeping with the notion of SDN, which seeks to eliminate adaptive routing. My suggestion? Make that an option, but also create new options for new services that weren’t designed based on the antiquated requirements of networking in the ‘60s and ‘70s.
I’ve always believed that we needed a two-layer model for SDN—an agile software-and-application-centric layer that focuses on service connectivity and a lower layer that’s focusing on reliable transport and suitable service quality. What links the two of them is policy, which gets me back to the notion of NFV. It may be that what we need for optics is more a part of NFV than a part of SDN, because in my vision we have “service networks” that absorb user requirements and aggregate them to drive “transport networks” through binding policies.
Every industry show is about revolutions because revolutions sell tickets. We can expect a bunch of purported revolutions from OFC too, but measure what everyone’s saying against my three points on value creations, and compare them to my model of layered SDN, before you buy into it. It could be that the story is just an attractive billboard.