Optical network analyzers have had to undergo major changes to keep up with new network applications. Users have made big investments in legacy synchronous optical networking (SONET) and switched digital hierarchy (SDH) networks, but demand for transporting Internet protocol (IP) traffic over these systems has led to new requirements that network analyzers simultaneously support application-side Ethernet and IPs, with attention to the unique transport needs of video and voice.
Multiprotocol analyzers can check Ethernet packet throughput and errors while simultaneously analyzing Ethernet over SONET transport.
This flexibility imposes new requirements on network analyzers. They must have the versatility to meet a variety of testing requirements, as well as cover all of the relevant wavelengths. A network analyzer with a modular design must accommodate separate optical transport network (OTN)/SONET/SDH and Ethernet plug-ins, so that both can be measured independently or simultaneously. With the emergence of Ethernet over SONET/SDH (EoS) and OTN analysis, modular design is imperative because users cannot afford to purchase multiple network analyzers.
EoS and OTN measurement requirements are different than those for traditional networks. OTN adds forward-error-correction test requirements plus two new speeds - 2.7 and 10.7 Gb/s. Although EoS allows service providers to accommodate Ethernet traffic, it adds both the advantages and complexities of virtual concatenation and its related link-capacity-adjustment scheme. EoS also adds testing requirements for its key packet-transport-protocol generic framing procedure.
Network analyzers must be capable of performing a variety of measurements on both line-side and application-side protocols, verifying end-to-end Ethernet/IP transport over SONET/SDH. The most important tests include throughput, framed and unframed bit error rate, jumbo frames, IPv6, and counters for the many kinds of errors that can occur. Meanwhile, voice- and video-IP transport impose requirements for carefully measuring end-to-end network latency and jitter, as well as quality of service.
Regardless of the type of optical network, analyzers must be able to measure jitter accurately. Defined as phase variation of the actual signal from its ideal position, jitter is an inescapable aspect of high-speed SONET/SDH and OTN and a source of unwanted errors at 10 Gb/s and above.
Three critical jitter tests exist: jitter generation, jitter tolerance, and jitter transfer. Jitter generation refers to the jitter present on the output of a single device, where it is very important that the test result not be affected by the intrinsic jitter of the measurement system. Jitter tolerance evaluates the extent to which a device operates without causing an error when the jitter amplitude is increased. Jitter transfer evaluates the extent to which the jitter applied to the device input is transferred to the output side.
Accurate jitter measurements at high speeds are difficult to make because of the extremely small times involved, but they are essential for implementing error-free networks. Jitter analyzers must also provide traceability to an industry standard to make measurements that are repeatable and accurate, thereby ensuring confidence in the measurements (see oemagazine, August 2004).
New optical network protocols and the new ways that engineers are using their legacy SONET/SDH networks to transport IP traffic are altering the means by which network analyzers should be evaluated. A modular design and the ability to measure multiple line-side and application-side protocols simultaneously, as well as the ability to perform jitter measurements with a high degree of traceable accuracy and repeatability, are the standards to which network analyzers should now be held. oe
Hiroshi Goto is an optical design engineer at Anritsu Co., Richardson, TX.