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Packet Bus Fault Isolation and Correction
555-233-123 9-8 Issue 4 May 2002
Maintenance of the Packet Bus
The following topics are discussed:
■
Comparison between the Packet Bus and the TDM Bus
■
Packet Bus Maintenance Software
■
Overview of Fault Correction Procedures
Packet Bus and TDM Bus: a Comparison
Although the Packet Bus is similar to the TDM Bus in many ways, there are some
important differences. For example, there are two physical TDM Busses in the
switch (refer to the TDM-BUS section in
Chapter 10, ‘‘Maintenance Object Repair
Procedures’’ , for more information), and one of these busses can fail without
affecting the other (although half of the call-carrying capacity is lost in this case).
On the other hand, there is only a single Packet Bus in the switch, and a failure of
that bus can disrupt all traffic on the Packet Bus.
In High or Critical Reliability systems, the Maintenance/Test circuit pack provides
Packet Bus reconfiguration capabilities. This allows the Packet Bus to remain in
service with up to three lead failures. There is no corresponding facility on the
TDM Bus, where the second physical TDM Bus continues to carry traffic until
repairs are completed.
In addition, the system response varies according to the type of bus failure.
Specifically, a catastrophic TDM Bus failure (one that affects both TDM Buses)
disables ALL traffic in the system, while a catastrophic Packet Bus failure affects
only Packet traffic. This means that all TDM traffic is unaffected, while all BRI and
ASAI traffic does not work. The significance of this distinction depends on the
customer’s application. (For example, a customer whose primary application
requires ASAI would consider the switch to be out of service, while a customer
with a large number of Digital/Analog/Hybrid sets and a small number of
ISDN-BRI sets would probably not consider the Packet Bus failure a catastrophic
problem.) The only way a Packet Bus failure can affect TDM traffic is via possible
impact on system response time in a large switch due to ISDN-BRI endpoint
maintenance running. This should rarely happen because the Packet Bus
maintenance software is able to prevent this impact for most Packet Bus faults
(see the next section).
!
CAUTION:
Since the correction procedures and some of the fault isolation procedures
for the Packet Bus are highly destructive to service throughout the system
(inasmuch as the procedures primarily involve removing circuit packs),
particular attention must be paid to nondestructive fault isolation. Also, for
the same reason, the time taken with destructive procedures must be
minimized. This is the major reason that maintenance of the Packet Bus and
of the Packet maintenance objects is described in such detail.