Short note: Stored program control

Stored Program Control (SPC) means that the exchange is controlled by a digital computer (processor) that runs a program stored in memory, instead of by hard-wired relay logic as in Strowger and crossbar exchanges. All digital exchanges use SPC.

What the program does

• Scans subscriber lines and trunks to detect off-hook, on-hook and digits.
• Analyses digits, finds a free path through the switching network and sets it up.
• Sends ringing, tones and signaling messages, and supervises the call.
• Records call data for charging (billing) and traffic statistics.
• Runs maintenance, fault diagnosis and administration tasks.

Types

• Centralised SPC: one main processor (usually duplicated for reliability) controls the whole exchange. Dual-processor modes are standby, synchronous duplex and load sharing.
• Distributed SPC: control is shared among many smaller processors (line-group processors, call processors, etc.). It is more reliable and easier to expand; most modern exchanges use it.

Advantages

• New services (call forwarding, call waiting, abbreviated dialling, conference calls) are added by changing software.
• Easy change of subscriber data and routing through a terminal.
• Automatic fault detection and remote maintenance.
• Fast call set-up and support for common channel signaling (SS7).
• Smaller size, lower power use and better reliability.

Disadvantages: high initial software cost, and a software fault can affect the whole exchange, so careful testing and processor redundancy are needed.