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Telecom Signaling

Telecommunication signaling is the transmission of control signals, and the processes that use these signals, to coordinate telecom networks and communication sessions.

There are different types of telephone signaling systems used in public telephone systems:

Private branch exchange (PBX) systems
Computer telephony equipments
In-band, sub-band, out-of band
Line side - rotary dial, DTMF, and flash signaling
Trunk side - Mulifrequency (MF) and E&M signaling
Common Channel signaling (CCS) technologies
Signaling system 7 (SS7)
Advanced intelligent network (AIN) control.

Multifrequency [MF] Signaling

5 different multi-frequency (MF) tones are used to send signaling (dialing digit) information on a communication line between end office (EO) switches.

The MF system uses 5 different frequencies combined to represent 10 keys; 700, 900, 1100, 1300, and 1500 Hz.

For example,

Dialed digits are gathered from a telephone that is connected to the end office switch.
A MF tone generator converts these digits to tones that are sent on a trunk line between end office switches.
The receiving end office switch uses an MF receiver to convert the tones back to the digit information.
This allows the receiving end office switch to determine which telephone (switch port) to connect to the communication trunk line.

E & M Signaling

E & M signaling uses separate lines to send control messages between switching systems.

For example, two PBX systems connected together using voice lines and E & M signaling lines

When a PBX system desires to use a communication line, it changes the voltage of the M line to alert the other PBX's E line.
When the other PBX detects this change of state, additional control messages may follow to allow the switching units to communicate with each other and route calls to the correct extension or port.

Next: Signaling System Number 7 [SS7]