Build-up of a call

1. Port controller of a digital line perceives, that the receiver of the terminal was raised up.
2. Port controller indicates it to the network controller.
3. Network controller stores the message for the switching field.
4. Software of the switching field asks for it, and recognizes, that it is an initiation for a call.
5. Switching field sends an answer message through the network controller to the port controller. First part of the message instructs the call initiative port to listen to the dialing sound in the assigned time-slot, and light the marking lamp in the terminal.Second part of the message instructs the initiative port to select a free time-slot, and the controller of the sound perceiving port to access a sound frequency touch tone.
6. When the user of the terminal dials the first number, the equipment transforms the analogue calling signal into a digital signal. Port circuit of the digital line puts this signal into the time-slot assigned earlier to the port.
7. The sound frequency perceiver, that observes this time-slot, translates the signal. Controller of the perceiving port sends the value of the number to the switching field through the network.
8. Switching field instructs the perceiver of the port not to wait for the dialing sound.
9. Switching field analyses the first numeric character, determines, the a call has arrived to another station of the system (dialing number of the devices in the labor begins with 3), and continues collecting the numeric characters, until it haven't collected enough for the identification of the station.
10. Switching field recognizes, that the called station is an equipment launching digital signals (see the note), instructs the proper sound frequency perceiver port controller to remove the percevier of calling signals from the given time-slot (note: if the dialled number is invalid, then the switching field sends a message to the initiative port to observe the sound indicating the invalidity of the number in the assigned time. (After this sign has arrived, hang up the receiver, and jump to step 19.)
11. Switching field determines, whether there is an accessible call indicator of the called terminal, and then sends a message to the port controller generating a sound signal to put the proper one from the ringing echo and the engagement signl into the time-slot assigned to the initiative port.
12. Switching field sends a message to the port controller in connection with the called station to switch the ringing unit on, and blink the signalling lamp of the called partner.
13. When the called partner picks up the receiver, the controller of the port indicates it to the switching field.
14. Switching field evaluates this message.
15. Switching field instructs the port controller to switch off the ringing unit, and operate the signalling lamp of the called partner continuously.
16. Switching field calls on the line port controller assigned to the called partner to select a free time-slot and observe the time-slot of the calling partner.
17. Switching field instructs the controller of the calling port to observe the time-slot of the called partner.
18. If one of the partners hangs up the receiver, then the controller of the suitable line port indicates it to the switching field.
19. Switching field interprets it as the end of the call.
20. Switching field instructs the line port controllers to make the engaged time-slots free, and switch the lamps indicating the call off.

Call procession data:

Number of calls transacted within 1 hour depends on the capacity of the processor and on types of calls in process. A system with a typical build-up is able to process 7200 calls in an hour, based on 36% of incoming, 36% of outgoing and 28% of bidirectional data flow.