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Duplicated Hardware
Issue 4 May 2002 6-9 555-233-123
Packet Bus Redundancy
The Packet Bus is used to provide ISDN-BRI signaling via the TN778 Packet
Control circuit pack. The Packet Bus is not physically duplicated in the same way
that the TDM Bus is duplicated. However, there are several spare leads on the
Packet Bus itself that are not normally used. The Maintenance/Test circuit pack
has the ability to test the Packet Bus and discover shorted or open leads on the
Packet Bus. When such a fault is discovered, the Maintenance/Test circuit pack
sends a message to each circuit pack that uses the Packet Bus, instructing that
circuit pack to use one of the spare leads in place of the defective lead.
Additionally, the circuit pack can recover up to three Packet Bus lead failures by
this mechanism. This provides high availability by decreasing the chance of a
blocking Packet Bus fault by two orders of magnitude.
Table 6-1 indicates when the TN771 Maintenance/Test circuit pack is required in
each cabinet or port network.
Table 6-1. TN771 cabinet/port network requirements
TN771
Maintenance circuit pack required
Model Reliability PACCON PPN EPNs
1
1 EPNs means all port networks in an EPN cabinet, that is, dual port networks may
require two TN771s in the cabinet, one per port network.
G3s, i, si Standard N N N
G3s, i, si Standard Y
2
2 For example, a PACCON board is required for BRI/ASAI or ISDN-PRI using PACCON
Y N
G3i, si High N N N
G3i, si High Y
2
Y N
G3i, si Critical N N N
G3i, si Critical Y
2
Y Y