Understanding high performance UTP cable

Contractors and users now have a valid reference for pass/fail criteria when field testing installed data/communications cabling.Installers and users of high-speed, 100-ohm Category 5 unshielded twisted-pair (UTP) cabling systems should know about the new standard that's been developed and adopted for testing this high performance cable used in local area networks (LANs). An installer should know

Contractors and users now have a valid reference for pass/fail criteria when field testing installed data/communications cabling.

Installers and users of high-speed, 100-ohm Category 5 unshielded twisted-pair (UTP) cabling systems should know about the new standard that's been developed and adopted for testing this high performance cable used in local area networks (LANs). An installer should know what can, and cannot be tested and certified; however, it's taken a few years for the standard to be developed.

Existing EIA/TIA standards

TIA/EIA-568, Commercial Building Telecommunications Wiring Standard (July 1991) specifies performance requirements for backbone and horizontal cabling, including the patch cords and panels, terminators and connectors. It also covers pin assignments and recommended distances and topologies.

The promise of 568 was to provide a quality standard against which any cabling installation could be rated. This was particularly important to someone planning for the future, someone who might not use the full capacity of a Category 5 cabling system for several years. (Category 5 cable can be used for data transmission speeds up to 155Mbps, at up to 100 MHz.)

The standard has no guidelines for testing the installed cable; it assumes that anyone who follows the procedures will achieve what was required. In practice, however, connecting and crimping quality varied, causing many installations that were theoretically Category 5 to deliver lower results.

So, not long after the 568 standard was finalized, an update was issued by TIA/EIA that provides information for high-speed LANs. The overwhelming acceptance of UTP as the preferred wiring method for horizontal runs from the wiring closets to work spaces led to the release of the TIA/EIA Telecommunications Systems Bulletin (TSB) 36, Additional Cable Specifications for Unshielded Twisted Pair Cables (November 1991).

But, even up to that point, all of the standards focus on the performance parameters of individual components, not on the performance of an installed link or cabling run (individual components connected together).

The new testing standard

To address this limitation, a new standard recently has been issued: TIA/EIA TSB 67, Transmission Performance Specifications for Field Testing of Unshielded Twisted Pair Cabling Systems (October 1995). It identifies what to measure and how to test installed links, and defines two horizontal link models, as shown in Figs. 1a and 1b. Also, four specific tests to make sure that the cable is properly installed are detailed.

The two links mentioned are the basic and channel links. The basic link (also called the contractor link), as shown on page 40 in Fig. 1b, should not exceed 90 m in length. It should include only the horizontal cable from the wall outlet, or transition point, to the first punchdown (typically the portion of the link for which the cable installer is responsible) plus the test cords supplied with the field tester.

The channel, which is also called the user link, as shown on page 40 in Fig. 1a, should not exceed 100 m in length. As shown, the basic link consists of Cabling Segment C, the horizontal wiring from the horizontal cross connect to the wall outlet in the work area, as well as two 2-m long test equipment cords. A field tester is connected to the basic link using these tester equipment cords.

The channel is a longer end-to-end run that includes a second punch block (in the horizontal cross connect) and a Jumper B. Note that Segment D is the transition cable from the wall outlet to an in-furniture or under-carpet connection.

One of the issues addressed by TSB-67 is the accuracy specification and measurement performance of the field testers. Annex A (a normative annex) of TSB-678 defines two levels of performance requirements; one is called Level I, and the more stringent and higher performance level is called Level II. While most manufacturers of test equipment only specify the performance of their testers for the measurement of the basic link, the standard states in Annex A that "the accuracy level shall be specified for both the basic link configuration and the channel configuration."

The basic content of TSB-67 is the four - part test, which consists of measuring the following.

* Attenuation, which is the reduction in signal strength over the distance the signal is transmitted. The conductor and insulation are the major contributions to cable attenuation. The jacket composition makes a minor contribution to attenuation in UTP cable designs. Attenuation is both frequency dependent and cable length dependent. In general, the higher the signal frequency and/or the greater the distance, or length, of cable, the greater the attenuation. Thus, the measurements are taken at various frequencies, or steps. The step size for attenuation is no more than 1 MHz.

* Near end crosstalk (NEXT), which is the unwanted signal coupling between cable pairs occurring mostly at the near end where the transmitted signal is applied and where the signal is strongest. Put another way, NEXT is a measure of how much signal energy is radiated from one transmitting pair and capacitively coupled to an adjacent pair (of conductors). Thus, NEXT is defined as the ratio of the strength of transmitted signal to the coupled signal measured on the adjacent receiver pair. The higher the value, the better the cable performance. Since it's frequency dependent, NEXT is also measured at multiple frequencies (steps) over a specified frequency range. Crosstalk is the major factor limiting UTP performance.

* Wire mapping consists of checking connector pinouts to detect faulty pair wiring on all four-pair connections (pins) of the cable. The most common errors are crossed pairs, where the polarity is reversed at one end; transposed pairs, where two connectors are wired to different positions at each end; and split pairs, where the continuity of a connection is correct but wires from two different pairs are used. [ILLUSTRATION FOR FIGURE 2 OMITTED] While the cable has correct pin-to-pin continuity, it will cause errors in data transmission due to an excessively high amount of crosstalk.

* Cable length, which is important because, if the cable length specification is exceeded, marginal performance may result. Length of a conductor is generally measured by a Time Domain Reflectometer (TDR).

The installer should be aware that a tester measuring cable length might have what is called a Dead Zone, because the unit sends out a signal and waits for a response (or echo). Problems arise when the signal travels only a short distance. If the tester is still emitting the signal when the echo comes back, the tester may not be able to read the response. Thus, at times it may not be possible to read the length of a short cable. A solution is to add a 3-ft or longer patch cable to the tester.

All handheld cable testers provide the four required tests as part of what is called an autotest, which means an installer has to push only one button on the instrument to automatically initiate all four tests. And, usually, the unit also performs other tests, along with the four required ones. All of the available testers can store the results of up to 500 autotests, with the test results capable of being downloaded to a PC or a printer. Thus, the installer can print out a test sheet that verifies each circuit, thereby certifying to the owner that the wiring was done correctly and that the network will work properly when it is up and running.

Understanding the products

Generally, these testers were developed before the TSB-67 standard was finalized, and they continue to have modifications and upgrades. But, because transmission measuring is a highly technical subject, marketing hype is confusing buyers. Thus, a buyer should carefully evaluate all of the products available before selecting a field tester, making sure to get a unit that has specific desired features. The choice of a Category 5 cable tester comes down to tradeoffs among a number of characteristics. For that reason, manufacturers are differentiating their products by touting a variety of unique features that relate directly to some aspect of the test standard.

All of the available test instruments are designed to reduce measurement error while still supporting both channel and basic link testing. Some makers have selected to avoid the modular-jack-contributed measurement error by using a special connector with lower NEXT.

One model has plug-in instrument modules, pointing out that the most significant error contributors are residual crosstalk and output signal balance. Another model allows the user to measure NEXT and Attenuation to Crosstalk Ratio (ACR) at both ends of an installed network cable link while remaining at one end of the cable.

One manufacturer claims that network managers (users) can determine whether their cable plants can support fast networking technologies. The device qualifies 20 LAN specifications during autotest, including the recently added 100VG-AnyLAN, 100BaseTX, 100BaseT4, 25M bps ATM and ISDN.

Another maker claims it has the first handheld tester using digital signal processing (DSP). DSP allows the unit to test the Level II accuracy of both the basic link and the channel, something denied to the other testers because of NEXT caused by RJ-45 (8-pin modular) connectors. The unit is also fast, a key feature when a large number of installed cables are involved.

Cable installation problems

Why all this concern over performance of the installed cable? Even though it may be delivered from the manufacturer meeting all specs, a cable can be easily damaged during installation. Thus, good installation practices include using gently curving turns to avoid minimum bend radii and loosely cinched cable ties to keep cable geometries from being deformed. Cable elongation is another pitfall to avoid. Additionally, the twist in the twisted-pair construction is an important feature in high performance cable construction. The twisted conductor pair cancels the electromagnetic interference (EMI) that accompanies current flows through the conductors. The tighter the twist, the better the cancellation. Thus, Category 5 twisted pairs shouldn't be untwisted more than 1/2-in. anywhere on the link. If the installer strips the cable and pulls the wires apart excessively in making the termination, the strict specs of Category 5 are violated, and the circuit will experience excessive crosstalk.

In voice circuits, it's common practice to strip off 3 in. of jacketing, pull the conductors apart for easy access, and then connect them to the punch block, wall plate or other termination. This is an unsatisfactory technique for a Category 5 installation.

Rewiring is an expensive procedure; but without a specification, accuracy statements about testers and a discussion of the uncertainties regarding their measurements leave contractors at risk of having to reinstall channels because of equipment shortcomings. Also, end users are not fully aware of the capabilities of their cabling systems. Although TSB-67 helps ensure proper channel measurements, it still needs to cover return loss measurement and ambient noise specifications, since they also affect channel performance. So, the standard will continue to evolve.

The new standard is available from Global Engineering Documents, 15 Inverness Way East, Englewood, Colo. 80112-5704; Phone: 800-854-7179.

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