The Foundation: What is Signaling?

In a telephone network, communication isn't just about voice. Before two people can talk, the network needs to establish, maintain, and then release a connection. This entire process is controlled by signaling, which is the exchange of information (other than user data) related to managing calls. It's like the nervous system of the telecommunication network, essential for its proper functioning.

Signaling primarily operates across three functional areas:

• Subscriber Line Signaling: Exchange of information between subscriber stations (telephones) and the telephone exchange (central office).
• Inter-exchange Signaling: Exchange of information between different central offices participating in a call.
• In-exchange Signaling: Exchange of control information between components within a single central office.

Types of Analog Signaling Mechanisms

Analog signaling relies on electrical currents or tones. Depending on the type of signal used, we can classify it into different categories:

• DC Signaling (Direct Current): This is the simplest method, typically achieved by interrupting or changing the direction of the DC current flowing through the subscriber loop.
  Example: Lifting the handset creates a DC current flow, signaling the exchange to pick up the line.
• AC Signaling (Alternating Current): This method uses specific tones or frequencies to convey signaling information.
  • In-band Signaling: Signals are within the normal voice frequency band (300-3400 Hz). These tones can sometimes interfere with speech.
  • Out-of-band Signaling: Signals are outside the voice frequency band, often at higher frequencies (e.g., 3825 Hz). This avoids interference with user data but requires special filtering.
• Types of Functions performed by signaling:
  • Supervisory (Line) Function: Detects and acts on changes in the line status (e.g., ringing, line seizure by exchange, hang-up).
  • Address (Dialing) Function: Transmits numerical address information (the dialed digits). Beyond simple addresses, it can also include supplementary signals for requests (e.g., send next digit) or acknowledgments.
  • Management Function: Involves transmitting information related to line management, such as tariff information, alarms, and blocking status. This information is vital for network operators.

Analog Call Setup: The Step-by-Step Process

The journey of a simple analog phone call from a caller to a receiver involves a coordinated exchange of signals, enabling the network to establish the connection.

Call Establishment Phases

1. Pre-selection (Idle State / On-hook): The calling subscriber's phone is initially in the on-hook state (handset down), and no DC current flows through the subscriber loop.
2. Service Request (Off-Hook): The subscriber lifts the handset, putting the phone into the off-hook state. The central office detects this DC current flow.
3. Dial Tone: The central office sends a dial tone (typically 400-450 Hz) to confirm that the line is ready for dialing. This is an in-band AC signal.
4. Address Signaling (Dialing): The calling subscriber dials the desired number.
   • Pulse Dialing: In older phones, digits are transmitted by brief interruptions of the DC loop current (e.g., '1' is one interruption, '0' is ten interruptions) at about 10 Hz.
   • DTMF (Dual Tone Multi-Frequency) Dialing: Modern phones generate unique pairs of specific audio tones (e.g., one from a low-frequency group and one from a high-frequency group) for each digit. This allows for encoding 16 different signals. This is an in-band AC signaling method.
     
5. Call Progress Tones to Caller: While the call is being routed, the central office sends various tones back to the calling party:
   • Ringing Tone: An interrupted tone (e.g., 400-450 Hz) informing the caller that the other party's phone is ringing. ITU-T recommends a ringing current duration of 0.67s to 1.5s, followed by a silent interval of 3s to 5s.
   • Busy Tone: An interrupted tone (e.g., 400-450 Hz, faster rhythm) indicating the called party is busy.
   • Congestion Tone: A rapidly interrupted tone (e.g., 400-450 Hz) if network resources are unavailable.
   • Routing Tone: A 50ms tone followed by a 50ms silence, guiding the call through the network.
6. Ringing to Called Party: The central office sends a high-voltage AC current (e.g., 80V at 15-25 Hz) to the called party's phone to make it ring. This is an out-of-band AC signal.
7. Answer (Connection): The called party lifts the handset, detected by the central office as an off-hook signal. The ringing current stops, and the central office establishes the voice path through its switching matrix.
8. Conversation: The analog voice signals are now directly transmitted between the two subscribers.

Disconnection Process (On-hook)

The call disconnection process is initiated when either subscriber hangs up their phone.

• Initiation: When a subscriber returns their handset to the cradle (enters the on-hook state), the central office detects the cessation of the DC loop current.
• Release Signal: A release signal is sent through the network to free up the reserved resources. The exchange will then typically wait for about 90 seconds for the other subscriber to also hang up. If the other party does not go on-hook within this timeframe, a busy signal may be generated or the connection might be force-released.
• Termination: Once both parties are on-hook, the connection is completely terminated, and all associated network resources are released, becoming available for new connections.