Non Telephone
As a method of in-band signaling, DTMF tones were also used by cable television broadcasters to indicate the start and stop times of local insertion points during station breaks for the benefit of cable companies. Until better, out-of-band signaling equipment was developed in the 1990s, fast, unacknowledged, and loud DTMF tone sequences could be heard during the commercial breaks of cable channels in the United States and elsewhere.
These DTMF sequences were sent by the originating cable network's equipment at the uplink satellite facility, and were decoded by equipment at local cable companies. A specific tone sequence indicated the exact time that the feeds should be switched to and away from the master control feed, to locally-broadcasted commercials.
An example of a cable company DTMF sequence code would communicate the following to the cable company's broadcast equipment:
SWITCH TO LOCAL NOW - SWITCH TO LOCAL NOW - PREPARE TO SWITCH BACK - PREPARE TO SWITCH BACK - SWITCH BACK TO NATIONAL NOW - SWITCH BACK TO NATIONAL NOW - "IF YOU HAVEN'T SWITCHED BACK TO NATIONAL NOW, DO SO IMMEDIATELY"
DTMF signaling in the cable industry went away because it was distracting to viewers, it was susceptible to interference when DTMF tones were sounded by characters in television shows (a character dialing a Touch-Tone telephone in a television show might convince the cable company computers to switch away from a "hot feed" to dead air), and the cost of human-imperceptible signaling technologies decreased.
In-band signaling applies only to Channel Associated Signaling (CAS). In Common Channel Signaling (CCS) separate channels are used for control and data, as opposed to the shared channel in CAS, so all control is out-of-band by definition.
In computer programming, an example of in-band signaling are magic numbers, used for signaling of file formats.
When out-of-band communication is unavailable, one of two techniques may be used to preserve network transparency.
- Encapsulation: The bundling of the control data in the packet's header and then removing the header (and/or footer) of the packet at the far end, restoring the data to be the same as the original.
- Bit stuffing: The insertion of noninformation or escape characters to modify, synchronize and justify the data so it never looks like signaling information (and remove the stuffed bits and escape codes at the far end, restoring the data to be the same as the original).
Read more about this topic: In-band Signaling
Famous quotes containing the word telephone:
“Women who marry early are often overly enamored of the kind of man who looks great in wedding pictures and passes the maid of honor his telephone number.”
—Anna Quindlen (b. 1952)
“A woman spent all Christmas Day in a telephone box without ringing anyone. If someone comes to phone, she leaves the box, then resumes her place afterwards. No one calls her either, but from a window in the street, someone watched her all day, no doubt since they had nothing better to do. The Christmas syndrome.”
—Jean Baudrillard (b. 1929)