PDF of this tutorialGiven that the reliability of any system is a function of the number of components in the system, and given that photonic networks are characterized by very high component counts, the potential for problems in such networks is very real. If any component does not function properly, the entire system could be affected. It is imperative, therefore, that the performance characteristics of network components be thoroughly assessed during all stages of development and deployment.
Two general types of testing are required to manage the complexity of photonic networks:
- Performance assessment—which looks at components and systems running at their maximum design limits in order to fully characterize performance margins and raw bit-error rates
- Conformance testing—which is more of a “go/no-go” approach where the main concern is whether behavior conforms to the relevant ITU–T, ANSI, ETSI, or other local standard
These tests are made during all phases of product and system development: design, integration, manufacture, installation and maintenance, and system tests.
Design
In design, network modules and submodules undergo both performance tests and conformance tests. In performance tests, design engineers examine the electrical and optical characteristics of the device under many different conditions in order to evaluate the total performance and performance margins of the component or element being designed. In conformance testing, a manufacturing test engineer operates in more of a “go/no-go” mode to determine whether the module or element operates within the standard specifications.
Integration
The same test instruments are used in integration as in design, only here the focus changes. The manufacturer is trying to integrate multiple modules into a particular network element. The modules must all operate within very narrow tolerances in power, signal-to-noise ratio, timing, etc.
Manufacturing
Once the component, module, or element has been designed and is ready for manufacturing, the manufacturer wants to know whether the individual module or component is good before shipping it to the customer. The modules are commonly tested only for conformance with relevant industry standards, rather than both performance and conformance. In many cases, after the individual modules are tested and assembled into a network element, the completed network element is tested again for conformance with the same industry standards.
Installation and Maintenance
At this stage, the network operator wants to be assured that the system can be installed and turned up quickly so revenues can be generated as soon as possible. The operator also wants to be assured that the system will run reliably and with little maintenance. The focus of the network operators is different from that of the equipment manufacturers. They want to be certain that the network provides the necessary, competitive level of QoS, but they do not want to bear the expense of testing for QoS in network elements. They want assurances that quality has been designed into every network element with room to raise QoS when customers demand it. Also, operators want a network that can be debugged quickly when being turned up, and restored to operation quickly when there are problems.
System Test
Both equipment manufacturers and network operators perform system tests to verify overall system performance. Typically these tests involve the characterization of overall system bit-error rates—the bottom-line assessment of system performance.
