DWDM creates new testing challenges
Greater system complexity and the lack of highly trained personnel have increased the number of requirements for remote test equipment.
Hale Farley
Anritsu Co.
Only a few short years ago, dark fiber was prevalent throughout the country, as telecommunications companies were still predominantly transmitting only voice over their fiber-optic cables. In the past two years, however, the heavy influx of data and video transmitted over fiber has created quite a bandwidth challenge to these telecommunications companies. To satisfy the market demand, companies must increase bandwidth over existing lines and enable two-way transmission over every single fiber in a cable.
A major contributor to this increased bandwidth demand is Internet traffic, as our desire to see pictures instead of just words is using all the capacity on many spans of the telecommunications network. The communications service providers` alliance with television broadcasters only increases the need for fiber-optic cable that can accommodate more bandwidth. In fact, it is estimated that telecommunications bandwidth demand will increase by a factor of three every year for the next 25 years. Using Moore`s law to the third power, which is double exponential growth, demand will increase 10 times within the next two years!
Several service providers are currently deploying dense wavelength-division multiplexing (DWDM) systems with as many as 80 channels in the erbium C-band in an attempt to meet the increased demand for bandwidth. Fortunately, such a channel count is miniscule considering that deployment may exceed 256 channels in the near future due to wider-bandwidth optical amplifiers, like those at 100 nm. In fact, some predict there will be optical-fiber manufacturing techniques for flat optical loss from 1300 to 1650 nm. If such a fiber becomes a reality--and there is no reason to believe otherwise--DWDM could reach more than 800 channels with 0.4-nm spacing by the year 2001. At OC-48 (2.5 Gbits/sec), data bit rate equates to more than 200 billion bits per second on a single fiber.
This increased capacity places a great deal of pressure on telecommunications companies because there is now a greater potential for costly errors. In some cases, such errors have been extremely costly. Recently, one outage for a backbone service provider cost $200 million, while a similar outage by a regional Bell operating company cost $20 million!
With such a potential for financial disaster, the bar has been raised for both telecommunications companies and test manufacturers. DWDM and its ever-increasing channel counts place new constraints on test instruments and force them to be of the highest quality. Test manufacturers must also design instruments that can be equally good in the field and the lab.
In addition to having superior specs, field instruments must be easy to use, because the telecommunications industry has reorganized and network managers are now expected to know more about the optical network with fewer personnel. Turning to measurement technologies with remote capability is the best answer, since today`s instrument is tomorrow`s optical-network element installed in the fiber plant.
Testing challenges
Testing the physical layer in fiber-optic networks is related to the fiber plant, which includes both active and passive elements. Network performance monitoring seems to be the biggest testing challenge. Service providers are asking both telecommunications hardware suppliers and test equipment vendors to partner with them, not only for installation and certification but for ongoing maintenance to meet this challenge.
In maintenance scenarios, network managers are now relying on advanced test instruments to determine the health of an optical network at all times. Remote fiber testing can help isolate a fault very quickly during a major interruption, but other tests must be done for a minor problem, such as one that affects a single wavelength of a DWDM system. Two types of testing are expected in the physical layer to identify such a minor problem.
Long-term testing of individual channels is the first of these test strategies. But long-term tests pose a challenge to network managers because they must look for trends over an extended period of time in a variety of parameters that define the network`s ability to carry digital traffic. These physical parameters include optical channel spacing, optical power level, optical signal-to-noise ratio (SNR), gain, gain tilt (flatness of optical amplifiers as a function of optical frequency), and deviation from the reference recorded when the equipment was first installed and certified. This performance monitoring must be done without interfering with traffic or affecting the power/loss budget. While bit-error-rate checks will identify an actual fault in the fiber plant more quickly than will tests derived from these physical measurements, such long-term testing and the parameter data they accumulate will uncover trends that could indicate an impending interruption of service before it occurs.
The second type of testing is troubleshooting faults in the fiber plant. To conduct such tests successfully requires a portable instrument that can measure many of the parameters mentioned here at a remote site. Additionally, remote control from a central office is highly desirable because more and more technicians lack the expertise to identify and repair faults themselves. This dilemma places the technical burden on the central office, where the responsibility to maintain the fiber plant is located. As a result, field instruments with "virtual remote" interfaces increase the value of these products greatly.
Keeping pace with technology
Standards organizations such as the Telecommunications Industry Association (TIA) and International Telecommunications Union--Telecommunication (ITU-T) attempt to write procedures that aid all parties: telecommunication-equipment suppliers, service providers, and instrumentation manufacturers. The organizations are forced to perform such tasks quickly to meet the rapid deployment demands of service providers who must satisfy the desire for additional data bandwidth. To this end, both organizations have agreed to 50-GHz (0.4-nm) channel spacing, and several equipment suppliers are already deploying these systems.
One source of concern over the new channel spacing is the supervisory, or telemetry, wavelength. The existing standard is 1510 nm, but there are systems in the field with 1480- and 1625-nm supervisory wavelengths. For the service provider to be independent of equipment manufacturers, a common understanding on interoperability must be reached.
The variety of channel spacings now in deployment or development--which include 0.4, 0.8, and 1.6 nm--presents the largest measurement challenge for the equipment vendors, service providers, and instrumentation suppliers. One way to eliminate potential problems with channel spacing is to use an optical spectrum analyzer (OSA) during installation and certification. Some OSAs are equipped with 1450- to 1650-nm wavelength-measurement ranges and a dynamic range greater than 70 dB. They also may have a defraction grating with a double-pass monochrometer, a technology that meets the demands of DWDM systems with channel spacings down to 0.4 nm.
Application firmware permits OSAs to measure all the physical parameters necessary to install and maintain the health of the fiber plant. These parameters include channel spacing, optical power level, gain flatness, SNR, and others to analyze the DWDM spectrum (see Fig. 1).
Many OSAs also incorporate a built-in super-luminescent diode reference light source that allows them to measure other parameters of the fiber plant, including reflectivity, directivity, and isolation. Such a wide range of analysis capability in one instrument is invaluable in light of the fact that many service providers expect to have the ability to monitor more than 120 channels of DWDM, with more than 250 channels deployed in the near future. To protect service providers` investments in their monitoring gear, test equipment manufacturers have made firmware upgrades available.
Component testing
OSAs can measure fiber components accurately, as well. Figure 2 shows both the active and passive elements in a DWDM system. Erbium-doped fiber amplifiers (EDFAs) generate a high level of noise, called amplified stimultaneous emission (ASE). This noise level creates a challenge when measuring the true gain of an optical amplifier in the presence of self-generated noise. To achieve the necessary measurement performance, an external modulator pulse technique is integrated with some OSAs. The purpose of this light-modulator is to optically switch at a rate faster than the meta-stable state optical amplifier can react, yet slow enough to allow the optical measurement. This technique stabilizes noise and is better suited for DWDM testing than the traditional fitting technique.
With the pulse-measuring technique, ASE levels at the signal wavelength can be measured directly to produce an excellent modulator on/off extinction ratio. This method eliminates discrepancies between fitted curve and actual noise, which are a common problem when using the fitting technique. Because the ASE level is accurately measured, polarization dependence and input loss are optimized and reliability is assured. As a result, the small difference between wavelengths of multiple signals found in DWDM systems can be measured. These measurements are independent of channel count (see Fig. 3).
Some instruments also feature three memories and three traces. In addition to using this feature for additional analyses--such as reflection attenuation, insertion loss, isolation, PMD, and EDFA noise frequency gain--the extra memory allows the OSA to be used as a field instrument, which is especially important in today`s applications. Typically, the third memory is used for field use, since it can store data for maintenance testing. Add this capability to RS-232 and/or GPIB connection, and the instrument can be controlled remotely from a central office. With this capability, an experienced technician can monitor several lines from the central office and dictate to the less experienced field technicians what must be done and where.
The increased capacity required of fiber-optic networks in today`s society has placed demands on manufacturers, service providers, and test manufacturers. By using technology to its greatest extent, fiber lines can operate to maximum capacity. Test instruments must incorporate the technology necessary to ensure optimum fiber performance during installation. They must also allow telecommunications companies to maintain their reorganization policies by allowing key individuals to remain in the central office and permit less experienced people to be field operatives. u
Hale Farley is a product marketing engineer for Anritsu Co. (Richardson, TX).