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Data Scrambler Using Table Look Up Procedure

IP.com Disclosure Number: IPCOM000089743D
Original Publication Date: 1977-Dec-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 3 page(s) / 43K

Publishing Venue

IBM

Related People

Favre, P: AUTHOR [+2]

Abstract

The purpose of this article is to present a method of scrambling binary data that employs a mixture of table look-up and sequential approaches.

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Data Scrambler Using Table Look Up Procedure

The purpose of this article is to present a method of scrambling binary data that employs a mixture of table look-up and sequential approaches.

A scrambler is an element which is used in a high speed synchronous modem to make the data communication system able to pass any data sequence. The scrambler is placed between the data source and the transmitter to process the transmit data into a random binary sequence. At the receiving end of the communication link, a descrambler is inserted between the receiver and the data sink to de-randomize the data.

Fig. 1 shows an example of a basic scrambler based on the primitive polynomial 1 + x/-6/ + x/-7/. It comprises a 7-bit shift register and logic gates arranged as shown in the figure. Control signals C1 and C2 are defined as follows: C1 = 0 when all bits in the shift register (B1-B7) are "1"; for all other combinations C1 = 1. C2 = 1 when all bits in the shift register are "0" ; for all other combinations C2 = 0.

Fig. 1 shows an example of hardware design directly derived from the mathematical model of the scrambler. The scrambling operation can also be performed through the use of a program or a microprogram.

A first approach is to use a table look-up procedure with a table of 2/8/ = 256 entries (shift register 7 bits + data in). This approach provides a fast execution but requires a large amount of memory.

A second approach is to calculate sequentially the scrambled data as follows:
1. (Data In) XOR (Z(-1)) --> (Data In) = X 2. Determine C1 3. (C1) and (X) --> D
4. Determine C2 5. (D) or (C2) --> E 6. Shift right and put value of E into shift register 7....