To accomplish these objectives, a line test system must meet certain standards. It must generate accurate, consistent data and measure many different parameters so the workforce that relies on the data can use it with confidence. It must make the data accessible to teams in different departments. The system's components must also be easily integrated into the telephone network so that it can interoperate with switches, DLCs, and any other types of equipment from different vendors. Finally, the line-test system should offer complete testing coverage of the network.

The Four Types of Line Testing Delivery

Copper Bypass Pairs

Copper bypass pairs are installed between a host switch with a test device and a DLC (see Figure 1). Remnants from an earlier generation, when all telco transmissions took place over copper wire, copper bypass pairs are now used in fewer and fewer networks. These pairs have two major drawbacks. First, their inability to allow testing over anything but copper wire is an obvious disadvantage as more and more telcos use digital microwave or fiber optics. Second, the results derived over them are inherently inaccurate because the bypass pairs add resistance, capacitance, impedance, and noise to the subscriber loop.

Figure 1. Copper Bypass Pair

Digitized Bypass Pairs

Digitized bypass pairs are installed between a host switch and a DLC system (see Figure 2). This type of system electrically detects the outgoing signal a test unit sends. The system digitizes the signal, transmits it to the far end, and attempts to replicate it at that end. It then digitizes the signal at the far end and sends it back to the test head. The major shortcomings of a digitized bypass pair are an inability to transmit complex signals accurately and reflect conditions at the far end. Compatibility with different vendor test units could be a problem as well. This limits the equipment's ability to do noise or load coil tests or anything beyond simple tests for basic DC troubles. This method could test more accurately if they were able to send a larger number of bits over the channel during a test. To do this, however, it would need more bandwidth than is usually available for testing. Providers, anxious for as much revenue-producing bandwidth as possible, are reluctant to allocate additional bandwidth for testing functions.

Figure 2. Digitized Bypass Pairs

Switched-Based (Built-in) Test Equipment

Many switches have built-in line test equipment. This test equipment is generally very limited in the types of tests it can perform. Because it is built by switching manufacturers for whom testing is a sideline, the equipment can be problematic. Service providers often have many different types of switches, each with its own syntax, command structure, and tests; learning how each switch's test equipment operates is difficult and time-consuming for employees. Also, with different brands of switches, differences in measurements arise, providing numbers that are not directly comparable. Built-in line test equipment has another disadvantage in that it is not sufficiently automated. On some types of built-in line test equipment a line equipment number is required to do a test; on other types, a phone number is necessary. In many cases, these different types of data must be correlated by hand—a very time-consuming and expensive task. Built-in test equipment is also expensive to run, because it requires a highly skilled operator who has been trained and certified to work on a particular switch. Yet another shortcoming of built-in test equipment is that it cannot test inward on a line. Built-in test equipment is typically less powerful, less accurate, and less versatile than a stand-alone line test system.

A Stand-Alone Line Test System

The stand-alone line test system is rapidly gaining in popularity because it performs a wide range of tests that have become more critical in the effort to reduce costs and improve customer service. It consists of a remote test unit that can be located at the key transition point in a telephone circuit—the source of dial tone to each copper pair. Testing at this point maximizes the types of tests that may be run, the accuracy of the tests, and the uniformity of the data. In each of these areas a stand-alone line test system offers significant advantages compared to copper bypass, digitized bypass, and built-in test equipment.

After surveying the four methods of line testing and the business objectives line testing helps to meet, we must examine the criteria for evaluating each of the line test systems. The first criterion to be explored is uniformity in testing.