|
Principal Sponsors:
 | Digital Subscriber Line (DSL) Testing |
8. Gaining Access to the Copper Loop for Test
There are two questions relevant to access: where can a test device establish metallic test access, and which OSS has the information to identify the access point? This section discusses metallic test access requirements, and the following section discusses the OSS interface requirements.
Metallic Test Access
The objective of preservice qualification is to determine if an existing copper pair can support xDSL transmission. As the majority of copper pairs in a residential area are in service and bearing switched voice traffic, the test access points for preservice qualification are located at the switch and DLC. To prequalify existing residential loops, the most convenient access point is the switch (estimates place the number of DLC loops at less than or equal to 30 percent). The NTT access at the switch places the highest restrictions on the testing that can be performed. The bypass-pair access on DLCs should not restrict tests but does add considerable complications for the TOS controlling the metallic test access. The access methods listed above assume that the pair being tested is already providing service. In other words, spare pairs cannot be tested without manual access at the MDF. These tests, which provide preservice qualification, require metallic test access with full-splitting capability on the customer's line. After an xDSL service has been provisioned, the types of test access expand to include number test trunk (NTT), DLC metallic test access unit (MTAU), DSLAM, and external MTAUs. In the case of ADSL, metallic access to the customer's loop implies access on the loop side of the POTS splitter, because access on the CO side of the POTS splitter would inhibit wideband noise measurements. The performance requirements for metallic access that support the preservice loop-qualification tests are summarized in Table 3.
The metallic test access methods for both preservice loop testing and in-service testing and monitoring are discussed in the following section.
Switches
Access at this point limits the tests outlined above. The limitation arises because the switch-test access would most likely be on the CO side of the POTS splitter, and the frequency range for the test access is typically limited to the voiceband, less than 4 kHz (see Figure 7). This will not support detection of bridge taps, depending on the frequency range supported, and would not support wideband noise measurements.
DLCs
The bypass pair–test access for switched-voice services would be used for preservice qualification, which is controlled by the pair gain test controller (PGTC) under the direction of the testing operating system (TOS) (see Figure 8).
A different test access is necessary for in-service testing because most legacy TOSs do not support ADSL testing. Some DLCs provide a metallic test access on the customer's side of the POTS splitter. The access could be controlled by the DLC's internal MTAU using transaction language 1 (TL1) commands with the access point selected with access identifier (AID) in the shelf-slot-channel format.
DSLAM
This type of access would be useful for in-service testing (see Figure 9). Some manufacturers of DSLAMs provide a metallic test access capability. There are currently no standards for controlling this test access.
External MTAUs
This type of access would be useful for in-service testing. This external MTAU could gain access using TL1 commands and be wired into xDSL circuits at the MDF. These devices must support the entire frequency range required in xDSL loop qualification.
OSS Interface
From the perspective of testing and monitoring, OSS systems can be broadly classified as supporting two types of circuits, designed (e.g., special services, T1 circuits) and switched circuits. For services targeted at residential users, the predominance of pairs in service (e.g., pairs the LEC would like to prequalify for xDSL) will be supporting switched services. In today's environment, switched services are tested with existing POTS TOS, such as Lucent Technologies' mechanized loop testing (MLT), using NTT access at the switch or the bypass pair of a DLC. The NTT access is addressed using the telephone number while the bypass pair is controlled by the pair gain test controller (PGTC).
For designed circuits (special services), a work order record design (WORD) document is created in TIRKS that will list all of the equipment in the circuit. The TIRKS WORD documents should contain the necessary test access information prior to the circuit being provisioned. If the LECs were treating xDSL services as special services, the test access information would be required for all devices that address an access point using an AID, such as ngDLCs, MTAUs, and DSLAMs.
| 
