Digital Switching Systems: Space, Time & Signalling Systems

31 Digital Switching Systems — Space Switching

Types and characteristics of space switching network

The space switching network operates with inputs and outputs organized in N time slots. Information from input time slots is routed from every input mux to a specific output mux without modification of the time position. Gates in crossbars are opened only for the duration of the corresponding time slot.

• Time slots: The network works at inputs and outputs with N time slots.
• Routing: Information of input time slots is routed from each input mux to a specific output mux without changing the time position.
• Crossbar gates: Gates in crossbars are opened only for the period of the time slot.
• Eight-bit words: Eight-bit words are carried through the space digital network typically in serial form.
• Control of space networks: Control memories are associated with outputs (S0) or with inputs (Si).
• Control memory size: The number of words stored in the control memory is specified by the number of time slots (N).

32 Digital Switching Systems — Time Switching

Types and characteristics of time switching network

Time switch: A time switch enables modification of the time position according to connection requirements. The 8-bit word received from time slot i of an input mux is delayed in the time switch so the 8-bit word can be sent to an output mux in time slot j (connection i-j).

• Implementation: Time switches are realized by RWM (Read-Write Memory) memories.
• Call memory: 8-bit words from an input mux are stored in this memory.
• Record types:
  • Marked as Tr switch if the switch has cyclic record and regulated reading.
  • Marked as Tw switch if the switch has regulated record and cyclic reading.

33 Digital Switching Systems — Signalling Systems

Used signalling systems: CAS and CCS

Signalling is the representation of a specific group of control signals and the transfer of these signals through the network with a view to the establishment, maintenance and cancellation of a connection.

Condition for international connections: Unification of signalling systems — from CCITT No.1 to No.7 (SS7).

Types of signalling

1. Subscriber signalling (analog attachments)
   • Line of calling subscriber: calling, dialling (MFC — Multi Frequency Code), termination.
   • Line of called subscriber: subscriber is called, login, termination.
2. Subscriber signalling for digital attachments

   DSS1 (Digital Subscriber Signalling System).

3. Internal signalling

   Signalling within an exchange (sometimes marked as interstage signalling).

4. Network signalling

   Signalling among exchanges. Types include:

   • CAS (Channel Associated Signalling): Associated to speech channels; it is carried either through the speech channel or through an associated signalling channel that belongs to the speech channel.
   • CCS (Common Channel Signalling): Signalling through a common signalling channel. Signalling system SS7 is based on this principle. The common signalling channel can be an arbitrary channel of a PCM 30/32 system.