Service driven optical networks

Telecommunications Americas, June, 2004 by Sean Buckley

Industry pundits have pointed out that the theme for service providers at this year's SUPERCOMM will shift from an "opex-savings" mode to a "give-me-services-at-the-lowest-possible-cost" mode. One of the technologies that could make a significant contribution in the metro access network is CWDM (coarse WDM), which offers a low-cost instrument a carrier could use to augment its service delivery repertoire.

While DWDM was clearly the winner in the long haul and the metro core, CWDM is increasingly becoming a cost-effective means to deliver metro access services and, in some cases, to extend the reach of a fiber network.

"It was odd that CWDM came after DWDM, but a lot of the initial demand for wavelengths happened in the backbone where the cost of fiber was expensive. This recovery that's happening in optical is being driven in the access and metro networks," said Neil Ransom, CTO, Alcatel. "As demand picks up in the metro, a different kind of technology is needed. You don't need hundreds of wavelengths; what you need is technology that can compete with an overlaid fiber that's not that long. That means the cost has to be very low and the number of wavelengths does not need to be that big."

What contributes to the low-cost of a CWDM system is its architecture. Where DWDM packs multiple tightly spaced channels between each wavelength (1.6 nanometers), which was fine for the long-haul and metro core, CWDM uses wider or coarse channel spacing--20 nanometers. Because it uses lower-cost, uncooled lasers, CWDM is ideal for the shorter distances in the metro access arena. The case for CWDM got a further boost last fall when it became an international standard under ITU-T G.695.

While demand is not skyrocketing, it is a market with growth potential. Infonetics Research has forecast that the metro CWDM market is set to jump to $243 million in 2006.

Extending the Loop

Although the fiber glut in the long-haul network is well documented, fiber in the last hundred feet in residential neighborhoods and suburban business parks is often scarce. CWDM is being used by various operators for fiber pair-gain, DSL loop upgrades, and in some cases, wireless backhaul.

Faced with a budget crunch, but a demand for service in a specific segment, the justification for new fiber or new equipment is not there. CWDM infrastructure vendors such as Metrobility and RBN (Redfern Broadband Networks), which has garnered OEM agreements with AFC and Marconi, have found a play for such applications in various cable provider and ILEC accounts. Meanwhile, emerging vendor Next G Networks is touting CWDM as a solution for wireless backhaul. As a fiber pair-gain option, CWDM can be deployed quickly to alleviate fiber exhaust.

One case in point is Telecom Ottawa. When the operator got a request for two new out-of-region customers, Telecom Ottawa turned to Meriton Networks, which enabled the operator to extend its GigE service over a 70-km CWDM span cost-effectively. While CWDM has been limited by distance, there has been pioneering work to extend its reach. By adopting 3R regeneration (reamplification, reshaping and retiming), a carrier can extend CWDM's reach while still taking advantage of the technology's economics.

For traditional operators that have existing fiber-fed DLC (digital loop carrier) cabinets, CWDM is an economic means to migrate to FTTx or to upgrade existing plant to support various flavors of DSL or business Ethernet. Carriers such as BellSouth, which has a large number of fiber-fed DLCs, is using CWDM to backhaul its DLC cabinets in certain areas. Likewise cable operators, such as Cox and even overbuilders such as RCN, have used CWDM to transport both their CLEC business traffic and video on demand.

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"In the residential case, where carriers have deployed DLC networks, for example, it's infrastructure reuse; in the business arena, it's just enough, just in time," said Richard Lauder, CTO of RBN. "If a carrier sees a profitable business in a specific segment, they are more likely to want to reuse installed fiber for upgrading a POTS network to DSL."

Business Class Access

Similar to DWDM, CWDM can be leveraged to deliver a host of enterprise services, including SAN (storage area network), transparent LAN interconnection and Ethernet services over one fiber. While the penetration for CWDM has not reached great heights, the technology is finding a sizeable niche in cases where a DWDM system would be overkill. In addition to the often-overlooked university campus segment, which is building its own fiber networks, CWDM will likely be applicable to all three market segments:

* SMBs--point-to-point connections for a single customer's Ethernet or storage applications that require no more than two to three wavelengths);

* Medium-to-large enterprises--multiservice networks using ring and hub architectures to carry multiple services and customers over the same infrastructure e.g., Ethernet, voice and data;

* Large enterprises--hybrid CWDM/DWDM configurations, where a carrier deploys CWDM with a migration path to DWDM as service demands increase.