Using test to break the bandwidth bottleneck

By STUART FERGUSON, Agilent Technologies New equipment capabilities require new, more capable test equipment.

New equipment capabilities require new, more capable test equipment.

Stuart Ferguson
Agilent Technologies

For service providers, today's challenge is to make profit and survive. However, data-inefficient metro networks now form a bottleneck between revenue-earning services and the unrealized potential of the core network. Testing is a major component in the effort to break this metro bottleneck.

Originally deployed for time division multiplexed (TDM) voice traffic, the metro transport network consists of many individual devices with specific TDM-orientated functions. In parallel, a separate network deployed for Internet Protocol (IP) traffic has carried out data routing functions. Over the years, a complex layering of transport and data technologies has evolved in the metro as data traffic dramatically grew. Now, not only is it a complex environment, the cost of upgrading and managing the network is considerable.

In response manufacturers are now being driven to produce a new generation of SONET/SDH device, often referred to as transport multi-service provisioning platforms (MSPPs). The way forward is a consolidation of individual network devices that combine elements such as add/drop multiplexers, optical and digital crossconnects, Ethernet switches, and DWDM terminals. This effectively reduces the number of individual network elements, thus cutting capital expenditure (capex) and operational expenditure (opex) and simplifying network management.

Though this reduction in network complexity solves some problems, the increasing complexity of devices presents new issues. Testing the plethora of functions now emerging in the transport MSPPs presents new challenges. For example, these devices now include data interfaces such as Gigabit Ethernet (GbE) and provide the direct mapping of services into SONET/SDH via new encapsulation technologies such as ITU-T G.7041 generic framing protocol (GFP) and ITU-T X.86 link access protocol for SDH (LAPS). These encapsulation technologies map the incoming variable rate Ethernet traffic efficiently into the fixed rate SONET/SDH signal. Once encapsulated, a new virtual concatenation technique and new contiguous payload structures (e.g., STS-24c/AU-4-8c and STS-9c/AU-4-3c) are used to "right-size" SONET/SDH to make the most efficient use of the bandwidth. These next generation devices also have high port densities and complex switching fabrics, so manufacturers need multi-port test platforms that can fully stress and validate metro network equipment systems in real-world situations with realistic traffic profiles.

Each stage has its focus

Testing of these devices needs to verify operating performance, including transmission capabilities, basic error and alarm handling, and the ability to efficiently transport data within SONET/SDH. Vendors must also verify error-free mapping/de-mapping to SONET/SDH, switching integrity, correct path provisioning, and automatic protection switching capabilities. The degree to which testing is carried out will vary along the product lifecycle from R&D to system verification testing (SVT), into production and ultimately to installation and maintenance in the field.

During R&D and the early stages of product verification, testing focuses on assessing compliance to internal design criteria, and to external international requirements standards such as GFP and LAPS. Compliance to these new Ethernet encapsulation schemes ensures network equipment manufacturers (and service providers) can be confident of Ethernet over SONET/SDH interoperability with other vendors' equipment. Analyzing overhead values and interactions, alarm detection and handling functions, and error-free transmission are all essential tests.

Many of these standards are new and evolving, meaning that they have to be interpreted, understood, and tracked. Failure to correctly interpret and comply with these standards can lead to potentially costly re-work. And it's this type of situation where test equipment can be used to find defects before the service provider, or worse before the service provider's customers, discover them.

Once individual chips, modules, and line cards are designed, debugged, and integrated, the challenge for network equipment manufacturers developing these complex products shifts to getting a clear understanding of how next-generation SONET/SDH devices would perform under realistic network conditions. This is the job of the SVT department. They test the overall performance of each network element (software and hardware) created in R&D to ensure that the network elements function properly together as part of a system.

SVT requires many new test features to accurately simulate the network. Multiple ports need to be stimulated simultaneously to verify MSPP loading characteristics. Test equipment must not only load a mix of interfaces but also a mix of payload structures within any transport stream. Test times under such circumstances would be infeasibly long without simultaneous measurement capability across all interfaces and payloads. These capabilities can be used to stress the complex switch fabric within MSPPs and ensure essential automatic protection switching functions minimize the disruption to customer traffic in the event of a severe network failure.

New test equipment will bridge both SONET/SDH and Ethernet test requirements. That, too, represents a challenge: Many service providers are undecided on who will install and maintain these new SONET/SDH devices. It could be the datacom groups, who are familiar with Ethernet, or the telecom/transmission groups more familiar with SONET/SDH. While many in the industry believe that it is most likely to be the latter, the consequence is that engineers who have an understanding of SONET/SDH will now be confronted with Ethernet and have to deal with new protocols and protocol analyzers.

In brief, new test tools that are capable of offering insight into complex data encapsulation techniques, mapping structures, and service protocols, across varying interfaces from SONET/SDH to Ethernet, are the only way to guarantee confidence of capitalizing on the metro's opportunity by getting quality product to market fast. And these test tools are only now becoming available.

It's no wonder that this network and device consolidation presents a daunting test challenge. To be confident of product quality in this increasingly complex environment, comprehensive testing is required to prove functions behave as expected if next-generation devices are to ultimately alleviate the metro bottleneck.

Stuart Ferguson is a product marketing manager within the Telecomms Networks Test Division of Agilent Technologies.

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