Twisted Pair Distributed Data Interface (TPDDI) is a standardized variant for low-cost connections with 100 Mbit/s via TP cable. TPDDI uses TP-PMD for the media-dependent layer and specifies as data connector and transmission media the RJ45 connector with STP or FTP cable ofcategory 5 as well as the Sub-D connector with STP cable 150 Ohm, corresponding to IBMcable type 1.
Functionally, TPDDI transmits the data rate of 125 Mbps in Non Return to Zero Inverted( NRZ-I), resulting in a center of gravity frequency of 62.5 MHz. After conversion to NRZ coding, the signal is scrambled to reduce frequency excursions. Scrambling achieves a uniform frequency distribution, especially for repetitive signal patterns.
To keep crosstalk as low as possible, especially in UTP cabling, the scrambled signal is further reduced in transmission frequency to 31.25 MHz using Multi Level Transmission( MLT). A scrambled signal in MLT-3 coding is thus transmitted via the transmission medium. For a Cat. 5 cable, this measure improves the crosstalk from 32 dB to 35 dB and the signal-to- noise ratio from 13 dB to 19 dB compared with an NRZ-I transmission.