New crossconnect system heralds all-optical networks

July 1, 1997

New crossconnect system heralds all-optical networks


Lucent Technologies, Warren, NJ, put a new all-optical crossconnect system through its paces at last month`s supercomm `97 show in New Orleans, LA. According to analysts, the advent of the system and the eventual introduction of competitors should boost the application of optical networking as soon as communications carriers determine out how best to work at the optical layer.

The Lucent system derives in part from the company`s work within the Multiwavelength Optical Networking (monet) Consortium (see Lightwave, April 1997, page 1). In fact, with the exception of enhancements aimed at the commercial market, the switches shown in New Orleans were identical to those in use by the monet project, according to Lucent`s Bill Gartner, director of product development, and Neil Jackman, member of the technical staff in the Optical Cross Connect Development Group, who spoke to Lightwave shortly before the show. Each of the two systems shown at supercomm supported four fibers in and out, with eight wavelengths traveling on each fiber, for a total of 32 wavelengths entering and leaving each system. At a transmission rate of 2.5 Gbits/sec, that represents more than a million circuits through each crossconnect system, say the Lucent representatives.

Lucent housed each crossconnect in a bay 7 feet high, 1 foot deep, and 26 inches wide, which held the switch fabric, input/output packs, monitoring system, and display. Monitoring represents an important characteristic of the optical crossconnect. The systems in New Orleans featured 192 signal-monitoring points.

The ability to monitor the network--and to restore it when outages occur--is an area of concern for service providers. Lucent will focus on this when it markets the new crossconnect (see Fig. 1 on page 1). "We believe that restoration is probably one of the early drivers in terms of how this product will be deployed. So it`s a key area for investigation with our customers," says Gartner. "A lot of our customers today are performing restoration first of all at the Sonet [Synchronous Optical Network] layer, where things typically get restored in 50 msec. And also at the DS-3 [44.736-Mbit/sec] layer using crossconnects. Typically, that is a very slow restoration process, ranging from seconds to hours, depending on the customer, where individual DS-3s are restored around the network."

Unanswered questions

The all-optical system`s ability to restore service in 10 msec using the optical layer compares favorably with both Sonet-based and electrical restoration approaches. While such speed makes a strong case for optical crossconnects in theory, the practice of applying this technology remains less than clear-cut. In fact, Lucent demonstrated a variety of restoration options at the show, ranging from an approach where the new crossconnect participated completely at the optical layer--detecting failures and automatically switching wavelengths to a spare facility--to a scenario in which it played a passive role while faults were dealt with at the Sonet layer (see Fig. 2).

"One of the interesting questions about optical networks and the optical layer is what role they would play in restoration within the customer`s network," admits Gartner. "And I don`t think we have the answer to that question yet."

The problem, say observers, is that communications providers have little experience with working at the optical layer and therefore are unsure of how best to exploit it. "I think there`s some amount of time that has to be spent with this, going into the field, and people designing networks around it," explains Scott Clavenna, a senior analyst at Cambridge, MA-based Pioneer Consulting, who received a pre-show briefing about the new optical crossconnect. "With dense wavelength-division multiplexing [dwdm] systems, it was pretty obvious; you just increase the capacity on a long-distance point-to-point trunk, and eventually you add optical add/drop multiplexers. And that had an existing model in Sonet to follow, and it made sense how to use it. But with this kind of crossconnect, it gets a little different because it asks the carriers to rethink what their networks look like, how they want them structured in the long term, and what an optical layer actually is."

Yet, while carriers grapple with "where to put these crossconnects, where they are most efficient, and how many end-users are required to support them economically," according to Clavenna, these systems definitely will play a role in future networks. "Carriers are really using this to differentiate themselves," he says. "A lot of what they`re doing is saying to business customers or whoever their customers are: `This is a much more reliable system; I can guarantee you restoration in milliseconds, or I can give you dedicated fiber-optic services or dedicated wavelengths versus what we were offering you previously or what our competitors are offering you.`

"So I would say, then, that a lot of the effort is going to be focused on not just technology --just say, okay, we`ve got an optical crossconnect, how do we use this now?--but how do we use it so that it gives us a competitive advantage over the other guy?"

These technological and competitive discussions will take place within a general trend toward more reliance on optics within communications networks in general.

"As customers deploy more and more dwdm systems, it will be very natural for them to want to restore traffic at the level of the failure," offers Gartner. "So if a wavelength fails or a fiber carrying multiple wavelengths fails, it will be natural to restore traffic at that layer--at the wavelength layer or at the optical layer, as opposed to trying to do that at the electrical layer, down at the DS-3 or STS-1 [52-Mbit/sec] layer."

"This trend toward optical networking is becoming inevitable, I think, especially for long-distance companies and applications in metropolitan area networks where you have a number of users in a downtown corridor, all interconnected over an optical network," explains Clavenna. "Wavelength-division multiplexing [wdm] and then optical crossconnecting and switching is just such a more advanced, long-term solution than to just keep trying to ratchet up Sonet systems or building redundant fiber systems."

The introduction of equipment like the all-optical crossconnect marks an important milestone in an optical-networking trend whose core currently resides in backbone applications, according to Clavenna. "This signals the core sort of spreading beyond just the very high-speed backbone of the long-distance network, more into the local and the metropolitan area." Clavenna says that he previously had anticipated that optical crossconnects would not debut in service until the beginning of the next century. However, he now feels that initial applications could occur as early as the end of next year or the beginning of 1999.

For their part, the Lucent representatives said that the system should be ready for limited initial production by year-end, with significant deployment anticipated by year-end 1998.

Even as fiber optics extends ever further from the backbone, however, Gartner says that conventional crossconnect systems will remain necessary for the foreseeable future.

"We don`t see much cross-elasticity between an optical crossconnect and, for example, a broadband crossconnect. You`ll still need to ultimately manage the traffic at an STS-1 or an electrical level," he says. "As an example, if you`re moving a wavelength through an office, today you might bring that wavelength down to the electrical level, do DS-3 level grooming through a crossconnect, and then bring it back up to the optical level and move it through the office. It might be more natural to do that at the optical layer, just move that wavelength from one fiber to another fiber, and never go through an electrical crossconnect. But the signals that are terminating at that office, you`ll still want to have that electrical crossconnect there." q

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