What you should know about testing Category 5 cabling

Dec. 1, 1997
Understanding the use of Cat 5 cabling and how to test its performance are two important aspects in telecommunications today. Category 5 unshielded twisted pair (UTP) is constructed of four pairs of 24 AWG copper conductors with thermoplastic (insulated conductors enclosed by a thermoplastic jacket).Telecom consultants recommend that Category 5 UTP cabling carry both voice and data in a building's

Understanding the use of Cat 5 cabling and how to test its performance are two important aspects in telecommunications today. Category 5 unshielded twisted pair (UTP) is constructed of four pairs of 24 AWG copper conductors with thermoplastic (insulated conductors enclosed by a thermoplastic jacket).

Telecom consultants recommend that Category 5 UTP cabling carry both voice and data in a building's structured wiring system. This way, the circuits can be reconfigured to accommodate all future needs while also providing the ultimate in circuit allocation flexibility. Many users have been following this advice.

Analysts project more than 100-million Cat 5 installations yearly at the end of this decade. And the contract documents for these systems increasingly are calling for the testing and/or certification of the installed cable to make sure that any current or future high-speed LAN technologies will successfully run on these wires. Thus, testing confirms that the cable can perform properly based on proper installation and the characteristics of the cable.

The Telecommunications Industry Association (TIA), a subgroup of the Electronic Industries Association (EIA), has created standards for structured cabling that define a generic cabling system for many popular network applications.

Specifically, the TIA/EIA-568A Commercial Building Telecommunications Wiring Standard gives performance requirements for backbone and horizontal cabling, including the patch cords and panels, terminators and connectors. Originally passed in July 1991, this standard has been revised, and now defines what electrical characteristics the cabling should have. In addition, the standard spells out the electrical parameters that determine transmission performance, such as bandwidth, attenuation, crosstalk and signal to noise ratio. Also, the standard incorporates TSB 67, which has information on Category 5 field testers, the test instrument pass/fail criteria for each of the parameters, the test configurations, and minimum tester accuracy for certifying Category 5 cable.

Four test functions that must be done on Cat 5 cabling are: * Attenuation-This 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. The measurements are taken at various frequencies, or steps. The step size for attenuation is no more than 1 MHz.

* Near end crosstalk (NEXT)-This is unwanted signal coupling between cable pairs occurring mostly at the near end to the cable where the transmitted signal is applied and where the signal is strongest. Or 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, so the higher the value, the better. Because it is 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, in which the polarity is reversed at one end; transposed pairs, in which two connectors are wired to different positions at each end; and split pairs, in which the continuity of a connection is correct but wires from two different pairsare used. While this cable has correct pin-to-pin continuity, it will cause errors in data transmission due to an excessively high amount of crosstalk.

* Cable length-is important because if the cable length specification is exceeded, marginal performance may result.

Before continuing, let's stop and study why specific test results for a number of cable characteristics are so important today. First of all, LAN cabling systems continue to require higher transmission rates. With newer applications and the integration of the Internet into the business operations, a telecommunications information manager must know how well the cabling plant will perform.

Also, with so much talk about the development of faster, higher throughput networking standards, and promulgation of new and confusing technical terms, the user should understand the difference between Mbps (mega bits per second) and MHz (megahertz). Mbps is a measure of the rate at which binary data can be transmitted; this rate is expressed in millions of bits per second and abbreviated as Mb/s or Mbps. On the other hand, MHz expresses a frequency of a pure sine sinusoidal signal. A bandwidth of 100 MHz indicates that a cable, or cable pair, can transmit sinusoidal signals with a frequency of up to 100 MHz, within an acceptable level of performance. The performance is defined in what is called the Bit Error Rate (BER).

The specific relationship between Mbps and MHz for a network cabling system is determined by the signal encoding schemes used for the binary data. The single encoding scheme or protocol for 10BASE-T Ethernet systems and 16 Mbps Token Ring networks imposes a one-to-one relationship between bandwidth and data rate. This means that a 10BASE-T Ethernet system requires a cabling system supporting a 10 MHz bandwidth (the transmission performance is specified in the IEEE 802.3 standard). Similarly a 16 Mbps Token Ring requires a cabling system that supports a 16 MHz bandwidth (the transmission performance is specified in IEEE 802.5). This one-to-one relationship between the two terms is not a general requirement but is imposed by the Manchester encoding schemes used by the two protocols.

For example, the Fast Ethernet standard called 100BASE-TX specifies a different signal encoding scheme (called MLT-3), which enables it to transmit 100-million bits per second with a bandwidth requirement of 80 MHz (specified in IEEE 802.3u).

Let's go on. The higher the data rate in Mbps, the shorter the time frame for transmitting one bit and the faster the rise and fall times for the pulses being transmitted. Thus, a good way to check if a cabling segment can transmit such short pulses with fast rise and fall times is to inject a sinusoidal signal with the proper frequency.

Therefore, a sinusoidal signal that is transmitted must have a specified level of quality. The quality is determined by attenuation, crosstalk and/or the combined effects of these two parameters, called Attenuation to Crosstalk Ratio (ACR).

All high-speed networking standards define a Signal to Noise Ratio (SNR) and a maximum noise threshold. But pair skew and propagation delay characteristics are important supplemental requirements of 100BASE-T, 100-BASE-VG and for ATM above 100MHz. Pair skew applies to technologies using multiple pairs for signaling. Essentially, signals are divided between conductor pairs and must be reassembled at the receiving end. If they arrive at different times, skewing of the signal occurs.

Propagation delay, also called velocity of propagation, refers to the time it takes for the signal to travel to the receiver. The receiver has built in equalizers, which attempt to amplify the received signal based on an assumption of the attenuation of the transmitted signal.

Tester manufacturers differentiate their products (and they continue to have modifications and upgrades) by talking about a unique feature, or features, related to field testing the cabling system.

All of the Cat 5 hand-held testers mentioned below do the four required tests as an autotest, which means that pushing only one-button initiates all four tests-automatically. A test unit can store the results of up to 500 autotests, and the test results can be downloaded to a PC, or a printer.

Cable installers must carefully consider their specific needs when choosing a cable tester, recognizing the efficiencies/economies gained if an organization standardized on a particular model (the field personnel become familiar with the product and can do their work faster). For example, cable installers usually require Level II accuracy to test basic links. Level I often suffices for network managers who use cable testers to troubleshoot channel links. A basic link is much shorter than a channel link; it does not include the user's patch cord and other components.

*The LANcat Vx tester from DataCom Technology, Everett, Wash., gives installers the ability to run two-way, half duplex voice communications over the test link. This Level-II-compliant tester includes a microphone and earplug with audio jack. The unit also features talk pair, auto-discovery and call alert. It is very fast in doing the Level-II-compliant autotest, including automatic 2-way NEXT measurements.

*The DSP 100 Digital Lan Cable Meter from Fluke Corp., Everett, Wash., extends measurement capability to 155 MHz and measures delay skew. It is the first hand-held tester to use digital signal processing (DSP). An upgrade to the tester includes an optional measurement capability to 155 MHz, using ACR as an indicator of cable performance.

*The Penta Scanner 350 from Microtest, Phoenix, Ariz., offers performance grading by defining Category 5 links in quality bands, each of which is 3 dB in width at 100 MHz. Each band represents a signal power difference of two times the next lower band. Performance is determined by worst-case peak values. The tester can measure length even into a hub and on individual wire pairs.

*The WireScope 155, from Scope Communications, Northborough, Mass., offers a skew reading that will identify cables that could pose a problem when the network operates. The unit measures one-way propagation delay for each of the four pairs and reports total delay and delay skew between pairs. Category 3, 4, or 5, screened, or shielded cables are tested by plugging in a specific test adapter.

*The Lantek Pro XL from WaveTek Corp., San Diego, Calif., can speed up the testing process by doing an autotest in less than 25 sec. The unit also runs line-map, direct-current loop resistance, length, capacitance, dual near-end crosstalk, attenuation and ACR measurements.

The manufacturer recently introduced the family of LT 8000 testers, which allows the user to select a particular model with only the features that are needed. Until recently, these instruments have had prices near $5000, but the LT 8000 Cat 5/Class D tester breaks the $2000-price barrier for a full featured LAN cable tester.

About the Author

Joseph R. Knisley

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