International Telephone Ring-Back Signaling Reference

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The Ring-back signal is used in Public Switched Telephone Networks to indicate to the caller that the called number is not busy, and that the line is being "rung" or signaled that an incoming call is present.

In most cases, the ring-back signal has the same cadence as the ring generators used in that country, but the ring-back and ring generators are usually not synchronized with one another.

In modern telephone switching systems, the frequency of the ring-back signal and that of the ring generator are rarely the same. For example, in North America, the standard ring generator on non-party lines sends a signal to the called telephones at 20Hz (nominally at 90VAC), while the ring-back signal sent back to the caller contains both 440Hz and 480Hz tones.

On party lines, different ring generator frequencies were commonly used to activate the ringer in only one telephone on the party line. This selective ringing was accomplished either by a frequency filter at each residence or by a ringer mechanism in each telephone that would only successfully vibrate enough to strike the bell in the ringer assembly if the magnetic coil received the AC ringer signal at the exact frequency that it was tuned for.

The caller may or may not hear a different ring-back signal when calling different parties on a given party line. This is usually dictated by the type of telephone switching system being used at the called parties central office.

In the very old cross-bar and manual switchboard systems, the caller would actually monitor the ring-generator signal being sent to the called line. The 20hz signal would be sent to the called line, while higher frequency harmonics created by the mechanical ring-generator would be what the caller would hear, as the actual ring frequency is below what will pass across the PSTN.

For most Americans, their first contact with a noticibly different ring-back occurs when they hear it in a movie or television program set in the United Kingdom, with the UKs distinctive double ring cadence. So strong is the assumption that such a ring-back must be related with Europe and other countries that US movie makers frequently use the UK ring-back for sound effects in film or television, regardless of what is actually correct for that country.

As with most telephone signaling standards, countries that used to be colonies or protectorates during at least part of the twentith century tend to have telephone signaling similar, if not identical to the parent country. For example, the United States, Taiwan and South Korea have virtually identical signaling conventions, as do the United Kingdom and Singapore.

This information is provided on an "AS IS" basis and has no warranty. Most countries have regulatory authorities who will have the latest specifications and compliance information.

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[Copyright 1995,2000,2001,2002,2004 Frank Durda IV, All Rights Reserved.
Mirroring of any material on this site in any form is expressly prohibited.
The official web site for this material is:  http://nemesis.lonestar.org
Contact this address for use clearances: clearance at nemesis.lonestar.org
Comments and queries to this address: web_reference at nemesis.lonestar.org]