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SDN optical transport networks: Programmability, but first, challenges

SDN can be used to optimize optical transport networks, but first, engineers must learn to apply it to the diverse architectures of operator networks.

SDN technology has primarily been applied to clouds and large data centers. More recently, interest has turned to extending programmability to the optical transport network.

SDN's promised advantages -- optimizing network resource usage, simplifying and accelerating service creation and enabling applications to specify resource requirements -- can also apply to optical transport networks. But first, SDN must be pushed beyond the ability to handle basic Ethernet infrastructure, and into the more diverse architectures of optical networks.

Transport networks were originally built to support long-distance voice traffic with high standards of reliability, but they were not meant to deal with highly dynamic capacity demands. Traffic was predictable with spikes at certain times of the day or the year.

But in the age of the cloud, providers distribute compute and storage resources around the world to provide 24/7 uptime in case some event causes one facility to shut down. Compute load is shifted from one cloud facility to remote facilities as demands outstrip local resources. Storage devices are backed up to remote facilities to protect against catastrophic loss. This wide distribution of resources has changed the demands placed on transport networks, making SDN an attractive alternative to current management technology.

Optical transport network SDN presents challenges

Clouds and data centers handle a single type of traffic carried over a single network technology: IP packets and Ethernet. Hardware components are limited to switches and routers, with both functions often combined in the same unit. Units from various vendors differ in how they are configured and managed, but all work with IP packets and implement well-understood protocols.

Transport networks are more complex. They vary in terms of basic network architecture and the types of cross-connects that act as switches between links:

Synchronous Optical Network (SONET) ring-based networks offer extreme reliability due to their ability to rapidly switch traffic from the working ring to the protection ring in the event of a failure. The downside is that only half of the total capacity is normally in use. Earlier optical mesh networks did not offer the same level of reliability as rings, but recent developments have enabled mesh networks to equal the reliability of rings. Mesh networks are more efficient in their use of network resources because less than 50% of capacity is held in reserve.

Optical cross-connects (OXCs) terminate links and switch data streams toward their destination. OXCs de-multiplex incoming optical signals and convert optical data streams to electronic format. They then use electronic switches to determine the outgoing link. Signals are then converted back to optical and placed on the appropriate output link.

Add-Drop Multiplexors (ADMs) add or drop a data stream from a multi-wavelength data stream and redirect it toward its destination. Reconfigurable Optical ADMs (ROADMs) are more flexible than earlier ADMs. Data streams can be added, removed or re-directed without interfering with other traffic, and re-configuration can be done remotely.

The diversity of network architectures and equipment types means that an SDN controller must recognize the capabilities and restrictions of each. A single source to destination path may cross multiple links. A controller must optimize resource allocation based on the capabilities of the installed network at each point along the path.

Optical transport network management tools are the hardest challenge

Implementing optical transport network SDN will require updates to both network equipment and management software. Network components designed for a more static environment will need to be replaced with units that enable rapid re-allocation of resources. But any replacement of the complex management technology in operator networks carries the possibility of disrupting revenue flow from the infrastructure.

Operations Support Systems and Business Support Systems (OSS/BSS) software will also require extensive modification. These systems integrate network management functions with the business software crucial to service provider operations. Device management interfaces must be added to provide an SDN controller with access to devices.

More on SDN and optical networks

SDN in optical networks means control and scalability

How SDN could bring automated provisioning to optical networks

The rise of multi-layer SDN

Changes in configuration and addition of services must be reflected through interfaces to the BSS to generate accurate customer charges. OSS/BSS systems have evolved over many years, and modification must be conducted with extreme caution to avoid disrupting ongoing operations.

Despite the complications, SDN will eventually create opportunities for service providers and their customers. Both benefit from the ability to quickly allocate capacity and services in response to demand. Customers' IT operations will benefit from increased available capacity and services. Service providers will benefit from reduced expense as management and configuration tasks are increasingly automated, as well as by the ability to charge customers from the newly available capacity and services.

Network equipment vendors have also recognized the benefits of SDN. They have begun to develop products with the added flexibility and management interfaces needed for SDN. A group of vendors are working with the Open Network Foundation to develop an Open Transport Switch (OTS) that will hide the complexities of the underlying transport network to provide what appears to be a seamless set of network resources.

About the author:
David B. Jacobs of
The Jacobs Group has more than twenty years of networking industry experience. He has managed leading-edge software development projects and consulted to Fortune 500 companies as well as software startups.

This was last published in March 2014

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