对语音/数据/视频网络施工进行认证测试的重要性
Electrical Contractor 杂志阐明了为什么对电缆施工进行认证测试是必不可少的以及正确进行测试所需使用的工具。
- Electrical Contractor：福禄克网络公司铜缆测试市场部经理 Hugo Draye 认为，要了解已安装链路是否满足某个传输性能标准或是否能提供所需带宽，唯*的方式就是在施工完成后进行测试。
详见原文……
Certification Testers
by Jeff Griffin
Installers of voice/data/video networks know there is more to do than connecting system components and verify they are functioning. Before a new system can be put in service, certification tests must be conducted to confirm the network meets applicable industry standards.
The only way to know an installed link meets a certain level of transmission performance or will deliver the required bandwidth, is by testing after installation is complete, said Hugo Draye, copper test marketing manager, Fluke Networks.
“Certification testing serves as a record of the installed cable’s performance to verify that the cable system meets or exceeds the performance level that is required for the network,” said Tim Kopp, Greenlee Textron VDV technical application specialist. “This record is often required to obtain the cable manufacturer’s advertised warranty, and it can be used to help troubleshoot the system at a later date if the performance degrades for any reason.”
To efficiently and accurately perform certification testing, installation personnel must understand the testing processes and the equipment necessary to do the job properly. What testers will be needed depends on network design and the types of cabling it includes, and it is not unusual today for networks to incorporate twisted copper, fiber optic and coaxial cabling.
“For a typical commercial building, installers may need to install multiple cable types to support telephone network (CAT 3 cable), copper cable for a data network (CAT 6 cable), fiber optic (multimode) backbone between floors and coaxial cable for a video-security network,” said Caroline Chen, an Ideal Industries product manager.
Tests vary with types of cabling
Required certification tests vary by system and cable type.Twisted-pair testing requirements have many parameters due to the multiple conductors. Wire map, length, attenuation, near-end cross talk (NEXT) and return loss are just a few tests required for certification.
“Coaxial and fiber are more straightforward due to the cable design. Coaxial testing can vary with the application, but attenuation, length and noise level are common examples of coax tests. Fiber optic cable testing typically consists of length, attenuation and propagation delay,” said Kopp.
For twisted copper cable, certification testers evaluate performance based on industry standards including NEXT, FEXT (far-end cross talk and return loss. Test equipment directly measures these parameters and calculates others from these measurements. Calculated parameters are attenuation-to-cross talk ratio (ACR), equal level far-end cross talk (ELFEXT), and power-sum parameters such as PSNEXT, PSELFEXT and PSACR.
Certification testing uncovers mismatched, faulty or noncompliant components, and cabling anomalies undetectable by visible inspection or simple continuity testing.
“Even though all components in a link meet their respective performance standards, and the simpler verification tools (continuity or wire-map testers) provide a passing result, the link can still fail to meet the link-transmission specification because of poor workmanship in terminating the cabling at the connecting points,” said Draye.
The most important link characteristic—signal strength at the link end—decreases with the length of the link, Draye said. Therefore, the total length of the cabling link and, more important, the total signal loss through the link [called insertion loss, formerly called attenuation] must be measured and verified.
Testing fiber optic cable is simpler. Multimode optical fiber and single-mode optical fiber are used. Multimode fiber links require a different test set than single-mode links. The wavelengths and launch conditions are different.
“We do not have to worry about a complicated signal-to-noise ratio model that gives rise to a large number of test parameters,” said Draye.
“The noise disturbance in fiber can almost be neglected. Instead, bandwidth limitations are an inherent characteristic of the type of fiber used and the manufacturing process of the fiber,” Draye added. “Tier 1 fiber optic field certification is limited to signal-loss measurement, verification of the polarity of the pair of fiber strands and a length measurement.
“Recently, new standards have been developed that define a Tier 2 test suite for fiber optic links. Tier 1 has traditionally comprised the field-certification requirements for fiber optic links. Tier 2 adds a qualitative ‘picture’ of the link by collecting an OTDR trace of the link-under-test,” said Draye
Coax test standards are much less precise. The more prevalent deployment of coaxial cabling today supports video distribution such as cable or closed-circuit TV.
“These industries have not defined a generic cabling structure that would lend itself to application of independent certification of the transmission medium,” Draye said. “Instead, the performance requirements for coaxial cable for video distribution must meet proprietary or situation-specific consureditions. Legacy Ethernet installations using 10BASE-2 (‘Thinnet’) or the older 10BASE-5 (‘Thicknet’) remain viable and must conform to the requirements of the applicable section of the IEEE standard 802.3. Certification tools to troubleshoot legacy network systems suffice to test coaxial installations.”
Multipurpose Testers
As with other products used for VDV and electrical work, manufacturers are offering a growing selection of instruments that can perform multiple tests, and Chen said many multifunction testers are available that can test all network functions.
Kopp said most multifunction testers can perform at least minimum (continuity, attenuation) testing on all three types of cable. Depending on what types of systems are using the cabling, a multifunction tester may or may not perform all the required tests for the cable application.
Twisted-pair certification testers equipped with the proper options can test all three cabling types discussed, excluding Tier 2 testing of fiber optic links, Draye said.
To gain full advantage of multifunction integration, the device must perform the test and certification of each cabling type in full compliance with the applicable standards in order to truly gain the advantage of this multifunction integration.
When determining whether a multifunction tester is preferable to using individual testers, Draye advised users to evaluate the time required to make the switch from one mode of operation to another. When the tester allows a quick and easy switch, the user benefits from this flexibility and can eliminate many extra steps. However, he added, most important is testing an installation to the level the client paid for, and functionality should never be allowed to trump performance and accuracy.
Documentation required
Documenting test results is essential.
“Certainly a meter must be able to perform all the tests required by standards and also must come with software enabling the user to print reports to document that all the links have been tested and that they meet requirements,” said Scott Black, product manager, ExTech Instruments.
Therefore, appropriate software is necessary to test results and prepare necessary copies of test results in printed or electronic formats. Some testers accommodate removable memory cards that increase the device’s memory capacity and allow the option of storing project data on individual memory cards.
Changing standards
The Telecommunications Industry Association (TIA) within the framework of the Electronics Industry Association (EIA) is the major North American industry organization that develops, publishes and maintains cabling standards, and those standards change with new technologies.
“It is safe to say that new standards are coming,” said Kopp. “Whether they are coming in the next 12 months remains to be seen. One only needs to look back at the timeline of the latest Category 6 ratification to see that new standards can take years to be implemented. Category 6 had been ‘right around the corner’ for years before finally being ratified in July 2002.”
Chen noted the United States standards committee is reviewing a draft CAT 6a test to 625 MHz and may move to 650 MHz. In Europe, the ISO-F requires testing to 600 MHz.
A recent change in the fiber optic world created the differentiation between Tier 1 and Tier 2 testing, Draye observed.
“The ‘talk of the town’ in the data communication market is 10 gigabit Ethernet over twisted-pair copper cabling,” he continued. “This work has by no means reached the level of a standard yet and will likely not become a standard before 2006. Twisted-pair links will need to be tested beyond the current 250 MHz upper limit for Cat 6. We will very likely see requirements to test the link parameters up to 625 MHz. Therefore, to ‘future-proof’ your investment, it would pay to look at a tester that supports these higher frequency measurements right now or that can be upgraded for a modest amount to support the higher frequencies when this capability is needed.”
About This Author
Jeff Griffin
GRIFFIN, is a construction and tools writer from Oklahoma City.
Phone: 405-748-5256
Email: up-front@cox.net