Telephone local call message flow or signal flow
This article covers telephone local call message flow. It covers signals exchanged between calling terminal, switching system and called terminal for a local call.
Figure depicts typical signal flow for a local telephone call. Signals sent in the direction away from the caller are called forward signals and those sent towards the caller are called backward signals.
As shown in the figure, forward signals pass from the caller to the exchange and from the exchange to the called customer. Backward signals pass from the exchange to the caller and from the called customer to the exchange. There is handshake protocol between the systems i.e. each signal produce a response in the opposite direction. Hence will verify the correct operation. The following are the signal flow or message flow for this local call. Refer telephone system tutorial describing electronic telephone block diagram and functions of telephony system.
➤Calling terminal initiates 'call request' signal. This is answered by 'proceed to send' signal from the switching system.
➤The 'address' signal is answered by the 'call status' signal.
➤The 'answer' signal is response to 'alert' signal.
➤The caller responds to answer signal by way of starting the conversation.
➤Backward clear signal is a response to forward clear signal and viceversa.
• For a telephone call over a junction between two exchanges, the actions between the customer's calling signal and connection to an outgoing
line occur at the originating exchange.
• This exchange send a seize signal to the terminating exchange.
• After the originating exchange has sent address information to the terminating exchange, the actions from receipt of the address information to alerting the called customer take place at the terminating telephone exchange.
• When the called customer sends the answer signal, which is extended till originating telephone exchange. This initiates the supervisory functions.
• At the end of the telephone call, clear signal originated by the caller is extended forward to the terminating exchange; and that from the called customer is sent back to the originating exchange. Both the exchanges then release their respective connections.