Question

Name the switching schemes used in a digital exchange. How call processing takes place?

Answer 1

The different switching systems used are:

1. Strowger Switching System

2. Cross bar Switching.

3. Electronic Switching System

Basic Call Procedure: Fig. Shows a simplification diagram illustrating how two telephone sets (subscribers) are interconnected through a central office dial switch. Each subscriber is connected to the switch through a local loop. The switch is most likely some sort of an electronic switching system (ESS machine). The local loop are terminated at the calling and called station s in telephone sets and at the central office ends to switching machines.

 

When the calling party’s telephone set goes off hook (i.e., lifting the handset off the cradle), the switch hook in the telephone set is released, completing a dc path between the tip and the ring of the loop trough the microphone .The ESS machine senses a dc current in the loop and recognizes this as an off-hook condition. Completing a local telephone call between two subscribers connected to the same telephone switch is accomplished through a standard set of procedure that includes the 10 steps listed next. 

1. Calling station goes off hook. 2. After detecting a dc current flow on the loop, the switching machine returns an audible dial tone to the calling station, acknowledging that the caller has access to the switching machine. 3. The caller dials the destination telephone number using one of the two methods: Mechanical dial pulsing or, more likely, electronic dual-tone multi frequency (Touch-Tone) signals. 4. When the switching machine detects the first dialled number, it removes the dial tone from the loop. 5. The switch interprets the telephone number and then locates the loop for the destination telephone number. 6. Before ringing the destination telephone , the switching machine tests the destination loop for dc current to see if tt is idle (on hook) or in use (off hook). At the same time, the switching machine locates a signal path through the switch between the two local loops. 7. (a) If the destination telephone is off hook, the switching machine sends a station busy signal back to the calling station. (b) If the destination telephone is on hook, the switching machine sends a ringing signal to the destination telephone on the local loop and the same time sends a ring back signal to the calling station to give the caller some assurance that something is happening. 8. When the destination answers the telephone, it completes the loop, causing dc current to flow. 9. The switch recognizes the dc current as the station answering the telephone. At this time, the switch removes the ringing and ring-back signals and completes the path through the switch, allowing the calling and called parties to begin conversation. 10. When either end goes on hook, the switching machine detects an open circuit on that loop and then drops the connections through the switch.