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Reliability Systems: A Maintenance Aid
555-233-123 6-10 Issue 4 May 2002
Duplication Concepts
High and Critical Reliability systems employ several concepts of duplication.
These are described in the following sections.
Memory Shadowing
Most of the time the Standby SPE is in a mode known as Standby Mode (see
‘‘SPE Modes’’ section) in which it is ready to assume the role of the active SPE. To
be ready to assume the role of the active SPE, PR-MEM must be an up-to-date
reflection of active SPE Memory. This is accomplished by the memory shadowing
mechanism.
The SHDW-CIR (Common Shadow Circuitry) on the Active SPE TN772 DUPINT
(Duplication Interface) detects all memory writes that the active SPE PROCR
(Processor) makes to Active SPE Memory. The active SPE Common Shadow
Circuitry sends this information to the Common Shadow Circuitry on the Standby
SPE Duplication Interface via the ICC. The Common Shadow Circuitry on the
Standby SPE Duplication Interface then writes the data to Standby SPE Memory.
This logical connection between Active SPE Memory and Standby SPE Memory
is known as the SHDW-LNK (Memory Shadowing Link).
Although most of the components making up the Memory Shadowing Link (that is,
Active SPE Memory, both sets of Common Shadow Circuitry, and Standby SPE
Memory) are tested and maintained individually, the integrity of the entire logical
connection is also tested and maintained to make sure that the shadowing
mechanism is functional and that Standby SPE Memory is the same as Active
SPE Memory. Therefore, this testing is done under the guise of the Memory
Shadowing Link.
SPE Modes
As described earlier, the Active SPE is the SPE that is currently responsible for all
call processing, administration, and maintenance activities being performed by
the switch. The Active SPE is always in Active Mode.
The role of the Standby SPE is to be ready to take over as the Active SPE in the
event that the current Active SPE fails. The Standby SPE does not perform Call
Processing or Administration activities. It does perform Maintenance activity, but
this is carried out completely under the direction of the Active SPE in order to
monitor and maintain the health of the Standby SPE.
The Standby SPE is always in one of three modes: Standby, Maintenance,
Busyout, or Down. Usually the Standby SPE is in Standby Mode. In this mode, the
SW-CTL (Switch Control) on the Standby SPE Network Control circuit pack is
isolated from the TDM Bus so that it does not attempt to control the bus and
interfere with the Switch Control on the Active SPE Network Control circuit pack.
Similarly, the Standby SPE Packet Control circuit pack is isolated from the Packet