Introduction to Data Communications
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i. Ringer Unit

The ringer is a device to alert you to an incoming call. It interprets the ringing voltage from the Central Office. Originally, the ringer was a electromagnetic bell but today, most ringers are electronic devices.

The Central Office sends:

• a 90 to 120 VAC ringing voltage
• Frequency of 20 Hz
• Cadence for North America is 2 sec On/ 4 sec Off

ii. Hook Switch

The hook switch is a switch that is activated by lifting the handset off the cradle. The position of the hook switch determines whether the telephone is waiting for a call or actively using the line. The Off-hook position informs the network of a request for use. The On-hook position releases the use of the network.

iii. Dialer Unit

There are two types of Dialer Units: Rotary Dial and Touch Tone. Rotary Dial are the old put your finger in the hole and spin type. The rotary dial operates by toggling the Hook Switch on and off.

Touch Tone is the modern method where 2 frequencies per push button are sent. Touch Tone is a trade name, the correct name is DTMF (Dual Tone Multi Frequency).

iv. Hybrid/Speech Network

The Hybrid/Speech Network performs several functions:

• It converts the Tx/Rx 4 wires from the Handset to the 2 wires for the Local Loop.
• It interfaces the signals from the Dialer Unit to the telephone line.
• It provides auto line compensation for line length to keep the volume constant.

Introduction to Data Communications
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