2 Digital subscriber line VDSL2
2.6 Transfer security

Transmitted data sequence is scrambled in PMS-TC block. Scrambling at the transmitting side (and descrambling at the receiving side) is intended to eliminate a periodic sequence of transmitted data stream. Scrambling/descrambling is usually implemented as a shift register with established feedbacks. Scrambling the descrambling operation must be of course completely inverted, so that always occurred to restore the original sequence of data.

Randomization of transmitted sequence of data results brings lower demands on the required frequency bandwidth of the transmission path.

For VDSL2 connection is made, like in other xDSL, security of the data sequence against transmission errors. For the detection of errors is used security using 8-bit word that is generated by calculating a classical cyclic code CRC (Cyclic Redundancy Check).

FEC is based on Reed-Solomon code RS. Error correction in conjunction with interleaving, provides protection against single failures and short burst errors.

When encoding using Reed-Solomon code are formed from solid blocks formed a number of data bytes, and a fixed amount of signaling bytes. The number of security bytes can be an integer ranging from 0 to 16. The full sequence data (including security) can have 32 to 255 bytes. Roughly speaking, the number of bytes that can be used to Reed-Solomon code fix is half than the number of bytes used security.

This method of error correction is very effective for its low redundancy (typically seven percent).

When determining the specific parameters for correction FEC and interleaving is a necessary compromise between the degree of error protection and overall transmission delay.