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Browse Prior Art Database

Self Timing Framing in a Digital Telephone System

IP.com Disclosure Number: IPCOM000078512D
Original Publication Date: 1973-Jan-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 2 page(s) / 31K

Publishing Venue

IBM

Related People

Besseyre, J: AUTHOR

Abstract

In a digital telephone system such as those using pulse code modulation, the status of each telephone set must be permanently transmitted on the line. Since that information is repetitive, it can be used to achieve frame synchronization. This idea may be implemented as follows.

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Self Timing Framing in a Digital Telephone System

In a digital telephone system such as those using pulse code modulation, the status of each telephone set must be permanently transmitted on the line. Since that information is repetitive, it can be used to achieve frame synchronization. This idea may be implemented as follows.

Let us assume a frame of sixteen bits with the first four bits being the frame header. The first bit indicates the idle or busy status of the terminal, the second and the third bits are coupled as follows: 0 0 voice only 0 1 data only 1 0 voice data 1 1 signalling and the fourth bit is a parity-check bit on the frame header. The following 12 bits will be voice or data bits but in any case random.

The figure shows the various possible frame configurations.

In most cases, the message will be of the type 1000 + 12 voice bits. The 1000 is both a framing character and a signalling character; this is a self-timing framing signal. When going to data or to signalling, one must change the header and send several times the corresponding new header. The receiver of the new frame first looses synchronization and then tries to establish a new synchro with a multipossible synchrocharacter (1000, 1001, 0001).

After a few frames, it finds the new framing and can then detect the new signalling status of the terminal. This process can be made quite simple if one looks only for a new synchro-character at the time where the framing header should have occurred....