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What is "packet optical" technology and what can it do for TDM networks?

Jason Marcheck

Editor's note: Packet optical networking solutions are on the telecommunication industry's radar as the solution to fast-growing packet traffic that travels over TDM-based SONET/SDH optical networks. Yet a definition of what packet optical really is hasn't emerged yet. What network elements will work best when deployed in public networks will become clear only over time. Already, two opposing camps have emerged. Current Analysis Principal Analyst Jason Marcheck is watching packet optical developments closely in terms of their impact on service providers.

With all due respect to 40G/100G transport, "packet optical" has come to represent the hottest trend in optical networking. But as just about every optical vendor on the planet races to address the trend, an absolute definition of what makes up a packet optical solution has yet to emerge.

To be fair, it's not for lack of effort on the part of industry analysts and equipment vendors. On one hand, various vendors, operators and analysts define "packet optical" as a standalone network element that combines reconfigurable wavelength division multiplexing (WDM) transport, integrated SONET/SDH support, connection-oriented Layer 2 switching and aggregation, and carrier-class OAM capabilities.

Nevertheless, even as more standalone network element solutions are announced, various players, including several very large optical equipment vendors, have so far refused to buy into

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the notion that a standalone network element solution is the approach that will ultimately win the day. As a result, two camps have emerged: those that favor the single network element strategy and those contending that a collection of "optimized" network elements makes the most sense in the long run.

Defining packet optical

In keeping with the heightened awareness of political correctness, I believe the best definition of "packet optical" should strike a middle ground between the single and multiple network element camps. For all practical purposes, because networks differ from carrier to carrier, attempts to fit packet optical into a definition focused on standalone network elements can be an example of the old adage of failing to see the forest for the trees.

Rather, it is more valuable to define packet optical in terms of specific features that must be met in order to qualify as a packet optical solution. This checklist of capabilities should include:

  • Reconfigurable WDM transport
  • Integrated SONET/SDH support
  • Connection-oriented Layer 2 switching and aggregation
  • Carrier-class OAM capabilities

Yet while the attributes many operators would like to see in their packet optical candidates are agreed upon easily enough, the market has not weighed in on how it prefers to implement those capabilities. To this end, if an optical vendor can offer its customers these capabilities in a coherent, centrally managed solution, it matters less whether the solution consists of a single network element or a combination of more traditional network elements, such as a reconfigurable WDM platform plus a Carrier Ethernet routing platform.

Lest we put the cart before the horse, however, it might be useful to take a step back and briefly examine how the packet optical craze came about.

What does packet optical replace?

At a high level, packet optical provides the migration path to replace SONET or SDH, depending on which side of the pond you live on. SONET/SDH is considered inadequate to handle the emerging needs of tomorrow's next-generation networks. Most industry observers agree that in order to scale networks cost-effectively to support the impending onslaught of traffic that Carrier Ethernet business services and IP-based multimedia services will unleash on the networks, transport networks will at some point have to become more efficient at handling packet traffic.

Packet optical promises to enable this by giving operators the tools to move beyond the scalability constraints that the SONET/SDH hierarchy imposes on transporting packet traffic in TDM wrappers. The result, according to the script at least, is the ability to transport IP/Ethernet traffic in its native format while maintaining the robust OAM features that SONET/SDH provides.

But returning for a moment to the single vs. multiple network element debate, in terms of value proposition, the two approaches are relatively clear. The single network element approach argues that one device can allow a carrier to migrate from a purely TDM-based network to a purely packet-based network. Anything in between gives network operators the freedom to migrate from today's SONET/SDH networks to packet optical at the rate that makes the most sense at any point in their metro/regional networks.

Along those lines, the "all-in-one" approach would theoretically lead to a collapsed network architecture with fewer elements that could lead to simplified operations, maintenance and management - all of which reduce OpEx and make delivering services more profitable.

The opposite approach argues that optimizing at both the WDM and Layer 2 switching layers helps carriers leverage what they already have deployed in their networks, leading to cost savings via investment protection. In addition, this approach would provide advantages in terms of scalability and product maturity that single network element solutions currently lack.

Why packet migration now?

As most of us know, rumors of SONET/SDH's death have been greatly exaggerated. The tried and true TDM-based optical transport technology has been battling rumors of a gradual 15-year decline for most of this decade - with the latest estimates holding that SONET/SDH will be with us for a long time to come. Anyone care to guess how long? Fifteen years or so.

So, given SONET/SDH's demonstrated staying power, the question becomes: Can we believe that packet optical is ready to step in and provide the solution to the impending network woes that the past dozen or so years of monolithic SONET/SDH hierarchy have imposed on network operators? Or will packet optical disappoint? Is packet optical too unproven, too loosely defined to provide a clear vision for network operators to get behind and implement on a large scale?

At the end of the day, despite incongruous approaches, it is clear that TDM-to-packet migration is a trend that network operators must plan for because it provides needed efficiency, and that optical vendors must address because operators will want it. As rollouts occur, competing vendor offers will no doubt be refined and, in some cases, redefined.

The guess here is that no one approach to TDM-to-packet migration will satisfy all of the operators all of the time. To that end, the onus will be on equipment vendors to be nimble enough - or have a broad enough portfolio - to meet a range of TDM-to-packet migration timelines and/or network design strategies.

The task for now is to keep a close eye on early Carrier Ethernet network rollouts, like BT's 21CN (21st Century Network), for clues as to whether or not network operators find it ultimately more economical to work within the framework of what most already have deployed (i.e., optical transport platforms and routers) or to work toward a more streamlined network that relies on equipment that can house both Layer 2 and WDM transport capabilities in a single network element.

This article first appeared on searchtelecom.com