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Reliability Systems: A Maintenance Aid
555-233-123 6-2 Issue 4 May 2002
Under normal conditions, the system operates using both TDM Buses. If the
system detects transmission problems on one of the buses, it takes that bus out of
service and operates with only one bus, but also reduces capacity. However, if the
system detects a problem with one of the Tone-Clock circuit packs, it is able to
take that circuit pack out of service without any impact on system capacity or
performance.
The Packet Bus is NOT duplicated in the same way that the TDM Bus is
duplicated (that is, there is one Packet Bus, while there are two TDM Buses).
However, there are several spare leads on the Packet Bus that are not normally
used.
In a High or Critical Reliability systems, the TN771 Maintenance/Test circuit pack
is required in the PPN, but is optional in EPNs. In Critical Reliability systems, the
TN771 is required in both the PPN and each EPN. This circuit pack detects faulty
Packet Bus leads and instructs all Packet circuit packs (for example, ISDN-BRI,
Packet Control, or TN570 Expansion Interface) to use up to three spare leads in
place of defective ones.
The rest of this chapter provides a detailed descriptions of the High and Critical
Reliability systems that are available as well as of the Standard system that is
available across the board. The chapter explains the duplicated hardware and
includes a description of the TN772 Duplication Interface circuit pack. It provides a
look at the following concepts relevant to High or Critical Reliability systems:
memory shadowing, SPE modes, and SPE-interchanges.
In addition, this chapter contains technical descriptions important to the general
maintenance of High and Critical Reliability systems, such as a list of the
appropriate system technician commands, SPE circuit pack LED strategies,
installation and replacement procedures for SPE Control Carrier circuit packs, and
for the Inter-Carrier Cable (ICC).
This chapter uses the terms “Standard Reliability system” or “Standard system” to
denote a system without duplication.