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A brief, introductory telecommunications signaling tutorial April 30, 2009

Posted by HubTechInsider in Telecommunications.
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Signaling is the process of sending information between two parts of a network to control, route, and maintain a telephone call. For example, lifting the handset of a telephone from the receiver sends a signal to the central office: “I want to make a telephone call”. The central office sends a signal back to the user in the form of a dial tone, indicating that the network is ready to carry the call.
The three types of signals are as follows:

* Supervisory signals. Supervisory signals monitor the busy or idle condition of a telephone. They also are used to request service. They tell the central office when the telephone handset is lifted (off-hook requesting service) or hung up (on hook in the idle condition).

* Alerting signals. These are bell signals, tones, or strobe lights that alert end users that a call has arrived.

* Addressing signals. These are touch tones or data pulses that tell the network where to send the call. A compuer or person dialing a call sends addressing signals over the network.

Signals can be sent over the same channel as voice or data conversation or over a separate channel. Prior to 1976, all signals were sent over the same path as voice and data traffic. This is called in-band signaling. In-band signalling resulted in inefficient use of telephone lines. When a call was dialed, the network checked for an available path and tied up an entire path through the network before it sent the call through to the distant end. For example, a call from Miami to Los Angeles tied up a path throughout the network after the digits were dialed but before the call was started.

Prior to the proliferation of voice mail, between 20% and 35% of calls were incomplete due to busy signals, network congestion, and ring-no-answers. Therefore, channels that could be used for telephone calls were used to carry in-band signals such as those for incomplete calls, dial tone, and ringing. Multiplying this scenario by the millions of calls placed resulted in wasted telephone network facilities.

In addition to tying up telephone facilities, in-band signaling sets up calls more slowly than out-of-band signaling. To illustrate, the time between dialing an 800 call and hearing ring-back tones from the distant end is the call setup part of the call. Call setup includes dialing and waiting until the call is actually established.

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