Scrambler (telecommunications)
A scrambler (German scrambler ) is used linear feedback shift registers (LFSR) or fixed tables to a digital signal according to a relatively simple algorithm convert reversible. What is achieved is a change in the bit sequence or the generation of a new bit sequence in order, for example, to meet a specific requirement on the transmission channel via which the binary data sequence is to be transmitted. Generally speaking, the digital signal is converted into a pseudo-random signal with the same information content and the same bit rate. A scrambler based on fixed tables or LFSR does not represent a usable encryption of data because of the simple and known procedures .
application
One of the aims of the method is to adapt the user data sequence to the transmission properties of a transmission channel for the purpose of data transmission . For example, certain transmission channels, such as a radio channel, cannot transmit bit sequences with equal components. With the help of an appropriate scrambler, DC components can be removed from the transmission sequence. The output signal of the scrambler has a more continuous spectrum than the input signal, which avoids energy peaks that can cause interference in neighboring signals. For example, a scrambler is used in the data transmission method ADSL and in ISDN . Video or audio data are also scrambled for channel coding; one example is the serial digital interface , which transmits digital video data without compression.
realization
A scrambler is implemented using a linear feedback shift register (LFSR). In most cases, the maximum possible code length per shift register length is used. A distinction is made between the following structures:
Synchronous (additive) scramblers
Synchronous or additive scramblers require a defined start value other than 0 in the LFS register, and the receiver must be informed of the exact code phase position of the transmitter by suitable measures, such as a special sync word. If the receiver does not know the correct code phase position, it cannot decode the scrambled data signal correctly.
The advantage of synchronous scramblers is the independence of the LFSR from the user data stream. In addition, possible bit errors in the transmitted user data sequence are not duplicated during the descrambling process. Disadvantages are the additional logic that is necessary for recognizing the sync word and the slightly larger transmission capacity ( overhead ) on the transmission channel for regular transmission of this sync word.
The structure of the scrambler in the transmitter and the descrambler in the receiver is identical for this type.
Self-synchronizing (multiplicative) scramblers
Self-synchronizing or multiplicative scramblers do not require a defined start value or a sync word in order to synchronize the code phase of the receiver with the code phase of the transmitter. The start value of the LFSR can also be any; setting all registers to logical 0 or logical 1 is also permitted. The self-synchronicity function is achieved in that the user data sequence acts directly on the content of the LFSR.
The dependency of the scrambler on the user data sequence is disadvantageous. In this way, certain user data sequences can "wipe out" the scrambler completely. This means that the scrambler then only generates a static 1 or 0 statically until the next user data bit change . In addition, transmission errors propagate with self-synchronizing scramblers: A single bit error at the receiver is at least doubled in this type of scrambler, depending on the type of shift register. This fact must be taken into account when selecting the appropriate error-detecting and correcting line codes .
The self-synchronizing scrambler and descrambler as used in the modem standard V.34 are shown on the right. It can be seen that the descrambler loads the receiving user data sequence directly into the LFSR - bit errors thus propagate over several places, up to a maximum of the length of the LFSR. Therefore, in order to minimize the error propagation when using self-synchronizing scramblers, efforts are made to use non-primitive generator polynomials for the LFSR with as few feedback points as possible.