Telecom Basics: Signal Processing, Signaling Control, and Call Processing, Third Edition by Lawrence Harte

Author:Lawrence Harte [Harte, Lawrence]
Language: eng
Format: azw3
Publisher: UNKNOWN
Published: 2017-03-11T05:00:00+00:00

Figure 6.5, DTMF Dial Operation

Flash Request

A system special service request feature that is used to indicate that a subscriber has a desire to recall a service function or to activate a custom calling feature (such as a call transfer request).

A flash feature service request can be created when the user initiates a short on-hook interval or through the sending of a special service request message. The short on-hook interval is created by a momentary operation of the telephone switch hook, during a prolonged off-hook period. The special service request message can be sent by a button on a telephone (such as a PBX telephone) or by pressing the SEND key on a mobile telephone.

Figure 6.6 shows how the flash signal (special service request) can be sent to the telephone system to activate additional call processing features. This example shows a flash feature is sent on an analog line by momentarily opening the current loop connection. When the loop current sensing circuit senses a brief open (no current flow) period, it creates a flash message that is sent to the call processing section of the telephone system. For digital telephones, the flash message is sent via a signaling message on the digital channel. This diagram shows that on an ISDN line, the flash message is sent on the D (signaling) channel.

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