Digital Is your outside plant ready?

As the demand increases for bandwidth-hungry applications such as Internet access, video on demand, distance learning and telecommuting, new digital services will be pushed further out on the cable plant. Large-scale deployment of these new digital services will require a faster, easier way to determine if a subscriber's existing POTS line can support high-speed requirements.

Two primary digital services being deployed in today's networks are ISDN and digital subscriber line. ISDN was introduced in the 1980s, and although the concept did not catch on immediately, it has now begun to gain in popularity. Telephone companies are beginning to offer ISDN and other digital services to many parts of the country.

In basic terms, DSL provides customers with the benefits of high-speed digital access to the telephone network. Depending on the type of DSL service, the data rate and loop requirements vary. Most DSL deployment is currently aimed at the business community, which primarily uses high bit-rate DSL and symmetrical DSL. Residential trials and commercial deployments are in the early stages, with asymmetrical DSL the prime offering. The advantage of DSL is its ability to remove data traffic off the public switched network, which was not originally designed for the long call lengths of data usage. Some services such as ADSL allow the standard analog POTS signal to ride below the digital signal on the same pair. At either end it can be split off from the digital portion, allowing the traditional reliability of analog POTS.

As these new digital services find their way onto twisted pair, they place increasing demand on the cable plant in terms of line availability and the quality of the service on those lines. Using twisted pair that has been previously used for voice transmission can create problems for the signals being passed within a digital system.

Two common elements in a voice circuit-the load coil and the bridged tap and lateral-will in most cases render the circuit useless. These elements must be removed or revised before digital transmissions can pass smoothly. The key to simplified deployment is to perform a logical sequence of tests to prequalify the line and locate any potential problems.

Loop length and load coils The maximum loop length depends on the type of service being deployed. ADSL and very high bit-rate DSL are variable rate services in which service quality and throughput will degrade over longer distances. Services can be affected by the actual cable length, gauge or diameter, bridged taps and laterals, and crosstalk from other pairs (Table 1).

The actual loop length can be measured using an open meter, a time domain reflectometer (TDR), or both. It is best to use both tools and compare the results (Figures 1 and 2).

A load coil is typically a 66 or 88 millihenry inductor that is used in analog telephone systems. Over long cable lengths, higher-frequency signals are attenuated because of an increase in capacitance. To counteract this capacitance, load coils are spaced along the line. This spacing creates a "tuned" circuit for voice frequencies ranging from 300 to 3000 Hz.

In the United States, load coils should be used on cables that are more than 18,000 feet long. Internationally, this might vary based on loss specifications of the system. In an H88 loading scheme, the first load coil is at approximately 3000 feet from the central office or exchange. Subsequent load coils are spaced approximately 6000 feet apart, although a coil may not be necessary over the last 10,000 feet to the subscriber.

Unfortunately, loaded analog systems and digital systems are not compatible. A carrier cannot pass digital and high-frequency signals through the coils.

However, using a load coil counter will help identify approximately how many loads are on the line. During deployment of digital services, it is important to remember that any number of loads are unacceptable. The carrier can use the load coil counter to check for one or more loads, then use the TDR to locate the first one.

The TDR is the only piece of test gear available that can simply and accurately locate load coils. A TDR sends high-frequency pulses down the line and senses reflected energy, much like radar. Because the pulses contain high-frequency energy, they cannot pass through a load coil. The coil represents a large increase in cable impedance and is similar to an open circuit waveform.

Generally, a load coil should be placed at approximately 6000 foot intervals for H88 loading, although the carrier will be able to see only the first one. To locate multiple load coils, the carrier can locate the first one and remove it. While there, the operator should connect onto the line and search again. This process can then be repeated for as many load coils that are on the line.

The maximum loop resistance varies depending on the type of service being deployed and the expected service quality. The actual loop resistance can be affected by the cable length, conductor type, gauge or diameter, and general cable condition.

Measuring the actual loop resistance for the digital line can be done using an ohm meter, with tip and ring conductors, and ground connection strapped together at the far end. Once the total loop resistance is measured, the carrier can estimate the length of the loop using an ohms-to-distance calculator that accounts for the cable type, gauge or diameter, and the temperature of the cable.

Bridged taps and laterals Bridged taps and laterals have been used for many years. A bridged tap connects another section of cable, known as a lateral, to provide POTS or analog service at various points along the main route of the cable. A lateral is commonly defined as any section of cable that is not on the direct path between the CO and the subscriber.

These laterals are acceptable in a standard POTS line, but they usually cause problems for digital services such as ISDN and DSL. The digital signal travels down the cable to the subscriber, but it is also transmitted down each section as well. Echoes from these laterals recombine with the original signal to the subscriber, causing errors in the signal.

Some digital technologies provide limited echo cancellation, but in most cases the carrier may still need to remove the laterals to provide reliable service. Other test equipment such as an open meter may be able to identify that there is extra cable between the CO and the subscriber, but only a TDR can locate each tap and determine the length of the laterals.

This is extremely useful because the carrier now knows where to go to remove the lateral and how long the section is that was just removed.

On a TDR waveform, the tap is shown as a downward spike with a straight sloping line that represents the lateral. The upward bump at the end of the straight line indicates the open end of the lateral. Each bridged tap and lateral will be displayed on the TDR waveform, so the more there are, the more difficult it may become to interpret the waveform (Figure 3).

The easiest way to eliminate this confusion is to locate the first tap and remove the lateral. Once this has been done, the carrier can shoot the line again and remove the next one. The carrier should repeat this process until all the taps and laterals within the limit have been removed.

To ensure a digital line conforms to its deployment requirements for bridged taps, the following three steps will improve a service provider's success rate:

* Step 1: Check closest bridged taps first and remove any bridged taps within the closest limit of each end.

* Step 2: Check longest bridged taps, then check to make sure the remaining long bridged taps do not exceed the maximum lateral length limit.

* Step 3: Check total bridged taps, and finally, verify that the total number of bridged taps on the line does not exceed the maximum limit. The easiest way to do this is to compare the TDR waveform with the open meter results.

Other potential pitfalls Before deploying digital services, carriers should also test for potential problems such as loop loss and crosstalk. The maximum loop loss varies depending on the type of service being deployed. Because the transmission frequencies vary between the different digital services, the frequency used to measure the loss will also differ. Measuring the loss of the digital line at the correct frequency will provide a general indication of whether the line has the potential to support digital services. It's a good qualitative test to confirm that all previous steps have eliminated any problem areas that the line might have.

In addition, most digital services are intolerant of too much crosstalk between the pair being used and other pairs in the binder group. A common source of crosstalk that is easily corrected is the split pair. Splits are wiring mistakes caused by splicing one wire of a pair to another wire in an adjacent pair. Resplits or corrected splits occur when someone recognizes which pairs are split and decides to match them back up farther down the line (Figure 4).

These splits and resplits can cause an apparently good POTS line to reject digital services. Traditional POTS test equipment makes finding a split or resplit difficult at best. Toning or complicated capacitance measurements are seldom practical or successful. If a service provider's TDR is equipped with a crosstalk mode, finding the splits and resplits will be even easier. By connecting the TDR's test and reference leads to the split pairs, the TDR will display only where the pairs are split and resplit. The rest of the cable is displayed as a flat line. This mode makes locating the split and resplit fast and simple.

Large-scale deployment of digital services will require faster, easier and more cost-effective methods to determine if the existing POTS line can handle it. By using the field test equipment available today, service providers can quickly pre-qualify the physical plant to determine where potential digital problems might be. Following a checklist and stepping through the test sequence will minimize deployment time.