Digital testers can perform a plethora of measurements to help keep your network running smoothly. The question is: Are your engineers and technicians using them?
Remember the good old days? Analog video offered different stages of picture imperfection. Customers might see some snow, but at least they didn’t miss their Sunday night show all together.
"With analog video, a TV set was the best test equipment you could buy. A TV set coupled with someone trained to view picture imperfections could tell you what the cause was," says Mark Millet, a senior system architect for Cisco Systems. With digital video, the picture either works or it doesn’t, and if it doesn’t, that means there’s been some sort of major failure. Today, of course, it’s not just a television picture at stake, but also premium video, cable modem and telephony services, and cable operators need more than just an old TV set in their test and measurement arsenal. The question is: Are they doing enough to ensure a stable network? You’ve got the power Consider the downstream. If the average power level of a digitally modulated carrier is not correct relative to what the amplitude of an analog channel on the same frequency would be, laser clipping or poor carrier-to-noise ratio might result, Ron Hranac, a technical leader for broadband network engineering at Cisco, reports. The average power level ought to be 6 dB to 10 dB below what the amplitude of an analog channel on the same frequency would be. "Take any test equipment that has a digital channel power function and use the push button automatic measurement to measure [the average power level]," Hranac says. Headend technicians need to measure signal levels on a daily basis, and installers need to take the measurements every time they are on a customer premise. One essential tool is a QAM analyzer, which can be used to automatically check average power levels. It also can measure modulation error ratio (MER) and both pre- and post-forward error correction (FEC) and bit error rate (BER). A troubleshooting tool Measuring average power level is a useful preventive maintenance technique. But a QAM analyzer also is an excellent troubleshooting tool. Sunrise Telecom Broadband provides an example. The owner of a "large metro system" asked the company’s techs to figure out what was causing random mass modem deregistrations. They employed one headend spectrum/QAM analyzer at a hub location to measure the return path, a second to verify the downstream signal from a specific node that fed from the hub in question and a third at the master headend. The first QAM analyzer monitored all four returns connected to a specific cable modem termination system (CMTS) return port. The second recorded MER and pre- and post-BER for one week, and the third monitored the input to the laser feeding the hub. By performing these measurements, the engineers determined that the emergency alert system (EAS) switch was causing the power in the headend to jump, which led to laser clipping. "The combination of being able to monitor a remote location over an IP connection, take MER/BER statistics over an extended time and the ability to do a digital power measurement of the entire spectrum provided the tools needed to solve this problem. All of these capabilities are often underutilized in a system," says Greg Potter, a Sunrise applications engineer. Digging deeper Routine testing for phase noise also is often overlooked by cable operators, despite the fact that phase noise is the number one problem diagnosed if calls are being dropped or if (with digital video) the picture is blocking, says Francis Edgington, a customer support engineer for Agilent Technologies. "The current instruments (that cable operators) have don’t have a good way of doing (testing for phase noise). You have to analyze the signal for a long enough period of time and look for a frequency or phase deviation of the signal," he says. A vector signal analyzer gives the operator the capability to sample for a lengthier time so that a frequency deviation or phase error will show up in the constellation. It also tests for distortions such as group delay and for composite second order and composite triple beat. "In the field, cable operators are starting to deploy low-cost QAM analyzer meters, which do a good job of detecting distortions between the headend and the customer’s house, but they can’t always determine distortions in the headend—more subtle distortions that can affect customers later on," Millet says. A peek inside One tool that goes a step further than a QAM analyzer is an MPEG stream analyzer, Bill Robertson, vice president of marketing for Acterna’s cable networks division, says. A Moving Picture Experts Group (MPEG) analyzer enables engineers to look at the quality of the data in the stream. "(An MPEG analyzer) worries about the content of a letter. A QAM analyzer worries whether the postman can make the delivery," Robertson explains. He adds that it also is useful as a monitoring tool. It can continually run hundreds of tests to verify network output. Maintain the system Jim Harris, marketing manager at Trilithic, on the other hand, is a firm believer in the mantra "maintaining the system will maintain the services." He uses the analogy of the interstate highways that were built in the 1940s and today are carrying vehicles of types and performance that couldn’t have been anticipated at the time these roads were constructed. What happens is that manufacturers design vehicles with the specifications of the highways in mind. "If program content and control signals are designed to work in a modern HFC network of known characteristics, then you can generally say if you maintain a cable system to those design specifications, the services that ride on it will work just fine," Harris says. Regarding the return path, ingress is the "biggest single upstream problem," he adds. "Random noise is sharing bandwidth with real digital signals…When a modem is slow, quite often the traffic is sharing bandwidth with noise—ingress—so a lot of packets have to be sent twice. If there are a lot of resends, it slows everything down," Harris says. His recommendation is for field technicians to be equipped with a return path analyzer. It communicates with the spectrum-measuring device in the headend to determine the soundness of the cable path. Hranac adds that some operators have found success in eliminating upstream ingress problems by monitoring downstream signal leakage to make sure that it remains at or below 5 microvolts per meter in both the network and the drops. Using what you already have While no digital test tool is as simple as the proverbial TV set that could be used to analyze analog problems, Millet notes that operators are neglecting tools they already have in their arsenals. The flap list of a CMTS, for example, logs the top 10 unreliable modems each day. By following this list on a regular basis, cable operators can determine where to troubleshoot. "Any problems are picked up sooner or later in the flap list," Millet says. Likewise, very simple instruments such as a signal level meter could be more useful if installers had more training, says Chris Swires, managing director of Swires Research, based in the United Kingdom. "Looking at digital systems in the U.K., if the networks are correctly set at proper levels and the signal-to-noise ratio is checked [by an installer], you will almost certainly get a good downstream picture. It should be perfect. It is as basic as that," he says. Monta Monaco Hernon is a contributing editor at Communications Technology. Email her at firstname.lastname@example.org. Digital Testing Brings Clarity With analog video, cable operators had certain warning signals that there was something wrong with the system, but digital video either works or it doesn’t. Cable modem service and telephony added in to the mix make the situation even more complicated. Testing becomes critical to ensure smooth operation. However, there are certain tests that cable operators may not be performing enough, for example, measuring phase noise and analyzing data from the CMTS. By adding these types of tests to their arsenals cable operators can reduce the number of customer complaints and on churn.