DSL tests focus on copper
Richard A. Quinnell, Contributing Technical Editor -- Test & Measurement World, 3/1/2006
Wireless communications seem to be getting all of the attention these days, but there still remains a huge market and installed base for copper-carried DSL services. The market's increasing interest in voice and video over broadband is changing the focus of field test for DSL, however. Instead of being satisfied with providing a "best-effort" DSL service, test is now focusing on maximizing the performance of the copper network.
The original test requirements on DSL installations were fairly basic. The two key questions were "Can the customer surf the net?" and "Is it fast enough?" Answering those questions involved looking at the cable's signal characteristics and verifying they were within set limits, then ensuring that the DSL modem could synchronize with the line card at the central office.
This "best-effort" approach held true as ADSL saw wider deployment, but it started to shift as installations migrated to ADSL2, ADSL2+, and VDSL2 to gain reach and bandwidth. The major discontinuity in test requirements came when service providers wanted to offer voice and multiple channels of video as part of a bundled service.
"One change was an increasing focus on service quality to support the more complex services of VoIP and IPTV," said Jon Beckman, product line manager at JDS Uniphase. "The other change was an ever-increasing focus on the physical plant—the cable network. The self-install model [for customer premise equipment] now demands a lot more of the copper plant."
The focus on service quality means that DSL field test equipment now requires more capability to measure and help diagnose problems. Tools have had to go from making simple good/bad decisions to helping correlate service quality with physical layer characteristics and to isolate the location of problems in the network.
Taking a close look at the network cabling is an integral part of that effort. "There is more focus on drilling down to the copper layer," said Beckman. "The evolution of DSL has driven an increase in the frequencies the cables must carry, from 2.2 MHz for ADSL to 30 MHz for VDSL. That means you need to look at the health of the copper plant to find problems such as corroded splices and the like."
New equipment to address all these needs must thus offer both greater automation and more capabilities in measuring physical characteristics. The HST3000 from JDS Uniphase, for example, can perform leakage, balance, and TDR testing on the cable; monitor datacom interfaces; test data layers such as PPP and FTP throughput; and serve as a Web browser to verify connections. It also offers VoIP phone emulation and a host of QoS measurement parameters, and it can emulate a set-top box to verify IP video services. Other similar products include the CoLT 450 from Exfo and the Tech-X field tester from Spirent Communications (see p. 60).
These products cannot do everything, of course, and to mitigate their limitations, many field units capture live data and store it for analysis in the lab. The units can also log data to help engineers analyze trends in the network and cable plant's behaviors.
Ultimately, however, these tools will continue to increase in both automation and in performance levels. As the xDSL installations get ever faster and more capable, so will the tools for field test.
