HP Labs Technical Reports

Some Further Analysis on Scramblers for FDDI on Shielded Twisted Pair and Data Grade Cable

Spratt, Michael P.



Abstract: The Fiber Distributed Data Interface (FDDI) was originally designed as a fiber optic Local Area Network. Recently, attempts have been made to transmit FDDI over copper cables, reducing costs. HPLB was part of a consortium of companies looking at the transmission of FDDI over Category 3 (voce-Grade) unshielded twisted pair cable. In the operation of the FDDI Media Access Control protocol, long sequences of the Idle symbol will occur, and to reduce radio frequency emission, the use of scrambling is required. This paper was presented to the February 1992 meeting of the FDDI committee in Austin, Texas, and further explores the scrambler systems and mathematics describing them, introduced in 'The Use of Scramblers with an Anti-Locking Circuit', first presented at the December 1991 meeting of FDDI. In this paper, the further issues covered are :- the locking of the scrambler by other FDDI line states than Idle, or indeed any repeating input data pattern, the use of an Anti-Locking Circuit at the de-scrambler at the receiver, as well as the scrambler, and the bound on the number of transition-free code bits generated by the transmitter system.

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