Optical Testing What Do System Ops and Techs Really Need?

The seemingly insatiable demand for more and more bandwidth is a significant driver for today’s optical testing and equipment market that, according to Frost & Sullivan, is expected to see revenues of more than $600 million worldwide by 2013.

Broadband customers want download speeds of anywhere between 50 Mbps and 100 Mbps to better accommodate applications like streaming video, online gaming and Web-based social networking on their PCs. Service providers, therefore, are pushing fiber closer to the home to help meet these demands.

"While operators compete aggressively to provide such services as digital TV and VoIP, the demand for testing the cables is expected to increase," said Srihari Padmanabhan, a Frost & Sullivan research analyst who estimates the global fiber network will reach 18 million homes by 2011.

Access, in turn, is forcing the metro core backbone to be more flexible, which means operators have to use reconfigurable optical add-drop multiplexers for dynamic bandwidth management and to upgrade from 10 Gigabit Ethernet (GigE) to 40 GigE.

"Everything has changed. We are seeing a paradigm shift,” commented Stephane Chabot, director/product line management for EXFO’s optical business unit. “The access market is driving networks to cope with demand.”

Testing Challenges

For starters, FTTx deployments that utilize a passive optical network architecture (PON) (in which one or two wavelengths are sent downstream on a fiber split to 32 or 64 homes) are more challenging than are traditional point-to-point deployments when it comes to qualifying a fiber or when troubleshooting.

"If you throw light on the network while operating an optical time domain reflectometer (OTDR) and if you are transmitting the test signal close to the wavelength the operation is using to transmit data, it will take every house down," said Tim Yount, marketing manager for JDSU’s North American fiber-optics test solutions business.

Choosing to “connectorize” (a mechanism of connecting two points so that they can be repaired easily) as opposed to fusion splicing during the design phase means more potential test-access points.

"If you have a connectorized system and a house is down, you can detach the splitter from the house, troubleshoot the link and not threaten service to other homes," Yount added.

And there have been improvements in out-of-band testing. With out-of-band testing, if the customer transmits at 1550 nanometers (nm), the test is run at 1625 nm. "It won’t affect the transmission itself, but you will still find if there is a macroband, for example. The higher wavelength is more sensitive, and you will see a loss very quickly," EXFO’s Chabot said.

Fiber Characterization

Dispersion becomes an issue when service providers move to transmission rates of 10 GigE, 40 GigE and faster. It could result in signal distortion and an increase in bit error rates, limiting the ability of a fiber to carry high speed signals, particularly over long distances. Fiber characterization prior to the installation or upgrade of network elements allows service providers to determine where dispersion might be an issue.

"If you are turning up a network to 10 GigE but you may roll out 40 GigE, you could qualify the fiber plant for 40 GigE even if you weren’t going to turn it up at that time,” Yount explained. “You could literally tag fibers so that, when you are ready to turn up a new link, you could look at which of the fibers you’ve already tested.”

While there are mechanisms to correct one type of anomaly (chromatic dispersion [CD], historically), the course of action when polarization mode dispersion (PMD) levels do not meet the system threshold has been to:

reduce the bit rate or to shorten the link;

find another, more suitable fiber; or

install new fiber.

To do the latter could cost a provider $100,000 per kilometer, Chabot said.

Newer Tools

Alternatively, newer testing tools are able to map PMD as a function of distance, locating high-PMD spans that can then be replaced or rerouted. "For 80 kilometers, you can map PMD every 100 meters. The solution could save millions of dollars with one testing by locating bad dispersion into a span," Chabot explained.

That said, smaller operators are making do with the testing tools they already own, due to cost. Alabama-based Troy Cable has some 700 to 800 miles of a fiber, comprising a fiber to the home (FTTH) infrastructure, a 10 GigE ring around some hubs and a SONET ring infrastructure with sister companies for backhaul.

Yet Chad Copeland, construction manager, said most of what his company does is standard loss and other OTDR-type testing.

Troy Cable doesn’t have an optical spectrum analyzer to test for PMD, due to the price, “but we’ve never run into an issue where we have had polarization or water peak problems," Copeland said, noting however that, as Troy looks to 40 GigE, it has been investigating the purchase of more specialized test equipment. Price remains an issue.

Beyond 40 GigE

Transmission speeds faster than 40 GigE would be even more susceptible to dispersion. However, according to Brandon Collings, CTO at JDSU’s communications and commercial optical products unit, 100 GigE has been possible for short distances by muxing together four signals carrying 25 GigE of data or 10 signals with 10 GigE of data for a total of 100 GigE.

"If you’ve qualified the network for 10 GigE, you’ve qualified it for 100 GigE,” added JDSU’s Yount. “If you are transporting four 25 gigabit lanes, you would want to qualify for 40 GigE. The standards numbers are for 10 GigE and 40 GigE.”

For long-haul and dense wavelength division multiplexing (DWDM) domains, there is a revolution of sorts going on regarding new modulation formats and coherent detection. Dual-polarization quadrature phase shift keying (DP-QPSK) packs bits closer together to fit 100 GigE into standard 50 GHz channel spacing. The coherent detection mechanism receives the highly compacted signal and digitizes it, using a technique called electronic dispersion compensation.

DP-QPSK means two phases into two orthogonal polarizations. Therefore, a traditional spectrum analyzer that measures the central wavelength for power will not work. The phase needs to be mapped as a function of amplitude, EXFO’s Chabot said, adding, "We can map all the different modulation schemes: the different phases versus the amplitude for coherent systems."

Once in digital format, PMD and CD can essentially be reversed, compensating for tens of thousands of picoseconds per nanometer of dispersion, JDSU’s Collings said, continuing, "When you have this capability, the need to measure dispersion and PMD in fiber is considerably less."

Field Trials

AT&T recently completed a field test of 100 GigE backbone technology on a 900 km transport link between Florida and Louisiana, using the Cisco CRS-3 Carrier Routing System.

Using an Ixia traffic generator and analyzer, AT&T tested the physical layer up through the packet layer. "We stressed the 100 GigE link with as many packets as possible. It was not so much about packets going through, but getting bits correctly from one side to the other," explained Martin Birk, principal member/Technical Staff at AT&T Labs.

In December 2009, Verizon turned up a 100 GigE optical system on its optical core network running between Paris and Frankfurt in Europe. Several months later, the carrier completed a 100 GigE optical transmission field trial on a 1525 km optically amplified section of its network in Dallas.

One testing challenge has to do with the need for guard bands between two modulation formats. "We have to validate the launch power and guard band and we need test equipment capable of doing that," said Christopher Mayer, Verizon’s vice president/systems integration and testing.

Another complicating factor is that, in the electronic space, there still is no 100 GigE ASIC, so lower speed lanes are used. In the physical coding sublayer (PCS), data is broken down into payloads: 20 lanes at 5 GigE each or four lanes at 25 GigE each, etc.

"PCS manages how data goes across the electronics and over the media,” said Charles Seifert, senior product manager at Ixia. “When it gets to the destination, there is a mechanism that identifies which lane the data came from and reassembles it into a packet that we can use."

The receive side needs to be tested to make sure the PCS mechanism is reading all the numbers and is reassembling them properly at the high speed. The health of the link between the sender and receiver also needs to be tested.

"The test tool needs to monitor in real time, detect errors and report," Seifert said. In addition, he said router manufacturers want to be sure the PCS mechanism is working, everything is synchronizing and there are no errors.

From a service-provider perspective, AT&T needed a test set that could look at PCS and troubleshoot if any issues came up during interoperability testing. "But once you work on the network level, there are so many issues that you don’t have the time to dive down into every little issue. PCS is more important to the manufacturer of network elements, and it is a very important test to do," Birk said.

Go/No Go

Indeed, Ixia’s test platform, used during the AT&T trial, had the capability to do Layer 1 to Layer 7 testing, including PCS. And modularity continues to be a buzz word in the industry, with JDSU and EXFO also touting platforms with multiple capabilities, depending on customer need. Yet others say the tide has turned, and service providers are asking for simpler, lower-cost, dedicated function, go/no-go testers.

"Logic would tell you that, as the demands in the network rise and the type of traffic becomes more critical to the user, you would want to test it more. At the same time, all service providers are under pressure to keep expenses in line," said Stephen Colangelo, director of marketing and business development at Anritsu.

In yet another twist, a representative from one of the larger MSOs who wished to remain anonymous, told Communications Technology he would like to see testing capabilities put into the equipment itself. "There are optical-performance monitoring modules that go into next-gen shelves, but handhelds continue to be needed,” the source said. “Once the equipment is installed, it would be nice to have all this at the ready and instrumented as the circuits are working.”

Lessons Learned

Troy Cable found out the hard way how important this is. "Early on, we burned one or two connectors due to cleaning — thinking they were clean without scoping," lamented Chad Copeland, construction manager. The company now scopes all connectors in the headend and transport area before plugging them in.

Monta Monaco Hernon is a frequent CT contributor. Contact her at [email protected].