The emergence of new cabling standards are testing the efficiency of today's field testers and furthering the development of testing procedures.
January 1, 2001
In the realm of telecommunications and network cabling, it is vital to have a standardization of products, systems and related testing procedures to assure the reliability and life of a system. Without standardization, networks would cease to co-exist and it would be impossible to certify or properly allow consistent communication between Local Area Networks (LANs) and Wide Area Networks (WANs).
System designers and installers no longer want to be tied to single-company proprietary schemes. They want choice, yet want to be assured that the equipment from one company will work with the equipment from another company. Therefore, structured cabling systems tied together by meeting specified standards assure that cabling will meet both today’s and tomorrow’s needs.
TELECOM STANDARDS HISTORY
Prior to 1985, telecommunications cable was ran on POTS (Plain Old Telephone System) or analog voice transmission lines, and the industry was not particularly concerned with high-speed cabling or standards. However, evolving technology and today’s gigabit speeds and high-bandwidth applications are pushing data transmission performance to over 100 Mbps+ data signals over UTP cable and 10 gigabits over fiber. Cable and system hardware manufacturers have developed products to allow the transmission of these data signals as well as the certification of these systems under the scrutiny of changing industry standards.
By the beginning of the 1990s, the newly formed EIA (Electronic Industry Association) and TIA (Telecommunications Industry Association) began defining standards for how buildings should be cabled for telephones and data. Both TIA/EIA-568 from North America and ISO/IEC 11801 internationally, were developed to address the cabling in commercial buildings.
Today the TIA/EIA, which is part of ANSI (American National Standards Institute), specifies telecommunications standards to outline the development, planning, design and installation of generic cabling systems that support a wide range of building sites and applications that include, voice, data and video. The TIA TSB-67 (Telecommunication Systems Bulletin) was published for North America to specify field test requirements for installed UTP that supports the cabling standards.
These standards define and specify performance of cabling and cabling links up to 100 MHz. TIA link performance standards are specified as Categories. International specifications use the “Category” designation for components and cables, but use the term “Class” for link performance:
TESTING AND CERTIFICATION
Cabling is the basic building block for LANs. Cabling challenges arise as these networks reach 1000 Mbps and beyond, especially in the realm of unshielded twisted pair (UTP), copper-based networks.
Cabling issues account for up to 50 per cent of all LAN failures, and the resulting network downtime costs organizations millions of dollars in lost productivity, idle resources and lost revenues. Therefore, it is essential to understand how to get the best performance from your cabling investment and correct faults as soon as they occur.
Installing and maintaining a reliable physical cable plant is crucial to the well-being of today’s mission-critical LANs. After the cabling system is installed, it should be tested and certified so that it meets the performance specifications of the most up-to-date standards. Field testing verification includes quick and simple tests to verify continuity. Certification, on the other hand, compares these measured values to standards-derived limits to see if the measurements are within the limits specified. Certification is especially important for high-speed applications.
Today’s sophisticated testers provide the installer with flexibility and simplicity in performing autotests. One challenge for tester manufacturers is to keep testers up to date with the latest standards. There is a great deal of confusion about cable measurements for high-speed cabling and how to interpret evolving standards. Following, are some of the most recent standard updates and the effects on cabling and testers that concern us today:
In February 2000, the latest addendum to 568A was published, which states that Category 5e is the accepted minimum horizontal data cabling and replaces Category 5. Previously, Category 5 testing requirements only encompassed a minimum of measurements, which included attenuation, crosstalk, length and wiremap. The requirements for testing and certifying Category 5e includes all of the tests performed for Category 5 and additional tests of delay skew, return loss for bi-directional signals, NEXT (Near End Crosstalk), ELFEXT (Equal Level Far End Crosstalk) and PSNEXT (PowerSum Crosstalk). With the addition of these new measurements and the change in standards, it is imperative that a field tester is up to date when automatically performing these tests.
The TIA 568B is a three-part addendum to 568A and addresses both UTP and fiber. The TIA 568-B-1 incorporates the Category 5e performance levels, as stated in 568A, as well as 50/125 mm fiber and allowance for alternate fiber connectors other than SC. TIA 568-B-1 also eliminates support for Category 5 in horizontal cabling and replaces it with Category 5e as the minimum acceptable performance level.
TIA 568-B-2 is tied in with B-1 to reference cable and connecting hardware requirements for 100 ohm UTP for both Category 5e and Category 3. This standard should be published in the first quarter of 2001.
TIA 568-B-3 includes specifications and requirements for both single and multimode fiber optic cable, connecting hardware and links.
Although many installers are specifying and installing Category 6 cable, the standards are still in draft form (version 7) and are currently being reviewed. When approved, it will become Addendum #1 to 568B. Current issues that affect Category 6 and testing procedures include: higher frequencies to be measured, agreement on pass/fail marginal bands, increased measurements, connector non-interoperability and additional memory needed for storing and documenting the much larger test results.
When comparing Category 5 to Category 6 testing, only 21 measurements were needed for Category 5. Looking at the worse case data points for attenuation, crosstalk, length and wiremapping determined whether the cable passed or failed up to 100 MHz for Category 5/5e.
With Category 6 there are more tests (see Category 5e in 568A) and more test combinations — 77 of them. These measurements require over 35,000 data points stored per test, so there is much more data to manage and a much greater possibility of failure. This means that testers must have the capability to record, store, download and manage this data quickly, without taking the tester out of the field.
BASIC AND PERMANENT LINK TESTING
Draft 5 of the Category 6 standard used the traditional Basic Link (BL) definition, but in Drafts 6 and 7 a change was made to the Permanent Link (PL), in order to comply with international standard harmonization efforts. The main difference is the change in reference plane between the PL and BL. When making PL measurements, field testers are supposed to factor out the effects of the adapter cord including length, delay, skew, NEXT, FEXT and return loss. Figure 1 (see p. 48) illustrates the change.
The essential difference between the two models is the addition of the cord between the field tester and the first connection. For Category 5 and 6 measurements, the test cord can be considered to have zero NEXT. Most field tester manufacturers use shielded Category 7 cable adapter cords. This cable has each pair individually shielded, with crosstalk so low that the cable itself does not add any NEXT to the measurement.
With the switch to the PL, the installer loses the beneficial effect of the attenuation in the test cord. This test cord reduces the apparent magnitude of the NEXT at the first connector by up to 2 dB at 250 MHz; the “free” margin is now gone. At frequencies of up to 100 MHz and Category 5 levels, this has not
been much of an issue; at 250 MHz with Category 6 signals, this effect becomes significant.
It can be difficult to keep track of this rapidly changing area of technology. Many of these new standards will put the field testers to the test with added measurements, higher bandwidths and a need for higher accuracy in field measurements. The tester that can best manage these differences, discern between the BL and PL, and make measurements with the highest level of precision, will provide the most efficient testing and certification.CS
Carol Everett Oliver, principal of Everett Communications, Ashland, MA is a freelance writer for Microtest and for the cabling industry. Mark Johnston, RCDD, is the director of technology development at Microtest, Phoenix, AZ. A voting member of the TIA, Mr. Johnston also participates in ISO and CENELEC cabling standards work.
16 MHzCategory 3Class CClass C
100 MHz Category 5/5eClass DClass D
250 MHzCategory 6Class EClass E
At a data centre, Rick Smith of Compel, Phoenix, AZ, certifies that every port meets the latest standards prior to the installation of networking equipment at a colocation site.