3 Digital Switching Systems

Example: The switching network, which is represented in Figure 3.9, allows for the connection of up to 100 subscribers. A maximum of five internal connections can be simultaneously set up, as well as up to three external trunk groups with up to five connections each.
For the case m = 10 inputs and n = 5 outputs, per switching matrix in a stretched arrangement in layer 1 m * n * 10 = 500 crosspoints are required.
In layer 2, the switching matrices also have m = 10 inputs and n = 5 outputs. The 5 switching matrices of this level thus have a total of 10 * 5 * 5 = 250 crosspoints.
In layer 3, the number of the crosspoints can be calculated from m = 5, n = 5 and the number of matrices which is five. This yields 5 * 5 * 5 = 125 crosspoints.
In total, 875 crosspoints will be required.
Hence because of internal blockage, it is not possible to create more than five connections for a subscriber group out of 10 subscribers which belong to one and the same switching matrix of the first layer. Furthermore, not more than five internal connections, and not more than five connections to the external trunk group, can be created simultaneously.
Two connections are displayed:

• 
  line 10 of the first matrix of layer 1 connects to line 1 of the second matrix of layer 1 (internal connection) and;
  
• 
  connection 3 of the 10^th matrix of layer 1 connects to line 1 of the external trunk group (external connection).
  

Figure 3.9 - Multi-layer switching network

By carefully designing the switching network layers and the connections between the layers, a compromise can be found between crosspoint number and blockage probability. This information on switching networks mainly refers to the switching of spatially separated channels, which can be implemented with Strowger selectors or co-ordinate switches.

But channels can also be in different forms. It is possible to assign a channel a fixed carrier frequency and switch this carrier in the switching system. Another possibility is the assignment of a time slot to a channel. In digital switching technology, the spatial and the temporal domains are utilised. In switching devices, spatial and time switching arrangements are often used in combination.