Loop testing refers to testing the transmission path between a service provider’s central office and a subscriber’s premises. Testing the local loop is as vital a step as configuring the DSL equipment or provisioning the circuits.
Loop testing has two different functions: electrical loop characterization and Centralized Repair Service Bureau. In the first function, the subscriber loop’s electrical characteristics are measured to verify that the loop is operational and to isolate and identify the fault when repair is required. In the case of a Centralized Repair Service Bureau, information is generated to enable informed inquiries about the subscriber end point status (the drop) and to let technicians be dispatched efficiently and effectively in response to a subscriber complaint or trouble report generated by mechanized test equipment.
Both these functional areas integrate three basic elements—parametric testing, transmission testing, and digital I/O testing. Parametric testing ensures modem interoperability at the physical level, including separating voice and data and minimizing signal interference and distortion. Transmission testing involves checking bit-error rate go/no-go tests that ensure that the modem works within specifications, with or without wire-line impairments. Digital I/O testing focuses on digital interfaces for subscriber/network connections. The idea is to verify correct functionality of the control and transfer of data through complex backplanes and digital interfaces.
Conventional outside plant designs might be many kilometers in length and traverse many facil-ities. Therefore, loop testing has traditionally required labor-intensive manual processes, such as making physical connections. Establishing a new xDSL service involves testing many lines—perhaps millions—as part of prequalification, which can add significant expense. Born of the need to reduce these costs, accelerate prequalification, and advance technology, mecha-nized loop testing (MLT) is increasingly common. MLT can reduce labor expenses, increase on-time service provisioning, and reduce outage times by automating test functions. MLT allows the provider to manage network-wide testing under a single, cost-effective central point of control. Personnel other than skilled technicians can run and interpret MLT tests.
ILEC Loop Management
ILECs initially relied on existing tools deployed to support POTS in the copper loop to support DSL deployment. Because the ILECs could use a line that was already in service for voice, some of the uncertainty about the loops being able to work properly for carrying DSL data
traffic was eliminated. Even now, most incumbents are still installing and are just beginning to use DSL-specific remote test equipment. Instead, the incumbents have relied on engineering inventories bolstered by low-frequency testing done out of the Class 5 switch. ILEC testing verifies loop length, and its design inventory indicates whether any impediments such as load coils or bridged taps are on the line. However, TeleChoice research has found that these inventories are accurate only between 60 and 80 percent of the time. When the line doesn’t support service even after it looked like it could, the ILEC’s procedures revert to the costly and time-intensive procedure of sending out a technician with handheld test equipment to identify and try to correct the problem.
CLEC Loop Management
CLECs have integrated DSL-specific remote test equipment into their networks. Although DSL test equipment cannot see through most splitters, this was not a concern, because the CLECs were deploying services on dedicated loops and therefore did not require splitters. They could colocate test equipment in front of their DSLAMs and gain an accurate picture of the capabili-ties and condition of the loops that were being handed over to them. Existing DSL test equip-ment cannot see through splitters in a DSL-over-POTS environequip-ment. Splitters create a need for additional wiring in the CO. With additional wiring, cross-connections are often mishandled, and when this occurs, providers have no direct way of knowing what the problem is. A few pro-viders, including at least one incumbent, are now placing DSL test equipment in front of the splitter. For competitive providers, this is a solution only if there is sufficient room to colocate a splitter in their cage.
Specifically for DSL, loop testing should take into account all the impairments and allow for reasonable remediation. The overall objectives of DSL cable qualification are to determine the classes and qualities of service that may be offered and to locate impairments that might degrade or prevent service so that they may be cleared.
The following sections describe the basic procedures and types of tests used in loop qualification.
Starting with the Existing Records
Initial loop qualification consists of a records check. Qualification testing of the line consists of sending a technician to the customer’s demarcation point and connecting an Asynchronous Transceiver Unit-Remote (ATU-R) test set for ADSL or connecting a 200 KHz or 400 KHz transmission impairment measurement set (TIMS) tester for SDSL.
The ILEC performs the records check, the first step in verifying readiness for basic service. A typical check determines the distance of the demarcation point from the CO. Other items might include checking the records for the presence and location of load coils and/or bridged taps.
Several parameters are used to qualify the local loop:
•
Frequency response•
Time domain reflectometry•
Noise metrics•
Discrete multitone testing•
Load coil detection•
Service tests•
Test equipmentFrequency Response
A measurement of frequency response indicates the line’s available bandwidth. DSL services can carry more digital information per second than dial-up modems because they utilize higher frequencies to carry the information. One of the most limiting factors for xDSL implementation is the local loop’s inability to carry the high frequencies required for DSL. For example, as discussed earlier, loading coils on a local loop cut off the high frequencies, thereby preventing ADSL operations.
Time Domain Reflectometry
Time domain reflectometry (TDR) is a cable-testing technique that was originally developed to detect faults on power transmission lines. It has been used in analog telephony for many years.
Only recently have TDR measurements become required for DSL deployments. With this test, a pulse of energy is injected into a line. When the energy pulse encounters the end of the cable or any other change in impedance (a short circuit, load coil, or bridged tap), part or all of the energy is reflected toward the TDR equipment. The signal reflection is measured to determine the distance to the fault.
Noise Metrics
Noise and impulse noise measurements let the service provider identify undesired intermittent or steady state disturbances that could affect data transmission between the provider’s serving office and the subscriber. Electrical disturbances can come from man-made or natural sources, such as the crosstalk energy from T1 or E1 lines. Impulse noise can intensify if water enters a cable sheath and acts as a partial conductor.
Discrete Multitone Testing
256-tone DMT loss tests measure the signal loss of each tone (every 4.3 kHz) and the noise in each band. The number of bits that can be carried per tone can be calculated. Conventional
testing of communications channel quality uses Bit Error Rate Testing (BERT), the measure of the ratio of defective bits passed to total bits, representing the network’s quality.
ADSL presents two problems for BERT testing. First, the analog sources of defective ADSL bins are masked during training, making BERT a measure of the effective error rate. Second, BERT uses a trained data pipe consisting of two modems and a physical link. As a result, BERT is not a measure of the specific modem being tested. Finally, with DMT ADSL’s many bins, BERT is unable to resolve bit error to a specific bin. Consequently, conventional testing via BERT is inadequate to detect subtle operational errors for DMT ADSL unless the errors are sufficient to substantially affect the basic data rate.
DSL providers can verify maximum data rates using SNR measurements with the missing-tone test technique. A missing-tone test, which can be implemented as a hardware/software solution, ensures correlation of a rapid functional test result with industry BERT test specifications. The missing-tone test is based on the measurement of noise in a missing tone (one of the tones in the DMT spectrum is turned off while all the others remain on) relative to the signal levels of the tones that are present. This causes the missing tone to expose residual noise.
Load Coil Detection
Most telephone companies have introduced practices for the systematic removal of loading coils from lines. Typically, only lines that are greater than 18,000 feet (5486 meters) contain load coils. Most test devices, both rack-mounted and portable types, can detect load coils by sending particular tones over the circuit and then measuring the results. Bear in mind that load coils absolutely prevent xDSL operation, because they block frequencies above the normal voice range. There is no way to counteract load coils other than complete removal, which is a time-consuming and labor-intensive task.
Service Tests
Specialized test sets can emulate the DSL modem. These service confirmation tests normally use the same chips and technologies that are used in modems. If a connection can be made, this type of test set can indicate the presence and service level (upstream and downstream connec-tion rates) of the line under test. Even though an LEC might not offer the higher rate at the time of testing, it might want the technician to log the maximum rate for future reference. If the maximum achievable rate is low compared to the expected rate, line maintenance might be in order before higher rates can be offered. If a line fault exists, however, only a cable qualifier can be used to determine the true source of the problem. Testing the line requires the provider to send a synchronization bit to the customer premises equipment. The SYNC signal is then sent back to validate service availability. Synchronization bits that are sent from the transmitter to the receiver and then back again are used to synchronize the clocks on both ends.
Test Equipment
The equipment used to qualify local loops covers a wide range of technologies. Rack-based IP multiplexer (MUX) devices can be used to test the local loop. As shown in Figure 2-11, handheld devices can weigh as little as a pound, and they might be equipped with an internal speaker and a 2.5 mm headset connection. Noise-canceling headsets are available for placing calls and for use while testing in high-noise environments.
Figure 2-11 Handheld Testing Devices Are Increasingly DSL-Specific
Some manufacturers have a complete software solution for testing the local loop. Technicians in the field run tests from handheld devices and communicate with a central office responder rack-based unit to run and report line tests. The office responder unit is the computer-based brain behind running automated local loop testing software. It stores all the test templates and determines which tests should be initiated. The responder unit then compares all test results to the individual templates for optimal service. The result is a pass/fail indication sent to the handheld device in the field.
Summary
In general, most potential DSL subscribers must be within 5486 meters (18,000 feet) of the central office equipment, whether in a traditional CO or in a remote terminal. This distance represents the traditional Carrier Serving Area (CSA) defined by remote terminals that extend voice service from the central office, and it has been accepted over decades for voice service.
The relatively recent use of DSL repeaters is extending this reach. DSL equipment can be installed in a remote terminal, but the equipment must be made compact and environmentally hardened. Even before the equipment is accepted, regulatory issues must be addressed to define colocation and line sharing between the incumbent provider and the competitive DSL provider.
More than a century of copper telephony infrastructure must be evaluated before DSL technol-ogies can be implemented over the existing POTS network. Legacy voice technology might present impairments that prevent DSL service entirely or at least inhibit optimal DSL service. In some cases, the cost to remove impairments might be so great that providers opt not to offer DSL in certain areas or might limit advertised bit rates. These impairments include the following:
•
Load coils (prevent DSL operation)•
Bridged taps (can lower the DSL bit rate and must be calculated to offer DSL service guarantees)•
Crosstalk and frequency interference (countered by careful separation of cables with different modulations)•
Copper impedance mismatches (must be documented and calculated for their effect throughout the copper plant)Countering impairments starts with modern DSL’s own automatic rate adaptation. Protection from unwanted noise is measured by the SNR margin. Although sophisticated error-correction algorithms are available (such as Reed-Solomon Encoding and Trellis Coding), these error-correction algorithms depend on the use of bit interleaving to spread out the error bits for maximum efficiency. Together, bit interleaving, with its redundant check bytes, and error-correction algorithm(s) create en route delay and processor loads that are unnecessary for certain types of traffic, such as standard IP data traffic, which can be retransmitted from end to end without taxing the processors and adding delay. This traffic rides in the fast path. Other types of traffic, such as streaming video, cannot be efficiently retransmitted in the case of received errors and must be corrected as much as possible en route. This traffic rides in the second of two paths, the interleaved path.
Review Questions
The following review questions give you a chance to assess how well you’ve mastered the topics in this chapter. The answers to these questions can be found in Appendix A.
1 Which of the following impairments always prevents ADSL implementation?
A AM radio interference B Impedance mismatch C Load coils
D Bridged taps
2 Which of the following is not one of the three primary methods of forward error correction in ADSL?
A Trellis Coding
B DMT
C Interleaving
D Reed-Solomon Encoding
3 Reed-Solomon Encoding divides the data frame into several parts that are called what?
A Cells B Code words C Tins D Codes
4 What does Trellis Coding do with small data errors?
A It retransmits them to the sender.
B It adds correcting cells and retransmits to the sender.
C It fixes the errors without resending them.
D Trellis Coding cannot handle small data errors.
5 Which of the following describes the interleaving process?
A It reorders bits so that errors due to impulse noise are spread over time.
B It reorders frames so that errors due to impulse noise are spread over time.
C It reorders cells so that errors due to impulse noise are spread over time.
D It corrects burst errors through mathematical reconstruction.
6 The SNR margin represents which of the following?
A Error correction
B Separation between the desired signal and the noise signal C Overcoming distance attenuation
D The precedence of signal purity over signal bit rate 7 True or false: Bridged taps prevent DSL operation.
8 Reed-Solomon Encoding is most effective on what type of errors?
A Large gaps in sequential bits B Bursty errors
C T1 binder group interference D Impedance mismatches
9 What makes it much easier for Trellis Coding and Reed-Solomon Encoding to correct errors?
A FEC
B SNR
C Interleaving D Crosstalk
10 What is one way to compensate for the amount of attenuation in a signal path?
A Add a repeater to the cable to boost signal strength.
B Boost signal strength by adding loading coils.
C Change the cable pair to unshielded twisted-pair to increase signal strength.
D Nothing can reduce the amount of attenuation in a line.
11 What form of crosstalk occurs when a signal is affected by leaking digital signal energy moving in the opposite direction?
A FEXT
B NEXT
C Front-end D FEXT and NEXT
12 Which of the following factors inhibits DSL service at the remote terminals?
A Access
B Environmental factors C Space
D All of the above
13 What is the primary function of digital loop carriers?
A To bundle multiple ISDN lines in a service area
B To terminate subscribers’ ISDN lines from the central office C To overcome the limitations of the central office coverage area D To add, move, and delete subscribers in a central office 14 Why must load coils be removed?
A They limit frequency response to below DSL frequency range.
B They create crosstalk.
C They add attenuation.
D They need not be removed completely for DSL service at lower bit rates.
15 Which of the following is not an objective of DSL cable qualification?
A Locate faults that cause bad, poor, or no service so that they may be cleared.
B Confirm the functionality of the physical line between the subscriber and the serving central office.
C Determine which service levels can be offered to the customer.
D Determine the number of devices required for service.
16 Which test works by injecting a pulse of energy into a line and timing the return of any reflections caused by cable abnormalities?
A Service
B TDR
C 256-tone DMT loss test D Frequency response
17 Which test indicates the line’s available bandwidth?
A TDR
B Load coil detection C Frequency response D Noise and impulse noise
18 Initial qualification consists of a what?
A Record check B TIMS test
C 256-tone DMT loss test D TDR test
19 Which test’s measurements let the service provider identify disturbances that could affect the transmission of data between the provider and the subscriber?
A Noise and impulse noise B Load coil detection C Frequency response D Service tests