White Paper : High - Speed Jitter Testing of XFP

Jitter is a key performance factor in high-speed digital transmission systems, such as synchronous optical networks/synchronous digital hierarchy (SONET/SDH), optical transport networks (OTN), and 10 Gigabit Ethernet (GigE). This paper outlines the differences between telecom and datacom jitter standards and describes the various jitter applications for compliance testing of 10G small form-factor pluggable (XFP) transceivers, which have become the dominant 10 Gb/s optical interface for telecom and datacom applications. Introduction Accurate jitter measurements are essential for ensuring error-free high-speed data transmission lines. Jitter which is any phase modulation above 10 Hz in a digital signal, is unwanted and always present within devices, systems, and networks. To ensure interoperability between devices and to minimize signal degradation due to jitter accumulation, limits must be set for the maximum level of jitter for an output interface as well as the maximum level tolerated at an input. Standards bodies determined these limits which can be divided into two categories: telecommunications and data communications. The major telecom standards organizations are International Telecommunications Union (ITU-T) and Telcordia, while the Institute of Electrical and Electronic Engineers (IEEE) is the main datacom standardization organization. Jitter Aspects and Characteristic Values for 10 Gb/s Telecom and datacom technologies use different timing methods. The system components in synchronous systems, such as SDH/SONET, synchronized to a common clock. In asynchronous and serial systems, such as 10 GigE, distributed clocks or clock signals recovered from the data provide the component timing. While it is important to limit jitter generated by components jitter transferred from one component to another is less important than that for synchronous systems, where jitter can increase as it transfers from component to component. Well-defined band-limited jitter generation, tolerance, and transfer requirements exist for SDH/SONET/OTN. White Paper: High-Speed Jitter Testing of XFP Transceivers 2 Table 1 shows how the specifications and test methodologies for jitter in 10 GigE differ from those for SDH/SONET/OTN transceivers. Both the specifications and test methodologies attempt to verify that the relative time instability of transmitted signals is not excessive. Telecom Datacom Technology 10 Gb/s type Bit Rate in Gb/s Transmission System Metro, long distance Point-to-point SDH STM-64 9.95328 Technology SDH/SONET/OTN 10 GigE LAN and WAN SONET OC-192 9.95328 Standards ITU-T G.783, G.825, G.8251 ITU-T O.172, O.173 Telcordia GR-253 IEEE 802.3ae OTN OTU2 10.709225316 Jitter Applications Jitter generation Jitter tolerance Jitter transfer BERT scan Stressed eye Not defined 10 GigE LAN 10GBaseR 10.3125 Jitter Tolerance and Transfer Sinusoidal Impairments (RJ, DJ) 10 GigE WAN 10GBaseW 9.95328 Table 1: Views of jitter and characteristic values for 10 Gb/s In SDH/SONET/OTN systems with regenerators, noise causes the greatest impairment and limiting factor for system performance. Jitter tolerance is measured using sinusoidal jitter. In Ethernet systems, jitter tolerance is measured using a stressed signal with combinations of impairments. Table 1 shows characteristic values for XFP transceivers, which support the established telecom standards STM-64 /OC-192 at 9.95 Gb/s, and OTU2 at 10.7 Gb/s. The 10 GigE datacom standards are supported at 9.95 and 10.31 Gb/s, respectively. These transceivers are pluggable optics, replacing legacy optical circuits with a lot of advantages: cost savings, very compact and White Paper: High-Speed Jitter Testing of XFP Transceivers 3 flexible design, exchangeability and direct replacement with equipment from different vendors, and hot-plug capable. SDH/SONET/OTN Jitter Measurements Three relevant test configurations for jitter performance measurements are: jitter generation, jitter tolerance, and jitter transfer. 1. Jitter generation: A certain amount of jitter will appear at the output port of any network element (NE), even with an entirely jitter-free digital or clock signal applied to the input, effect known as jitter generation. The NE itself produces this intrinsic jitter, for example due to thermal noise and drift in clock oscillators and clock data recovery circuits. Output jitter is the total jitter measured at the output of a system, specified in unit intervals (UI). One UI corresponds to an amplitude of one clock period, independent of bit rate and signal coding, displays results as a peak-to-peak value or root mean square (RMS) value over a defined frequency range. Peak-to-peak results provide a better measure of the effect on performance, as the extremes can cause errors, whereas RMS values provide information about the average total amount of jitter. 2. Jitter tolerance (maximum tolerable jitter, MTJ): A measurement that checks the resilience of equipment after the input of jitter, which is required to confirm that the NEs in the transmission system can operate error-free in the presence of worst-case jitter from preceding sections. Jitter tolerance is one of the most important characteristics of the clock recovery and input circuitry of network equipment. 3. Jitter transfer (jitter transfer function, JTF): A measure of the amount of jitter transferred from the input to the output of the network equipment. JTF is important for cascaded clock recovery circuits in long-distance transmission systems with regenerators and line terminals. In addition, the jitter transfer measurement is required to confirm that cascaded NEs in the transmission system have not amplified the jitter. Figure 1 describes the typical values/ranges for the three jitter characteristics.