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The cost of optical transceivers is a major concern for data center operators as they migrate their networks from 100GE to 400GE. To be competitive, transceiver manufacturers must find ways to drive down production costs. Like most new technologies, the price of next-generation optical transceivers tends to drop sharply after introduction to the market, and development costs amortize as volume ramps. Next-generation transceiver technology, such as 400GE, will reach mature pricing within a year of introduction. At maturity, the cost of transceivers is directly proportional to the complexity of the design and the number of optical components. Test time contributes significantly to the overall transceiver cost. More efficient testing of the broad range of transceiver data rates accelerates innovation and lowers cost.
The fiber-optic interconnections will not be realized without optical transceivers in data centers. In a data center, at least thousands of optical transceivers will be consumed to achieve the interconnections. Thus, optical module price and the costs related to it account for a large proportion of data center operating expenses. Therefore, choosing proper transceivers is of crucial significance.
Main components of optical transceivers
The components of driving the light through the optical cable are also called light sources for Fibre Optic transceivers. The most used light sources are LEDs and VCSELs (vertical cavity surface-emitting lasers). Even though they have the same purpose of converting electrical signals into optical light and vice-versa, they are quite different in their functioning. They are in fact small semiconductor chips and they are emitting light from the surface of the chip.
The job of an optical transceiver is to convert the electrical signal from a switch or router to an optical signal that can be transmitted and received over fibre optic cable.
Fibre Stub. A strand of fibre cable along which optical signal enters the transceiver. A small fibre stub is optimal to minimize signal attenuation.
Focusing Lens. Refocuses the light coming in (or going out) to maximize signal strength.
Isolator. Shields the transmitted signal from the received signal by reducing EMI (electromagnetic interference) within the transceiver. This improves signal strength in a highly compact form factor.
TOSA module is the Transmission Optical Sub Assembly module which converts the electrical signal to the optical transmission light that lands on the fibre. It is a small and expensive module and the materials and manufacturing used for it has to be of good quality standards in order to provide a long time stable optical signal. A TOSA contains a semiconductor laser diode (LD), while a ROSA contains a photodiode (PD), optical lens, preamplifier, and passive electrical parts.
TOSA
TOSA is the component inside the transceiver which is responsible for converting the electrical signal into an optical signal and then transmitting it over the optical fibre strand connected to it. The transmitter optical sub assembly consists of an electrical connection, a monitor photodiode, a laser diode, a housing which can be of metal or plastic and an optical interface. The TOSA is an essential component of every fibre optic transceiver.
ROSA
ROSA refers to Receiver Optical Sub-Assembly, the primary function of which is to convert the optical signal transmitted from TOSA into electrical signal. ROSA contains a photodiode (PD), optical interface, metal and/or plastic housing, and electrical interface. Like TOSA, the concrete components of ROSA depends on specific functionality and application of the transceiver. Other components such as amplifier may be included, aiming to reshape input signals degraded by long-distance transmission. The preamplifier converts a current signal to a voltage signal and amplifies the signal to a high voltage gain, while the post-amplifier equalizes the output signal of the preamplifier to an amplitude level suitable for input to the following digital circuit.
Select Transceiver with:
Energy efficiency is also a concern as to the cost of operation. Produced by different vendors, fiber optic transceivers with the same type may have different parameter values in power consumption, resulting from the manufacturing ability of the vendors. Due to the technology and craftsmanship, some modules manufactured by small-sized suppliers may have higher power consumption even if they can work normally.
Indeed, the difference in power consumption of two modules may be only a few watts or even a few tenths of a watt. However, the generated power consumption will be accumulated when there are a number of modules involved. If the power consumption of two 10G optical modules is 2.5W and 3W respectively, then the power consumption of the optical transceiver on a 48G switching board may reach 120W for the former and 144W for the latter. If a network device with 16 boards is inserted, then the total value will be 1,920W and 2,300W. Select the optical modules with lower consumption—reduce the power consumption to minimize the operation costs.
The space utilization of the data center is a main concern for many IT architects. When selecting from the modules with the same operating rate but in different form factors, the ones with smaller sizes will save the space as much as possible. Take two 40G modules as an example, a 40G QSFP+ optical module is roughly 12cm long and 1.8cm width, while a CFP optical module is approximately 14 cm long and 8.2 cm width. Given that there are 8 modules, QSFP+ modules require about 172.8c㎡ whereas CFP modules need around a total of 918.4 c㎡, both of the situations are not calculating the gap between the modules. As a result, fiber optic transceivers with smaller form factors can provide optimized space-saving solutions for a high-density data center.
Key to reduce test time and subsequently:
In the research and development (R&D) phase, starting with powerful design simulation software is the first step to ensure test efficiency and lower test cost. Today’s design and simulation software enable transceiver designers to optimize their designs, ensure performance and robustness, and avoid costly additional board design cycles. They can identify the most sensitive design components early on and decide how to set specifications to improve manufacturing yield. Once optimized, designers can test design performance using post-processing data analysis functionality without rerunning simulations.
Once deployed in data centers, marginally performing transceivers can bring down the network link, lowering the overall efficiency of the data center as the switches and routers need to re-route the faulty link. The cost associated with failed transceivers once deployed in the data center is enormous. In a hyperscale data center with more than 100,000 transceivers, even a small one-tenth of one percent failure rate would equate to 100 faulty links.
Industry standards organizations, such as the Institute of Electrical and Electronics Engineers (IEEE), International Committee for Information Technology Standards (INCITS) and the Optical Internet working Forum (OIF), generate and maintain optical transceiver specifications and define test procedures to ensure interoperability of modules from different vendors. The operating margins in 400GE optical links are the tightest of any generation yet, creating additional test challenges as small measurement errors can quickly consume the entire operating margin. Fortunately, the methods that describe how to characterize 400GE designs are becoming more stable, and engineers can review and follow the guidelines outlined in the standards when developing their transceiver characterization test plans.
Test automation software can reduce test time down from hours to minutes. Choosing automated compliance test software verified to test to the exact specifications of each technology standard is essential. Test automation software provides insights to the test engineer about any detected issues and can quickly pinpoint failures, saving hours of debug time. Test automation software guides the test engineer through setup, calibration and compliance measurements, and allows them to quickly run through test cases without being an expert on test procedures. More importantly, ensuring transceivers are compliant to standards will minimize the risk of interoperability issues with network switches and routers once installed in data centers around the world.
The results of automated tests are available to test engineers in real-time reports. These reports typically include details about the test setup and configuration, the measurements made, the pass/fail status, margin analysis, and output waveforms. With this information, it is easy for test engineers to replicate the test scenario later. However, sometimes the sheer amount of data collected is overwhelming for a test engineer to analyze and understand. Therefore, test automation software with integrated data analytics capabilities is ideal.
Data analytics tools provide insightful analysis of test results. Visualization methods, such as line and histogram charts, show pass/fail limits and statistical information so test engineers can see at a glance the performance of the device under test. Data analytic tools often store results in a cloud repository, making it fast and easy to share the results among global and distributed teams. Test engineers can quickly make design decisions with confidence that would otherwise take days or weeks for them to analyze and resolve.
Research and development of 400GE transceivers is well underway. Engineers are still struggling with how to test PAM4 modules, and 400GE standards are continuing to evolve. 400GE transceivers have a tight time-to-market window to meet the demands of emerging technologies such as 5G and IoT, as well as the massive growth of the public cloud infrastructure. Once 400GE transceivers reach the manufacturing phase, any issues found will mean a costly rework of designs. The ideal is to design for manufacturing. Several tools can help transceiver manufacturers create manufacturing-friendly designs. Costs increase significantly as issues are uncovered later in the development process.
The data center transceiver market is extremely cost sensitive and competitive. Test time is a significant factor that contributes to the overall cost of transceivers. By reducing test time, transceiver manufacturers can reduce costs and be first-to-market with next-generation transceivers. At each phase of the product development life cycle, there are test solutions that can be used to maximize test efficiency. These tools can shorten design cycles, dramatically improve productivity, ensure quality and significantly reduce costs.
With a limited budget, most managers will tend to purchase many compatible modules, which turns out to be a cost-effective solution. In addition, power consumption and form factors are also essential factors to consider saving the operation expenses and available space. All in all, adopt compatible optical modules with low-consumption and high-density to realize an optimal optical solution.
