PCM switching and stored program control

PCM (digital) switching

In PCM switching, speech is switched in its digital form: 64 kbps PCM samples are moved between time slots and highways by time and space switches, without converting back to analogue.

Advantages

• Direct integration with digital PCM transmission (E1/T1): no codec or 2W/4W conversion at every exchange, so no noise or loss build-up.
• Time switches made of cheap RAM; very few crosspoints; small size and low power.
• Four-wire switching throughout, so no hybrid echo problems inside the network.
• Easy to switch data and $n \times 64$ kbps channels; supports ISDN.
• Fast connection, high reliability, easy software control and maintenance.

Issues (problems)

• Synchronisation: all exchanges must be clock-locked; clock differences cause slips (lost or repeated frames).
• Delay: each time switch adds up to 125 µs; many in tandem add delay and need echo control.
• Codec per line: analogue subscriber lines need BORSCHT functions and a codec on each line card, which is costly.
• Jitter and wander on incoming links need elastic buffers.
• Control complexity: path search over slots and highways needs powerful processors and software.
• Interworking with analogue exchanges and signalling conversion.

Stored Program Control (SPC)

SPC is the control of a telephone exchange by a digital computer (processor) that runs a stored program, instead of hard-wired relays or electromechanical logic.

  [processor] <--> [program + data memory]
       |
  [scanners / distributors / signal units]
       |
  switching network  <-->  lines and trunks

• Scanners read the state of lines and trunks (off-hook, digits); the processor runs call-processing programs, consults the data memory (subscriber class, routing tables, charging) and orders the switching network through distributors/markers.
• Centralised SPC: one main processor (duplicated in standby, synchronous-duplex or load-sharing mode). Distributed SPC: many processors, each controlling part of the exchange or a set of functions.

Benefits: new services (call forwarding, waiting, conference, abbreviated dialling) by software change; easy changes of subscriber data and routing; automatic fault diagnosis and traffic statistics; centralised maintenance; accurate billing; faster call set-up; smaller size.