Specifications

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CSU/DSU MS DBU
O PERATION D URING C RITICAL T IMES
The following four conditions will cause a CSU/DSU MS DBU 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 over the copper wires used in the local loop. Sealing current
may also be used for local-loop testing purposes. An absence 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.
No Receive Signal
This is an indication that the copper pairs in the local loop may be either open
or shorted, or that the OCU in the CO is inoperative. In a private network,
this may indicate that the transmitter of the remote DSU is inoperative.
All-1s or all-0s Condition
This condition is usually generated by the network to indicate that some device
(or devices) in the network is inoperative. Upon detecting an all-1s or all-0s
condition, the CSU/DSU MS DBU initiates a handshake routine to determine
whether the remote unit’s DTE is the source of the all-1s or all-0s condition or
whether an actual network failure exists.
Answer Always
When this option is enabled, the unit goes into DBU mode if a call is present
regardless of the status of the dedicated circuit. It is recommended that the
DBU Passcode be used in this mode, since inadvertent calls could stop data
flow indefinitely.
NOTE
The DBU Answer Test is disabled if this option is chosen.
O PERATION D URING N ONCRITICAL T IMES
The CSU/DSU MS DBU may be configured not to enter dial-backup mode if Data
Terminal Ready (DTR) is low. This feature prevents the CSU/DSU MS DBU from
entering dial backup during noncritical times such as nights and weekends.
For more information, see Chapter 9.