5.3.2 Remote Digital Loopback (RDL)
The Remote Digital Loopback (RDL) test checks the performance of both the local and remote Model 2500RC Series',and the communication link between them. Any characters sent to the remote Model 2500RC Series in this test mode will be returned back to the originating device. For example, characters typed on the keyboard of the local terminal will appear on the local terminal screen after having been passed to the remote Model 2500RC Series and looped back (see Figure 12, below).
CSU/DSU
Digital Network
Figure 12. Remote digital loop
RDL Test Activation
The RDL test may be activated in one of three ways:
1)Toggle the front panel “Local/Normal/Remote” switch to the left hand side to the “Remote” position.
2)Activate the RL lead from the DTE (Note: in order to use this option, the DTE Loop Control option must be
3)From a terminal, first go to the Main Menu and select item 3 to display the Software Switch Menu (Section 3.2.4) In the Software Switch Menu, select item “d” to go to the Set Remote Digital Loop Menu (see below). To activate RDL, select item 1.
RDL Test Procedure
Once LAL is activated, the “Test” LED should be lit. Perform a BER (bit error rate) test on the system, using BER testers on both ends. If the BER test equipment indicates a fault and the Local Analog Loopback test was successful for both Model 2500RC Series units, you may have a problem with the line between the CSU/DSUs. You should inspect the line for proper connections.
5.3.3 Telco Testing
The digital service provider’s central office can perform CSU Loop and DSU Loop diagnostic testing. These diagnostics allow the central office to evaluate circuit operation without making visits to a customer’s premises.
CSU Loop
The CSU Loop is activated when the central office reverses the DC sealing current that flows between the TX pair and the RX pair. In this case, the Model 2500RC Series recognizes this and loops signals on the RX pair back to the central office on the TX pair (see Figure 13, below). While the CSU Loop is activated by the central office, the TM light is illuminated.
DSU | CSU | Digital Network |
Figure 13. CSU loop
DSU Loop
The DSU Loop is activated when the central office sends a DSU loop signal over the twisted pair wire. The Model 2500RC Series senses this signal and loops the digital data back to the central office (see Figure 14, below). While the DSU Loop is activated, the TM light is illuminated.
DSU CSU | Digital Network |
Figure 14. DSU loop
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