Service providers need a dynamic, application-aware network infrastructure that suits today’s cloud and mobility needs. Software-defined networking (SDN) and network functions virtualization (NFV) have great potential to improve the flexibility and responsiveness of networks, decrease both operating and capital expenses, and add new and innovative features to many different types of communications networks.
When applied to packet networks, the benefits are clear. Networks can be flexibly redefined and equipment repurposed quickly and easily to meet changing network needs. However, constructing a dynamic and flexible packet network on top of rigid optical transport does not yield an optimal solution, and it can impede the range of services that may be introduced.
SDN concepts were first applied to the packet portion of the network, since optical transport couldn’t be programmed. However, new advances in optical technology can allow network operators to program the optical layer. A few examples are:
- Software-defined optical port speeds, protocols, and wavelengths
- Advanced modulation and detection schemes, especially at 100 Gb/s and higher speeds
- A flexible wavelength grid rather than a fixed 50 GHz grid
- Flexible wavelength routing via dynamic ROADMs
- OTN grooming and switching
Employing these programming capabilities, the optical transport layer can be abstracted to a set of shared, common resources that can be used dynamically and on-demand.
The main reason to adopt SDN in the optical layer is to reach a unified, multilayer control. In such an environment, packet and optical networks are not managed separately. A global, central control is aware of the entire complex of packet and optical resources and is able to use them efficiently for flexible service creation. Having a unified control results in better resource utilization, simpler end-to-end orchestration, and rapid and dynamic service turn-up.
The OIF, in its document “OIF Carrier WG Requirements on Transport Networks in SDN Architectures ,” offers two models for SDN-controlled transport networks. In the first model, the SDN controller directly controls each node in the network, including the optical nodes. In this model:
- Each network element needs to support a transport programming interface, such as OpenFlow or Netconf
- The SDN controller communicates with each network element to provision an end-to-end service
- The SDN controller must be aware of the detailed topology and optical switch technology
In the other model, the controller controls a multidomain network and offers an integrated network view. The multidomain network is abstracted and appears as a single, flat network for applications. Each domain exercises internally a choice of control – dynamic routing, GMPLS, or SDN.
The second model represents a more evolutionary road toward optical SDN. The ONF work on OpenFlow extensions for transport networks is still underway. In addition, service providers tend to implement SDN in a staged approach to avoid network instability. As a result, SDN is deployed alongside existing network control protocols. This model is also in line with the MEF Lifecycle Service Orchestration (LSO) architecture, where hierarchical layers of element management, infrastructure management, and service management contribute to the end-to-end inter-operator LSO.
We can identify a few building blocks that are necessary for multilayer control:
- Automatic discovery of the network topology, providing updates when the topology changes
- PCE (Path Computation Engine) – computation of paths based on routing and optical domain constraints
- Allocating resources and provisioning circuits based on the computed path
- Automatically restoring service on a network failure by providing an alternate path
An important element of the multilayer SDN control is the domain gateway controller. This controller is able to mediate between a legacy optical domain and a higher, multidomain SDN controller, translating the controller’s commands to the domain protocol and relaying the detailed optical information to the controller via a standard northbound interface. The domain controller gateway holds both standard SDN northbound and southbound interfaces, such as OpenFlow, Netconf, and REST, as well as legacy interfaces such as SNMP. It also functions as a gateway for a domain controlled by distributed protocols such as GMPLS.
As SDN moves from hype to field trials and early adoption, standards bodies like the ONF , ITU, and OIF continue the work to define a common software interface to optical transport SDN. But meanwhile, vendors and service providers can begin extending SDN principles to the optical arena to achieve a unified, optimized packet and optical network. These efforts need to be done with care to make sure that they do not result in unstable networks long-term and that they are compatible with emerging standards.