In the Public Switched Telephone Network (PSTN), various signaling methods facilitate the establishment and control of telephone calls. These signaling types can be broadly categorized into two main groups: in-band signaling and out-of-band signaling.
In-Band Signaling:
In-band signaling involves the transmission of control information along with the voice signals over the same communication channel. This approach was traditionally used in early telephone systems. Two common types of in-band signaling are:
1. DTMF (Dual-Tone Multi-Frequency):
DTMF signaling, also known as touch-tone signaling, is widely used for telephone signaling. It employs two simultaneous tones to represent each key on the telephone keypad. These tones are then decoded at the switch to interpret the dialed digits.
2. Pulse Dialing:
Pulse dialing, also known as rotary dialing, is an older form of in-band signaling. It involves interrupting the current flow in the phone line to generate pulses corresponding to the dialed digits. Each pulse represents a specific digit.
Out-of-Band Signaling:
Out-of-band signaling separates control information from the voice signals, allowing them to travel on separate channels. This method offers more reliability and flexibility compared to in-band signaling. Two primary types of out-of-band signaling are:
1. Signaling System 7 (SS7):
SS7 is a widely used out-of-band signaling protocol that operates on a separate network for call setup, management, and teardown. It enables advanced features such as call forwarding, caller ID, and other supplementary services.
2. Common Channel Signaling (CCS):
CCS is another out-of-band signaling method that separates signaling and voice channels. It provides a dedicated channel for signaling purposes, improving efficiency and reliability in call setup and control.
In summary, the PSTN utilizes both in-band and out-of-band signaling methods to establish and manage telephone calls. Each type has its advantages and applications, with out-of-band signaling gaining prominence due to its enhanced capabilities and reliability.