Widespread installation of DSL has been impeded by physical constraints of the local loop (number of load coils and bridge taps, distance from the CO, etc.), by interoperability problems between products, and by spectral compatibility issues.

The problem with sending a high frequency over an unshielded pair of copper wires is that the electric field travels outside the wire. If the wires are too long, as most wires in local loops are, the signal at the end of the wire may become too weak to be understood. If the telephone company tries to mitigate this problem by increasing the power at the originating end, the signal tends to transfer itself to nearby wires in the same cable bundle. This transferring of signal is called crosstalk. In the telephone network, multiple insulated copper wires are bundled together into a cable binder. Adjacent systems within a cable binder that are sending or receiving information in the same range of frequencies can create significant crosstalk interference. This is because crosstalk signals combine with signals that were intended originally for transmission over the copper wire loop. As a result, a slightly different shaped waveform becomes transmitted.

While crosstalk was not much of a problem in the analog world, it has the potential to become a severe problem for DSL users. DSL requires much higher frequencies than analog; consequently, the number of lines subject to crosstalk or other interference increases significantly. Performance will be severely impaired if the DSL line is tied to a T1 line or other high-speed lines.

A second problem with deploying DSL is the use of bridge taps, which are branches that are attached to circuits to reroute the line to another location. According to Bellcore, 56 percent of the loop population has bridge taps. A bridge tap is any portion of the loop that is not in the direct path between the CO and the end user. For example, a bridge tap could be an unused cable pair connected at an intermediate point or an extension of the circuit beyond the end user's location. Telephone companies have used bridge taps so they could cheaply build in extra capacity in a neighborhood without knowing in advance whether the demand existed.

The number of load coils and bridge taps on the lines and the telcos' inability to locate them quickly without a costly analysis and mapping of all lines is a real problem without an easy solution. The local telecom customer's loop is said to have anywhere from 20 to 33 bridge taps along the line. About 20 percent of all lines also have load coils attached to them. Provisioning DSL service means removing all these load coils, bridge taps, and repeaters from the line.

Another way telcos try to improve the quality of the line and lower the costs when the line extends farther than 18,000 feet is to install repeaters and/or remote terminals (RTs) so that the signal can terminate at an intermediate point and then be backhauled to the CO or to a wire center over high-speed lines. These RTs are called digital loop carriers. A DLC is a remote unit that connects a number of subscribers to a CO. A single connection runs from the CO to a DLC. Copper pairs then connect the DLC to the end user. DLCs' key advantage is that they reduce the length and number of direct connections from customer premises to the CO.

One main obstacle to a successful deployment of DSL to the mass market is loop testing and qualification issues. Mass deployment of DSL technology is fraught with difficulties in addition to the technical limitations of the technology and the fierce competition by CLECs and cable providers. For carriers such as CLECs and ILECs to establish service, they must manage loop qualification and conduct monitoring, troubleshooting, and repair in a new local-loop environment that hosts voice as well as data services. The inability to prequalify copper loops remains a significant obstacle to a successful mass deployment of DSL. Prequalification is the testing of loops to determine if the loop is capable of supporting DSL transmission prior to attempting to provide the service to the customer and is critical because DSL deployment is dependent on the design and quality of the copper loop. The ability to qualify the loop without having to dispatch a technician to either the CO or the customer premises will result in significant cost savings to the service provider. As DSL growth begins to explode, automated testing and provisioning of loops is key to a successful deployment. The current manual methods of testing loops will not be able to keep pace with the explosion in demand for DSL. What might have been an adequate system when only 100 lines were being installed will crash under the heavy burden when thousands or millions of lines are installed.

Spectrum compatibility and management are critical issues that need to be addressed quickly. Development of spectrum compatibility standards is essential to reduce crosstalk and other forms of spectrum interference. The continued development of spectrum compatibility standards will help to minimize crosstalk, which often results in the degradation of the intended signal. Spectrum compatibility and management become significant concerns with the introduction of new high-speed services in a multiple-provider environment.

Spectral compatibility is a significant concern for carriers that are interested in deploying DSL. For example, if an ILEC and a CLEC offer DSL services that use different line-encoding technologies, and if their respective customers' loops are located next to each other in the same binder pair, the two technologies may unintentionally interfere with each other and interrupt the signals traveling over each loop.