The first telephone systems performed the mechanical switching of calls by human operators. The operators interconnected telephone lines by manually connecting cables at switchboards. These switchboards contained many wires that had plugs and the switchboard had many sockets for the plugs. To interconnect telephone calls at long distances, one operator would have to call other operators to setup the call. Setting up calls could be a complex process and this process got more complex as many more telephones were installed.
Switching systems have evolved many times over the past 100 years. The types of switching systems that are still in common use today include crossbar, time slot interchange (TSI), and packet switching.
Crossbar
Crossbar switches used mechanical arms to physically connect to wires (or busses) together. These mechanical arms (“Crossbars”) connect horizontal and vertical bars together to connect input and output lines together. Magnets are used to open and close the crossbar switch contacts.
Figure below shows a crossbar switching system. In this example, there is a matrix of lines (busses) where each input line can be connected to any output line. When a connection needs to be made, a mechanical switch connects one of the busses with the other busses. The disadvantage of this system is that the number of mechanical switches for connecting each input port to an output port exponentially increases with the number of ports that require connection. For example, a switch with 10 inputs and 10 output lines requires 100 switches. A switch that has 20 inputs and 20 outputs requires 400 switches.
Time Slot Interchange (TSI)
Time slot interchange (TSI) switching is a process of connecting incoming and outgoing digital lines together through the use of temporary memory locations. In the late 1960’s, mechanical crossbar switching systems began to change to TSI digital switching systems. A computer controls the assignment of these temporary locations so that a portion of an incoming line can be stored in temporary memory and retrieved for insertion to an outgoing line.
Figure below shows a TSI switching system. This diagram shows a simplified matrix switching system. Each input line (port) is connected to a multiplexer. The multiplexer places data from each port in time sequence (time slot) on a communications line (e.g., a T1 or E1 line). This time multiplexed signal is supplied to a matrix switching assembly. The matrix switching assembly core has two memory parts: a section that holds the pulse coded modulation (PCM) data and Control Memory - CRAM that holds switching addresses data.
The time slots (voice channels) from the incoming multiplexed sent through switch S1 to be sequentially stored in the PCM data memory. The data is later retrieved by switch S2 and placed on a specific time slot on an outgoing line. The outgoing multiplexed line is supplied to a de-multiplexer so each time slot is routed to an output port.
Packet Switches
Packet transmission is a mode of data transmission that divides messages or data into small increments (packets) that can be routed through a network. When the packets arrive at their destination, they are reassembled in the proper order to recreate the original message or data.
Packet switching can be connection based or connectionless. For connection based switching, a path through the network is established during call initiation and packets are continuously routed through the same path. For connectionless switching, each packet is given a destination address and the switching points in the network (switching nodes) assist in routing the packet to its destination.
Figure below shows two types of packet switching in a communications system. Diagram (a) shows that connection based packet switching sets up a communication circuit prior to transmitting packets that contain data. Diagram (b) shows connectionless packet switching. Connectionless packet switching requires intelligent switching nodes (routers) that can decode the destination address and select the forwarding route based on the results of the lookup in the routing table. This diagram shows that packets of data arrive at the switch. The routing switch extracts the destination address and possibly the type of message.
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