Built for the long haul
Tests ensure that Ciena's CoreStream optical transport systems communicate reliably between cities.
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 5/1/2005 2:00:00 AM
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LINTHICUM, MD—Whenever you place a long-distance call or use the Internet, you likely use a Ciena CoreStream optical transmission system without knowing it. Major telecom carriers throughout the world rely on CoreStream systems to carry voice, data, and video between metro networks in different cities. Part of that reliability comes from the intense testing that Ciena engineers perform during design verification at the component, subsystem, system, and network level.
Ciena takes an unusual approach to product development, as some of the people who define the technical details of new products and upgrades also perform design-validation tests. Greg Bartolomei, director of systems validation, and James Allard, senior director of systems and verification, are two key people at the beginning and ending of a product's development cycle. Allard works closely with Ciena's customers and with marketing to obtain feedback to define a new product or feature upgrade. After marketing develops a functional feature specification, Allard's systems-engineering team transforms it into an engineering requirements and specifications document.
When Ciena begins work on a new design, Allard assigns a systems engineer to follow the product throughout its development. Allard says the engineer lives in the engineering labs and is the "one neck to grab" when other engineers need a resource or if something goes wrong. "The process not only ensures quality," said Allard, "but it ensures that we developed the product we set out to design."
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| James Allard, senior director of systems and verification, leads the group that writes engineering specs; his group also has the final say on when a design is ready for production. |
After Allard's systems-engineering team writes a product's engineering specs, the specs are passed on to digital hardware engineers, software engineers, a power-supply specialist, and mechanical, reliability, network, and automation engineers who all contribute their expertise to the product's design. Because the heart of a long-haul transport system is its optics, engineers in the optics lab, electronics lab, lightwave systems lab, and systems-validation lab test the optics at the component, board, system, and network levels to verify that a product will meet Ciena's specifications. At the end of the development phase, Bartolomei and others validate the final engineering product design, and then Allard's group has the final say on whether a product is ready for production, based on tests from Bartolomei's group. The entire process can take up to four months.
It starts with optics
Photonics director Jean-Luc Archambault manages eight engineers in the components lab who evaluate optical products for new designs. Upon receiving engineering specs, engineers including senior principal engineer Dr. Doyle Nichols test optical receivers, transmitters, amplifiers, and passive components such as splitters. (For a complete list, see "Products tested in Ciena's components lab" at the bottom of this page.)
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| Jean-Luc Archambault manages the components lab, where engineers evaluate active and passive optical components. |
Ciena tests optical components to verify that they meet manufacturers' specifications and that they will perform properly in the company's equipment. "We compare our measurements against those of the manufacturers" noted Archambault, "If we see large discrepancies between our measurements and manufacturers' specs, we must resolve them. Often, we require component makers to add tests to their products."
The components lab has four test benches for active components, two each for evaluations at 10 Gbps and two for slower speeds (1 Gbps and 2.5 Gbps). On the day of my visit, Nichols tested an optical receiver at 10 Gbps. He often tests components at the slower speeds when they will connect to a tributary line.
To test a component, Nichols uses a test mounting board roughly 2 ft x 3 ft in size, on which he mounts the device under test (DUT), fiber-optic cables, attenuators, splitters, and connectors. Nichols knows the characteristics of all components other than the DUT, which ensures that no unforeseen variables will alter his measurements.
Nichols routes the receiver's electrical output to a clock-recovery circuit, which extracts embedded clocks from data streams. He runs the receiver's recovered clock and data stream into a bit-error-rate tester (BERT), an optical spectrum analyzer (OSA), and a digital communications analyzer (DCA). With these instruments, Nichols performs tests such as BER versus optical power and BER versus optical signal-to-noise ratio (OSNR). A receiver evaluation usually takes one or two days in the optics lab. (For details on how Nichols performs BER versus OSNR tests, see "Bits battle noise," Ref. 1.)
Optical signals must pass through dozens of amplifiers as they travel the long distances between cities. Ciena engineers typically perform more than 100 measurements when they qualify an amplifier. "Testing is an Nth-dimensional problem," said Archambault. "You can spend as much time testing as you want." While Ciena purchases most of the amplifiers it uses, Archambault said the company gets heavily involved in an amplifier's design. "Sometimes, we come up with our own preliminary designs and show them to our suppliers," he noted.
Homegrown testers
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| Figure 1. An optical switch routes a light pulse around a loop to simulate the effects of the pulse going through a chain of amplifiers. |
Amplifiers add noise to optical signals. To simulate a light pulse traveling through amplifiers, Archambault's engineers built a recirculating fiber-optic loop that contains one or more amplifiers (Figure 1). The switch forces the pulse through the loop many times. The loop simulates light passing through an amplifier or small chain of amplifiers each time around, letting the engineers conduct long-distance testing with relatively few amplifiers. A digital delay generator regulates the time the light runs around the loop. When the time expires, the switch lets the light pulse exit the loop. Engineers can test the effects of a pulse passing through 30 amplifiers by passing light 15 times through a loop that contains two amplifiers.
The recirculating loop is but one homegrown tester in the components lab. Ciena engineers also built an automated test system that measures how well an amplifier boosts optical 1's and 0's. Other tests the system performs include gain level, gain flatness, and gain accuracy. The engineers use a comb laser source to generate more than 2000 optical signals of different wavelengths across a 38-nm wavelength band. Using an OSA, they can take measurements at each wavelength.
Archambault's team also test dispersion-compensating fibers (DCFs), which compensate for dispersion of light as it travels through a fiber-optic cable. The engineers measure loss and dispersion as a function of wavelength, and they perform multipath interference measurements. Backscattering will occur in a DCF that isn't properly designed.
Besides performing bench-level performance tests, the engineers also test components for reliability by placing them in thermal chambers. Archambault pointed out that a component will undergo a thermal stress at 75°C, 90% relative humidity for about three weeks. Components also undergo temperature cycling from –40°C to 85°C.
Operational tests take place at temperatures from 0°C to 65°C. During these tests, an engineer drives a component with a pattern generator and measures BER. The engineer also looks at a component's eye diagram with a sampling oscilloscope to measure how a device's output changes with temperature. An optical switch, controlled by a PC, lets engineers run thermal tests on up to 18 devices at once.
Add the electronics
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| Senior director Cecil Smith manages 25 electrical, software, and automation engineers who develop products and test software in the electronics lab. |
Engineers in the electronics lab design the hardware and software that will integrate with the optical components evaluated in the components lab. Headed by senior director Cecil Smith, the lab employs 25 engineers who develop products and write test software. "To develop one board," noted Smith, "we may use an RF analog engineer, one or two digital designers, a mechanical engineer, a power specialist, and a PCB designer. Automation engineers write test code that automates engineering tests and production tests. Our people are very good at what they do, and we can often use a circuit design in more than one application."
Hardware and software development takes place simultaneously. Hardware engineers manually test the first iteration of a product such as a line card by testing the card's physical-layer functions. Figure 2 shows the test equipment on a typical engineer's bench. Engineers use the equipment to measure parameters such as signal power, jitter, and BER.
To perform these tests, engineers built a test bed that includes a CoreStream backplane. It also provides access to optical and electrical data streams that the card needs. Engineers start by looking at waveforms from the backplane and from the line card's electrical interfaces to its optical components. The test bed connects to a PC that exercises and calibrates the card. (Calibration involves setting the card's optical power levels and wavelengths.)
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| Figure 2. Engineers’ benches in the electronics lab contain optical and electrical test equipment. |
For time-domain measurements, Smith's engineers use DCAs to observe eye diagrams of transport signals in electrical form on the card and in optical form as the signals enter and exit a card's line interfaces. They look for reflections and check for correct waveform shapes, and they use logic analyzers to investigate data patterns. They then adjust drive levels, extinction ratios, or impedances to get the best jitter performance.
The engineers also measure a card's optical output power and make BER measurements on both transmitted and received data streams. To make the optical BER measurements, they use SONET test sets that transport payloads of pseudorandom bit sequences encapsulated in SONET frames. The frames start as OC-3 (155 Mbps) and OC-12 (622 Mbps) streams multiplexed up to OC-192 (10 Gbps) streams. They also add noise to the signals so they can measure a card's ability to accurately transport data under less than ideal conditions.
If a card requires design changes, its second iteration usually goes to the automation group, where engineers run the card through a series of automated tests. "We have an extensive set of I/O libraries that we use to bring up new cards," said Smith. Automation engineers use those libraries to write test code in LabView, and they often reuse the code in production test systems.
As software engineers complete software modules, testing moves from checking the hardware to checking that the hardware and software work together. For example, a line card must report errors if SONET frames fail to arrive at their destination. The software must also fill a payload frame when errors occur.
When engineers are convinced that a card and its embedded software meet specifications, they perform environmental tests. "Some countries don't have good environmental controls," said Smith. "Transport systems could reside in central offices that, although heated, could receive a blast of cold air if someone opens the door in the winter, so we test our cards for thermal shock." A typical test subjects a card to a temperature change of 40°C in one minute.
On to the systems
When a card passes its tests in the electronics lab, it's ready to run in a system and communicate over a simulated network. In the lightwave systems (LWS) lab, Dr. B. Sridhar tests cards and systems under simulated real-world conditions. He uses racks of test and network equipment, 30,000 km of fiber, and dozens of optical amplifiers to emulate networks in the field. "People characterize communications systems in terms of distance and capacity," said Sridhar. "We test line cards and systems to see how they perform under different conditions." Sridhar also uses the lab's network to test purchased optical amplifiers by looking at the signals they produce and by running BER tests.
From his network measurements, Sridhar can evaluate new products and can recommend to customers how they should configure new networks for optimal performance. Ciena has developed a network-design software tool that customers use to configure their networks with amplifiers and fibers. The tool is based on Sridhar's work at characterizing networks in the LWS lab.
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| Figure 3. In the LWS lab, engineers test the effects of amplifiers in an optical network. |
On the day of my visit, Sridhar tested a 93-channel system configured with 1600-km of fiber and 20 erbium-doped fiber amplifiers (EDFAs) spaced 80 km apart (Figure 3). Typical long-haul networks have amplifiers every 80 to 120 km. A CoreStream system can accommodate up to 192 channels, each on a unique wavelength, in a 10-Gbps link. A single EDFA can amplify all 192 wavelengths, eliminating the need for an individual amplifier for each channel. The test network needs two EDFAs, one for each transmission direction. The LWS lab's network also consists of patch panels that let engineers choose fiber types. A 1x16 optical switch lets Sridhar measure the noise produced by optical amplifiers along a fiber-optic link. Because each amplifier adds noise, the total noise in a network is proportional to the number of amplifiers and the amount of gain each one provides. In a network that must maintain a 20-dB signal-to-noise ratio, even 0.1 dB to 0.2 dB of noise from each amplifier can be enough to reduce performance.
Sridhar tests each transport system or line card with different vintages of fiber because each produces different amounts of dispersion in the optical signal. He then adds dispersion-compensation fiber to compensate for the network's dispersion, as Ciena's customers will do. "Once we design a transport system," said Sridhar, "we test it with different fiber types to ensure that it will work in the real world."
To perform his tests, Sridhar uses a tunable filter that selects each wavelength. He looks at each channel's optical power with an OSA. Although an OSA indicates a wavelength's power, it provides no information about the signal's integrity. Therefore, Sridhar uses a DCA to view a signal's shape and jitter in the time domain.
Because communications service providers demand that their networks operate at 10–12 BER or lower, Sridhar designs networks to run at 10–16 BER, which lets him attain a good margin of error. "We're trying to figure out how far we can go between amplifiers," noted Sridhar. "We look for the optimum configuration for each customer. More channels results in shorter distances because channels close in wavelength will more likely interfere with each other as distance increases."
Full circle
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| Director of systems validation Greg Bartolomei and his team perform system-level tests on Ciena's new products to verify that they meet engineering specifications. |
After a new product passes tests in the components lab, electronics lab, and LWS lab, it goes to the systems-validation lab for final approval. Bartolomei and Allard, who wrote the product's engineering specs, now test the product to verify that it meets or exceeds the system requirements and specifications.
Allard and his engineering team test new products under conditions that simulate a customer's existing network, bringing hardware, its embedded software, and system software together for the first time. In this lab, engineers verify that a product or product upgrade will work with the network-management software that Ciena's customers use. Ciena provides network operators with network-management software, but so do third-party developers. Some operators develop their own management software, and Ciena uses that software in the systems-validation lab to verify that products are ready for production.
Product upgrades must also pass tests in the systems-validation lab. "Customers don't need a new box to get the latest features," said Allard, "They can add the latest features to existing equipment in the field." Bartolomei and Allard verify all product upgrades because they have at least one of every Ciena product built over the last 10 years. Thus, they can emulate any customer's network in the lab.
As part of systems validation, Bartolomei and Allard configure optical switches controlled through a LabView program that simulates real-world hazards such as broken fiber-optic cables. When a simulated break occurs, they check that the system properly recovers from the break. They also check a system's physical layer to make sure it's compatible with the customer's network. "It's a luxury to be able to simulate so many customer networks," said Bartolomei, to which Allard added "we can quickly mimic a customer's network from the management software layer to the physical layer."
When Allard's group approves a product or upgrade, it goes to production. The systems that test production units were developed by automation engineers in the electronics lab, which completes the circle of light.
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The CoreStream system
Ciena's CoreStream optical transmission system uses dense-wavelength division multiplexing (DWDM) to deliver up to 2 Tbps of data in the C band. DWDM puts data from different sources on their own wavelengths of light and then multiplexes them on a single optical fiber for transmission over long distances. CoreStream contains optical terminals, amplifiers, and add/drop multiplexers connected through a chassis backplane. Optical terminals interface the system to OC-12 (1 Gbps), OC-48 (2.5 Gbps), and OC-192 (10Gbps) optical data streams where the system multiplexes the lower-speed tributaries into the OC-192 streams.
The system supports synchronous optical network/synchronous digital hierarchy (SONET/SDH), asynchronous transfer mode (ATM), and Internet Protocol (IP). It can send and receive data at 10–15 BER. Ciena also provides LightWorks ON-Center, a management software application that telecom service providers can use to manage their networks.
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Products tested in Ciena's components lab Active components
Amplifiers
Passive components
Modules
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