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