PDF of this tutorialThe following is a description of each component followed by an overview of how they can vary.
Test Access
As noted above, a DSL test system is justified because it reduces the number of truck rolls required to test and provision a new service. To achieve this objective, the means to physically connect a test probe to a copper pair in a remote central office is required. In a traditional plain old telephone service (POTS) network, test access is provided via the Class-4 or Class-5 voice switch. Unfortunately this access point is not adequate for most DSL loop testing because it only provides access to the voiceband portion of the circuit. For comprehensive testing, full broadband test access is required. With a few exceptions, the majority of DSL access multiplexer (DSLAM) vendors have not provided test access as part of their offering.
Key elements of a good loop management system are discussed as follows:
Vendor-Neutral Loop Access
To address the need for independent test access, service providers can turn to vendor-neutral loop access systems to provide test access. Many vendors augment test access with additional loop switching functionality to perform remote service changes or protection switching. Figure 1 shows a test access switch in place between the main distribution frame (MDF) in the central office and the DSLAM equipment.
Figure 1. Typical CO Test Environment
Test Head Options
Depending upon the manufacturer, a test head is either installed external to the test access switch or mounted internally in the chassis. The relative advantage of either approach depends upon the specific circumstances of the service provider. In general, an external test head solution provides the test planner with more flexibility. An unbundled test head and access switch enables the planner to select the best test equipment for a specific task and/or redeploy existing test resources already in place.
Switching Functionality
Loop management functionality of an access switch ranges across two axes and varies with respect to switching functionality and cost as shown in Figure 2. As noted later in this tutorial, a test planner should first evaluate the type of switching and test access functions required and then select the right access system for his requirements. By carefully evaluating requirements, test planners can determine the right size for their test access system and thus maximize the capital expenditure funds available.
Figure 2. Functionality
Control Integration
Control is an important element of any loop management system (LMS) and will determine how easily an integrated solution can be developed. Most LMSs provide multiple methods of connecting to the switch including Internet protocol (IP), asynchronous transfer mode (ATM), and dial-in modem communication ports. Industry standard control protocols such as simple network management protocol (SNMP), TL1, and Telnet, as well as simple command line interfaces, provide well-defined control interfaces for management via a common network (e.g., IP). Ideally the LMS will include pass-through ports for control of an external test head, eliminating the need for an additional communication link.
Functions of an external LMS include all or some of the following:
- Remote test and loop management
- Test access toward the loop
- Test access towards the DSLAM
- Split-mode for bidirectional testing
- Bridge the loop
- Equipment sparing
- Cross-patching
- Tone generation (independent of the test probe)
- Pass-through port for external test head control
Intelligent Test Device
A DSL test device designed for the central office (i.e., test probe) enables a network technician to remotely perform a variety of tests during the life cycle of a DSL service. The following tests fall into several categories. The following is a brief description of each one.
Prequalification Testing
This is a cursory test that predicts a loop’s ability to support DSL. This test augments customer loop records and provides a more accurate prediction of line performance.
Preinstallation Testing
This is typically performed after the incumbent local-exchange carrier (ILEC) has unbundled the loop and terminated it in the competitive local-exchange carrier’s (CLEC’s) equipment and before the line is accepted. Based upon the line characteristics in the interconnect agreement, a network technician will test the loop to verify the absence of elements that would limit the ability to support the ordered DSL service.
Preinstallation tests typically include both traditional POTS testing as well as a variety of broadband tests. POTS testing is useful for three reasons:
- DSL performance can be inferred from traditional metallic tests.
- Traditional narrowband test results are more widely understood and accepted.
- Central office technicians are better equipped to resolve traditional narrowband issues.
Single-ended broadband testing (such as time-domain-reflectometry [TDR]) is also important because it enables NOC technicians to accurately predict line speeds, determine loop length, identify bridge taps, and detect load coils to determine if the line will perform as ordered.
Spectral Interference Monitoring
DSL service can be severely compromised by disturbers (e.g., T-1 service) in the bundle. Disturbers may be present at the time of the cutover or introduced at some future time. Consequently, it is beneficial to conduct baseline spectral analysis at cutover and periodically monitor the line over the life of the service.
Post-installation Testing
In addition to spectral analysis, DSL service testing will be required periodically to resolve service disputes and to troubleshoot equipment failures. A series of broadband tests may be required to characterize the line, compare it to baseline measurements, and to identify cross-talk issues. Testing toward the DSLAM may also be required using a “golden” modem to verify line-card performance.
In summary, effective DSL test probes can provide some or all of the following capabilities:
- Traditional POTS tests (TIMS, c-msg, VF insertion loss, etc.)
- TDR measurements
- Background noise measurement
- Load coil and bridge tap detection
- Power spectral density measurement
- Identification of interferers and disturbers
- ADSL transmission unit–CO/ADSL transmission unit–remote (ATU–C/ATU–R)
- Longitudinal balance
Test Automation
Test management is the third critical piece in the DSL solution. The test operations support system (OSS) becomes the window through which the NOC technician views and manages the remote loop testing and also the most compelling reason to invest in a DSL testing solution. The information available to the test technician via the test OSS is used to make crucial decisions regarding line acceptance and service availability. How this information is presented, shared, and archived will impact the overall efficiency of the DSL service deployment.
The following are key functions of an automated test management platform:
Loop Access and Management
Through the test OSS interface, test technicians select which loop to test and what type of test connection to make (i.e., break toward loop, break toward equipment, split mode or bridge). For test access systems that offer additional loop management capabilities, these functions are also managed through the test OSS, allowing test technicians to make service changes or spare out faulty line cards.
Test Automation
Most tests performed by the test head actually consist of a number of individual tests. A test OSS will predefine a variety of test routines that can be invoked with a single command. Automation also enables supersets of predefined test routines to be created to quickly conduct a comprehensive battery of tests.
Real Time Results Reporting
To aid the test technician’s decision making, the test OSS will enable predefinition of acceptable test results and present the results in a simple statement (e.g., OK) or graphic symbol (e.g., thumbs up). Detailed reports are also available and useful in negotiating a resolution with the ILEC technician.
Results Sharing
Test OSS can form a part of an automated flow-through provisioning process. In this scenario, test results (typically a result summary) are shared with other OSS elements. Application programming interfaces (APIs) based upon open standards (i.e., SNMP, common object request broker architecture [CORBA], extensible markup language [XML]) enable the test OSS to share data with a larger pool of network management systems (NMSs) and element management systems (EMSs) systems.
Historical Analysis
DSL performance and characteristics have a tendency to change over time. External interference, as well as disturbers in the binder group, can be introduced after the initial installation and thus go unnoticed during cutover testing. Periodic line monitoring and comparison against initial test results can provide useful evidence of performance degradation. The same information can be used to negotiate a resolution and lead to faster service restoration.
Like loop access systems, test automation solutions range from stand-alone real-time test management solutions to more complex and fully integrated test OSSs. A modular building-block approach to the DSL test access system enables the test OSS to evolve over time to match functionality and complexity with the operational requirements.
