Tip of the Week: Mesh networking radically improves optical-network efficiency--when tools are easy to use

However, while modern optical networks are far more usable and cost-effective than their predecessors, today's DWDM systems still rely on an inherently inefficient ring-based architecture, leading to higher costs and a limited ability to scale networks. The emergence of optical mesh networking is poised to change this dynamic, but it will not truly take off until mesh-provisioning tools are just as usable as those employed in conventional networks.

Limitations of optical rings
For a service provider wishing to connect many sites across a region, ring-based architectures--the hallmark of optical networking for years--force a choice between two basic options: The provider can use fewer large rings to connect many sites, or it can use many smaller rings, with each ring connecting a few sites. Both options are problematic.

In the large-ring approach, each circuit from each node must traverse the entire ring, rapidly consuming the available capacity of the network (and ultimately requiring a turn-up of new fiber to meet growing demand, a costly, time-consuming proposition). However, while connecting sites with a greater number of smaller rings conserves fiber, the overall costs are even higher, since every connection between rings requires converting signals from optical to electrical and back to optical. Such "O-E-O" conversions require huge amounts of additional equipment, as well as manual reconfigurations at each site whenever a circuit is provisioned or changed.

What if service providers could connect all sites in a region with an IP WAN-like mesh, in which any site could communicate directly with any other site without having to pass signals through (and consume fiber at) every node on the ring? This is exactly the capability that new mesh ROADM tools provide.

The mesh approach
Instead of forcing traffic to enter and exit each site via "east" or "west" traffic paths along a ring, a mesh network can link nodes via multiple paths and pass signals in multiple directions via the shortest possible path. In a mesh, circuit paths are created at the sub fiber level, allowing service providers to establish multiple "virtual" rings across a single fiber on a span-by-span basis. With this capability, service providers can take the many-small-rings approach to connecting a region and use their fiber much more efficiently. At the same time, since virtual rings can pass wavelengths from ring to ring at the optical level, they eliminate the need for cost-prohibitive O-E-O conversions.

Mesh networks also allow service providers to more efficiently scale their networks to respond to growing demand. This is because, while multidegree nodes are typically described in terms of direction (with the node ostensibly connecting fibers from the north, south, east, west, etc.), these degrees don't have to be configured directionally. When service providers have a bottleneck on a particular span between two nodes, using an additional degree--effectively adding another fiber between the nodes--can immediately increase capacity. In fact, many service providers considering mesh technology are not designing their meshes based on geography but on expectations of where bottlenecks are likely to occur. With the ability to add degrees to a node over time, service providers can ensure enough fiber path diversity and eliminate the possibility of a single point of failure affecting other parts of the mesh.

Simplicity Is key
As a result of these advantages, more and more service providers are viewing mesh capabilities as a requirement for any new DWDM network. However, as service providers move to adopt mesh solutions, they should recognize that, in order to truly benefit from mesh capabilities, these solutions must be simple to deploy and operate.

The ideal mesh solution should provide all of the savings and design benefits of mesh without taking a step back from the operational and simplicity advantages of today's two-degree ROADM solutions. A truly viable solution must extend to multidegree meshes all the alarming, protection, and automated provisioning capabilities that service providers use in ring networks. Without this level of simplicity, service providers may deploy mesh-capable technologies, but the cost and complexity of operating them will undermine the potential benefits.

Ron Johnson, Product Manager, Optical Transport Business Unit, Cisco ronjohn@cisco.com.

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