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Chapter 1. Introduction
Entering Dial Backup Mode
When a condition for entering dial backup mode is detected, the
led
Alarm
turns on, and the buzzer sounds. The buzzer alternates between 30 sec-
onds on and 30 seconds off unless the DDS line is restored or it is disabled
by using the
key and selecting T
O
B
. See the section Front
Quick
URN
FF
EEP
Panel on page 3-10 for more information on the
key.
Quick
The following conditions can be enabled to cause the DSU III TDM to enter
dial backup mode:
Loss of Sealing Current
Sealing current is a low voltage DC current provided by the central office
(CO) to prevent corrosion of splices in copper wires used in the local loop.
Sealing current may also be used for local loop testing purposes. An ab-
sence of sealing current generally is an indication that the loop is open.
Out of Service (OOS) Signal
An OOS signal, generated by the network, indicates a device (or devices)
in the network is out of service. If the network has an excessive rate of er-
rors, the DSU III TDMs can no longer communicate, causing an out of sync
condition and resulting in dial backup operation if this option is selected.
No Receive Signal
This is an indication that the local loop copper pairs may be either open or
shorted or the OCU in the telco central office is inoperative. In a private
network this may indicate that the transmitter of the remote DSU is inop-
erative.
All 1s or all 0s Condition
This condition is usually generated by the network to indicate some device
(or devices) on the network is inoperative. Upon detecting an all 1s or all
0s condition, the DSU III TDM will initiate a handshake routine to deter-
mine whether the remote unit's DTE is the source of the all 1s or 0s condi-
tion or an actual network failure exists.
For more information see the chapter Configuring Dial Options on page 9-1.
61200063L1-20
DSU III TDM User Manual
1-5
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Table of Contents
Troubleshooting
