Now you see it, now you don't

As the communications industry edges closer to realizing the vision of Sonet-a state of full multivendor interoperability in which service providers are free to mix and match products throughout their networks-service providers need to understand how prospective Sonet vendors are actually achieving interoperability.

Service providers are faced with a data networking problem significantly larger than the issue of connecting equipment, and it requires that operational protocols throughout the entire network be carefully examined. If a vendor's plan doesn't adhere to the specifications now in place based on the work of the Sonet Interoperability Forum, Bellcore and the American National Standards Institute, service providers could be buying into network solutions that are incapable of providing the level of interoperability expected.

Service providers need to recognize that they play a significant role in promoting interoperability. Even if a vendor claims to have built its product in compliance with standards, service providers must question how interoperability was achieved. It's also important to determine if a third party such as Bellcore has tested and certified that the choices made by a vendor claiming standards compliance do indeed support the most open standards.

One example may involve interoperability across the Open Systems Interconnection protocol stack on the data communications channel (DCC), also known as the NE-NE interface. It's imperative for full application level interoperability that all seven layers are addressed. If interoperability to only the network layer exists, full interoperability between applications is not possible.

In some cases, a service provider may be content with the level of interoperability between two specific manufacturers. If the service provider decides to change vendors, however, it may find that the level of interoperability previously experienced has vanished.

That's why it's critical to understand a vendor's approach to multivendor interoperability for network management. Is it achieved through a pair-wise agreement between itself and another vendor? Or is interoperability attained through a totally open implementation in compliance with standards?

Their Own Rules In a pair-wise vendor agreement, two vendors decide to provide interoperability, and compatibility issues are worked out one-on-one. These types of partnerships are very limited in scope, and it can be very tempting to interpret standards in favor of a particular application because it's quicker and easier.

Pair-wise agreements provide service providers with short-term cost savings and quicker implementation schedules. In the long term, however, products may cost more, take longer to deploy in the network and limit the service provider's choice of vendors. New vendors entering the fold will be forced into additional pair-wise relationships that may not be efficient or cost-effective for the service provider.

In a perfect world, every vendor would build to the standards and interpret the standards the same way, and every product would work together immediately.

In the real world, working toward a standard equates to about 80% of the process. Therefore, the most feasible approach-referred to as totally open implementation in compliance with standards-is for all vendors to build to the standard, interpreting it the best they can, and then to conduct the final testing together in an open environment. While the final testing process may resemble pair-wise testing, it should be much easier to get products from two different manufacturers to interoperate properly when both have built to standards.

Lucent Technologies, in developing its DDM-2000 OC-3 multiplexer, and Tellabs, in designing its Titan 5500 digital cross-connect system, have adopted a totally open implementation in compliance with standards. The two companies recently announced an interworking agreement based on the Sonet standards (Telephony, Aug. 26, page 8). Because both manufacturers consistently selected the most open protocols, it took only a few days to get the two products to interoperate once testing began-a process that typically takes much longer if standards have not been closely followed.

Lucent and Tellabs are hopeful that their efforts will encourage additional manufacturers to adopt totally open Sonet implementation in compliance with standards. Other cross-connect manufacturers, for example, can be confident that if they build to standards, at least one multiplexer-in this case Lucent's-will support a high level of interoperability. The same holds true for other multiplexer manufacturers, which now know that the Tellabs cross-connect system has been designed to closely follow the Sonet standards.

Avoiding the Quick Fix From the service provider's perspective, it must be able to "plug and play" products from multiple vendors with the confidence that the products will interoperate. Free to choose the best from each product category, including add/drop multiplexers and digital cross-connect systems, service providers can finally create-and operate-world-class networks in an affordable manner.

Even though a service provider might be growing at a rapid pace and might be in need of a quick solution, it should be cautious about making a snap decision just because a vendor touts interoperability. Because pair-wise agreements generally result in pockets of interoperability within a network, service providers only take a small, incremental step away from a proprietary, single-vendor network. Therefore, it's up to the service provider not to accept these solutions at face value but rather to dig deeper and ask the right questions to determine how standards are being addressed in the vendor's proposed product (see sidebar).

Service providers also are encouraged to make their feelings known with SIF, whose primary focus is now network management. SIF is also working on information models for Sonet network elements and network management platforms.

The infrastructure implemented by SIF enables differentiation among vendors in the network management-to-network element relationship. The differentials equate to a transparency of service management protocols-the revenue-generating aspects of the service provider. When vendors agree on the protocol, new revenue-generating service offerings such as survivable broadband access will be free to run without the need for a single-vendor network to ensure compatibility across the network.

It will quickly become apparent to the service provider when a vendor has closely followed standards because interoperability with other products happens almost immediately. On the other hand, when a vendor has decided to interpret standards differently to make implementation easier or less expensive, the service provider won't receive all the benefits intended with Sonet.

A totally open implementation of standards might take more work in the short term, but the benefits of this approach will far outweigh the initial investment. And it's the only way a carrier won't fall victim to the "Now you see it, now you don't" illusion.

Srinivasan Ravikumar is a Distinguished Member of Technical Staff at Lucent Technologies, North Andover, Mass. Alan Repech is a Senior Product Planner at Tellabs, Lisle, Ill.