Pulse or digital communications – Synchronizers – Phase displacement – slip or jitter correction
Reexamination Certificate
2000-10-05
2003-07-01
Tran, Khai (Department: 2631)
Pulse or digital communications
Synchronizers
Phase displacement, slip or jitter correction
C359S264000
Reexamination Certificate
active
06587530
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to an apparatus and system that measures and outputs a first order assessment of signal characteristics and a determination of signal quality and integrity within telecommunications networks such as gigabit and multi-gigabit networks.
2. Description of the Prior Art
Gigabit and multi-gigabit Local Area Network (LAN) and Storage Area Network (SAN) markets are rapidly emerging industries, which are entirely dependent on inexpensive network media components, including both copper and fiber optics technologies. Since the late 1980's, the available high-speed fiber optic components have been inexpensive screened parts taken from compact disk (CD) production lines. CD laser technology is based on edge emitting Fabry-Perot laser diodes, which offer data communications bandwidth from hundreds of megabit/sec to gigabit/sec. Other low-cost technologies, such as super-luminescent light-emitting-diode (LED), have been incapable of achieving gigabit link speeds. Since LAN and SAN markets demand low cost, the technically more mature edge emitting Fabry-Perot and distributed Bragg reflector laser diodes that are used in telecommunications applications were not adopted.
The CD laser technology was embraced by a number of suppliers, who applied their respective level of sub-component supplier selection and parts screening processes. Some CD laser vendors did an excellent job in selecting sub-component suppliers, while others applied various levels of parts screening. In the end, low cost CD lasers have a fundamental characteristic: gradual reduction in “relaxation oscillation frequency.” Over time, these components experience un-damped ringing, resulting in drastically increased, measurable jitter on the link (as measurable by a digital-sampling oscilloscope). Some CD laser vendors have had to undergo full-scale replacement of their product in the field, while others have stated that it is just a matter of time before their product suffers from the same degradation. This represents several years of production from multiple suppliers, all of which requires support in the field.
Laser media suppliers are transitioning to Vertical Cavity Surface Emitting Laser (VCSEL) diode based technology, which is specifically designed and manufactured for the low cost, LAN and SAN data communications markets. This will allow much better resolution of issues, since vendors are no longer leveraging a fundamentally different, high volume, commodity industry. VCSEL technology has the potential to offer data communications bandwidth from 1 to 5+, gigabit/sec, and offers the initial promise to attain very high reliability figures similar to the early days of the telecommunications industry. On the cautionary side, VCSEL technology is still relatively new, and primary failure modes are still being characterized. Additionally, VCSEL manufacturers are transitioning from ion-implant confined device structures to oxide confined device structures. These device enhancements, coupled with overall manufacturing/materials cost cutting efforts and higher data rate applications operating at 2 gigabit/sec, represent areas of risk which may impact life-time characteristics compared to current practice and theoretical understanding.
However, beyond laser physics, there are a whole host of other materials, design, and manufacturing issues which have the potential to adversely impact signal integrity. Field service groups have already experienced instances of cracked lenses within the laser assembly, glue that fails over time, glue that has not been fully cured, as well as spattered glue on the lens, all of which which darken the transmitted signal at some time after installation.
The primary failure mode requiring new field instrumentation is degradation of the transmitting device, including the laser diode and associated drive electronics. This failure mode does not change average or peak signal power levels, but rather results in increased “jitter” within the digital signal. “Jitter” is an industry term which refers to the amount of variance within the rising and falling edges of the digital signal. As seen in
FIG. 2
, Jitter
23
is measured at the receiver, and appears on a digital-sampling oscilloscope as though the rising and falling edges of the signal are smeared across a broad area of the overall duty cycle. This smearing results in a more closed eye opening
21
and may be quantified by expressing the eye as the “percent eye closing” or “%eye.” Noise
24
is the signal unrelated to the signal of interest.
At the early stage of transmitter failure, the link may experience occasional bit errors, which may only happen with specific data patterns within an IO data-gram sent over the link, or with heavy IO load levels transmitted over the link. At the later stages of transmitter failure, the link saturates with errors, and becomes non-operational for transferring IO. For both of these increased transmitted “jitter” cases, the optical power level is within the original manufacturer specification, and field service is not able to use an optical power meter to distinguish an error free link from a link that is unusable. At the present time, there is no direct measurement technique suitable for use in the field which provides a measurement of signal integrity or transmitted “jitter” within gigabit and multi-gigabit networks.
Without this capability, field service is placed in the difficult position of trouble-shooting the onset of intermittent errors using only customer data transmitted through the network and analysis of error counters throughout the system. Trouble-shooting intermittent errors within an on-line system is very difficult, due to the random nature of hitting the right data pattern or load level which causes the error, as well as collecting and analyzing the correct set of error counter data, which identifies the most likely failing component or components. This error-counter analysis procedure is extremely time consuming and results in the identification of a small number of “most likely” failing component(s), rather than accurately identifying “the” one or more failing components. Additionally, this analysis depends heavily on the judgement of the field service person, who must make the decision whether to expedite a field service action based on a few increments in error counters or wait to see what happens next.
Once field service has the opportunity to take a portion of the system down for maintenance, they have a short period of time to complete any further trouble shooting to narrow the list of most likely failing components, replace the fewest number of components, and return the system to operation. It is at this point in time, that field service requires a low-cost, rapid, and deterministic pass-fail test to isolate the failing component. Without this capability, field service has no way to verify quality of the components being placed back into operation, and the system is exposed to the risk of oscillating through many cycles of maintenance actions. In the case of intermittent errors, this is especially unacceptable since it may take a long period of time to confirm whether the problem was fixed, based on the likelihood hitting the data pattern or load level, which triggers the error.
Automated and continuous collection of on-line diagnostic data has, to some extent, helped to reduce trouble-shooting time and improve accuracy. However, even with improvements in on-line diagnostics data collection, there is a need in the industry for a means to directly measure signal integrity in the field. Whether in the form of a portable, hand held field unit or incorporated into the communications equipment itself, the ability to rapidly evaluate the optic signal at the customer site is a key quality assurance factor.
All of these criteria point to the need for a means to directly measure signal integrity both in-line and in the field. Additionally, many of the risk factors apply equally to copper-based gigabit media.
SUMMARY OF
Conklin Troy R.
Hutchison, Jr. Harold B.
Garnett Pryor A
International Business Machines - Corporation
Paxson LLP Dilworth
Tran Khai
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