PDF of this tutorialExpertise in Test-Head-Transducer Design and the Link to Accuracy in Testing
Transmission, noise, and ISDN tests are very important in today's networks. To build a system that can perform such tests accurately, a manufacturer must have strong core competencies in test-head transducer design and measurement algorithms. To develop these competencies, a manufacturer needs extensive experience in developing line testing systems. Unfortunately, the test devices used in many digital switches have not been developed by manufacturers with expertise in line testing systems. Rather, they were developed as add-on features, providing only a limited set of testing options, and little or none of the data management capabilities needed to automate the testing effectively. A vendor with expertise in line test systems is able to provide additional tests that greatly enhance the diagnostic abilities used in measuring telephone lines.
Load Coils: Preventing Attenuation
Specialized knowledge in line test systems is also invaluable in developing systems that test load coils on long loops. Load coils are used to reduce attenuation on a copper pair, a condition in which the capacitance of the telephone line decreases the signal as it is transmitted over a distance. A user experiences this decrease as a loss of volume. Load coils are used in approximately 20 percent of the outside plant pairs in North America. Load coils are installed in specific patterns—typically every 6,000 feet—to transform a nonloaded pair into an inductive-capacitive filter. With load coils, voice-frequency signals are passed with lower attenuation than if there were no coils present. Proper loading is critical for data transmission in Internet access and in fax, data, and other nonvoice calls. The load coils, while reducing voice frequency (VF) in-band loss, have the side effect of increasing VF out-of-band loss.
Attenuation can be a very serious problem in transmitting voice and data. In a typical situation, a copper pair is located 12,000 feet from a central office (CO) and will use 300-Hz to 3,300-Hz bandwidth to transmit at normal voice frequency. The transmission will have some amount of loss, as a result of the resistance and capacitance of the cable. This loss is logarithmic in nature on nonloaded cable. Without load coils, the loss would be so severe that at 36,000 feet from the CO, callers would need to shout in order to communicate with each other, particularly if the transmission was at higher frequencies. It is essential that load coils be deployed correctly. If a cable splicer fails to install a load coil at an appropriate place, attenuation will take place. If the cable splicer puts two load coils where there should be only one, a double load results, with serious effects on transmission characteristics, especially for modems.
Load Coils and Digitized Bypass Pairs
Digitized bypass pairs cause difficulties during load coil tests, because the test-head transducer is not located where the copper pair originates. This means the digitized bypass pair cannot handle the frequencies—200 Hz to 10 kHz—required for adequate testing. It is necessary to have a frequency range of this size to identify correctly loaded-cable as well as fault-cable characteristics. This bandwidth range is wider than the one used for the voice frequencies that are normally passed via pulse code modulation (PCM) links of digitized bypass pair devices.
Stand-Alone Systems and Load Coils
Only a full metallic connection to the line under test, such as a stand-alone system test head provides, can give accurate loaded results.
