When the line is idle, the phone is on-hook and the hook switch is open. The capacitor (C) in series with the ringing detector does not pass d.c. current (from the battery). With no d.c. path through the phone, the loop current through the loop plant is near zero.
To initiate a call, the caller picks up the phone (goes off-hook), closing the hook switch and letting the speech network (hybrid) draw current from the battery. This current powers the phone.
The CO switch sees this loop current as a request for service and a notice to prepare to receive a dialed phone number. When ready, usually within 3 seconds, the switch gives dial tone and no more than 70 ms later is ready for the caller to dial.
During dialing and call processing, depending on the make of the switch, the voltage from the network may reverse polarity, change up or down, or disappear briefly. If the CPE opens the loop during dialing for more than 1 ms, that may be counted as a pulse; an open circuit of more than 100 ms may disconnect the call.
Audible call progress signals are usually given to the caller: dial tone first; then ringing tone to indicate the far end has not yet answered, busy signal, or re-order (fast busy). At the end of the call, the phone is hung up (goes on-hook) which stops the loop current. An open loop tells the switch the line is idle again and to clear the call.
On lines with extra features that are activated by a ‘hook flash’ like call transfer, call waiting, or 3-way calling, any on-hook period over 1.5 s is considered a disconnect. An open interval of 300 to 1000 ms is taken as a hook flash signal to invoke a service.
When the switch has a call for a phone, the switch alerts the called party by applying an a.c. voltage (up to 170 V a.c. RMS) to the local loop. Alternating current passes through the capacitor C to activate the bell or other ringer. Alternating current into a standard analog phone energizes electromagnets that shake a clapper which strikes mechanical bells. More recent phones and PBXs may recognize an a.c. ringing current or merely the presence of an a.c. voltage of the expected frequency and amplitude (above some minimum voltage, as low as 40 V RMS for a 20 Hz ringer).
When the call is answered, the hook switch closes the d.c. loop. Battery current trips a relay or electronic monitor in the switch, which disconnects the line from the source of ringing voltage (in under 200 ms) and connects it to the talk path.
This introduction is meant to teach you about the functions and technology of a Central Office.
Analog to Digital Bandwidth
The Telephone Network
A Topology of Connection
Network Hierarchy (pre 1984)
Network Hierarchy (post 1984)
North American Numbering Plan
The Subscriber Extension
Local Access and Transport Areas
Wiring Connections: Hooking Things Up
Types of Communication
Lines Vs. Trunks
Foreign Exchange Signal