Dual processor modes in centralized SPC

In centralised SPC, a single processor controls the whole exchange, so its failure would stop all calls. To give high availability (an exchange may be down only about 2 hours in 40 years), the central processor is duplicated. The two processors work in one of three modes.

1. Standby mode

 +-------+         +-------+
 | P1    |         | P2    |
 |active |         |standby|
 +---+---+         +---+---+
     +----[shared memory / disk]----+
           |
     exchange equipment

• One processor (P1) is active and handles all calls; the other (P2) is idle in standby.
• Both have access to a common secondary memory; the active one periodically copies the exchange state (call data, tables) to it.
• When P1 fails, P2 takes over, loads the latest state and continues.
• Cold standby: P2 starts from the last saved data, so calls being set up are lost. Hot standby: P2 is kept updated and takes over almost at once.
• Simple, but takeover takes time and transient calls may be lost.

2. Synchronous duplex (match) mode

 +-------+   comparator   +-------+
 | P1    |<------C------->| P2    |
 |  M1   |                |  M2   |
 +---+---+                +---+---+
     +-------- exchange --------+

• Both processors run the same program on the same data, in step, each with its own memory (M1, M2).
• A comparator checks their results continuously. One processor's outputs actually control the exchange.
• If the results disagree, a fault is suspected; both run diagnostics. The faulty processor is isolated and the good one continues alone, with almost no loss of calls.
• Gives fast fault detection and no takeover delay, but both processors do the same work, so no extra capacity, and a transient fault may be hard to locate.

3. Load-sharing mode

 +-------+   exclusion    +-------+
 | P1    |<-----ED------->| P2    |
 | calls |   (shared      | calls |
 | 1,3,5 |    data)       | 2,4,6 |
 +---+---+                +---+---+
     +-------- exchange --------+

• Each processor handles part of the traffic; an incoming call is given randomly or in turn to one of them.
• Both share the exchange resources and data; an exclusion device (ED) stops them from seizing the same resource at the same time.
• Each can handle the full load. If one fails, the other carries all the traffic (with possible congestion at peak).
• Gives higher capacity in normal operation and good reliability, but software is more complex.

Comparison