SDN architectures

Carrier SDN vs. Transport SDN

When software defined networking (SDN) started moving from the datacenter into the service provider WAN, I spent some time sorting out what to call this trend.

Is it Transport SDN? Is it Carrier SDN? For a time, I used both terms interchangeably, but after many discussions with colleagues at Heavy Reading and in the industry, I have realized that Transport SDN and Carrier SDN are two different things.

Carrier SDN as a term better fits the description of SDN applied to the service provider WAN. Transport SDN is more specific and applies to the application of SDN to the transport layers of the service provider WAN. Transport layers are layers 0 (DWDM, photonics) and layer 1 (Sonet/SDH and OTN).

The distinction between Carrier SDN and Transport SDN is not simply an academic one. The higher layers of the carrier network (Ethernet, IP, and up the OSI stack) will adopt SDN very differently than the transport layers. We saw this happening from the very beginning. Companies like Google and the Carrier Ethernet and cloud sides of network operators were talking about datacenter interconnection, OpenFlow, and the benefits of SDN in generating new services revenue. But the transport sides of network operators were talking about GMPLS, Path Computation Element (PCE), and the benefits of lower capex and opex.

These transport-minded folks have taken a beating over the past two years, as some OpenFlow proponents derided them for being dinosaurs trying to resist the inevitability of progress.

But the reality is not that simple. Verizon's Optical Transport Networks Director Glenn Wellbrock summarized the distinction perfectly at Light Reading's Packet-Optical Transport Evolution conference in May in his keynote on the topic. "Transport is not generic, and we've got a ton of issues in an analog domain. We're messing with physics," he stated.

The addition of the analog to the digital domain in the transport layers is, according to Wellbrock, the primary reason why transport SDN must evolve differently than higher layer Carrier SDN. For better or worse, the ways of dealing with analog attributes in optical networks are very vendor specific, and it is not possible for a generic type of controller to understand all of these differences. Nor is it possible for network operators to remove all of their transport gear from their networks and replace it with an uber-standardized type of generic optical hardware.

This reality -- and not simply a knee-jerk resistance to change -- is what is driving transport SDN evolution down a different path than the higher layer SDN that functions entirely in the realm of 1s and 0s.

-- Sterling Perrin, Senior Analyst, Heavy Reading

Dredgie 8/23/2013 | 12:03:06 PM
Re: Multilayer 'Carrier' SDN Uh-oh. What does that mean for 'Carrier Ethernet', then? 'Ethernet for Carriers'? :-)
sterlingperrin 8/23/2013 | 11:31:36 AM
Re: Multilayer 'Carrier' SDN Dredgie, good question, and i appreciate your interest in my post. As you point out that multi-layer approach would cross both realms. Whether you can apply a single name to that, I don't know. But I do feel the distinctions between how L0-1 and L2-3 would be handled still applies, if it is going to be open.

There is alot of interest among incumbent suppliers in do-everything approaches that are proprietary in the end. To me, this is the opposite of what the SDN trend is all about and I don't think it will be widely adopted in the end - regardless of what these architectures look like on paper.

Back to nomenclature, I have also been told that I should use the term "SDN for carriers" because they use SDN both in the DC and in the WAN...


Dredgie 8/21/2013 | 12:06:25 PM
Multilayer 'Carrier' SDN As someone who has to continually make the distinction between Data Center and 'Carrier' SDN, while also having to explain why it is so critical to the evolution of the network operator WAN,  I'm glad someone is thinking about this!

One question I will ask – what nomenclature would you then give to a multilayer 'Carrier' SDN function that (logically or otherwise) handles L0 thru L2 (.5!)? Both Carrier and Transport, by your definition.  It looks like companies like Ciena are stuck on 'Carrier', in those cases. But maybe I answered my own question in my... errr... question.
cugini 8/21/2013 | 5:38:10 AM
Transport SDN Right, the PCE technology is clearly evolving towards what can be called SDN (see the recently introduced stateful and active functionalities).

Let's see what happens to GMPLS..
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