Bipolar violations

A: Briefly, a digital signal consists of ones and zeros, which represent changes in voltage along the cable. A bipolar violation is two consecutive positive digital ones, but to explain how that occurs requires some background.

Q: What are bipolar violations, and how are they used in the signaling process of a circuit?

Jim Dixon

Telepex Inc.

Jackson, MS

A: Briefly, a digital signal consists of ones and zeros, which represent changes in voltage along the cable. A bipolar violation is two consecutive positive digital ones, but to explain how that occurs requires some background.

Unipolar line coding is as simple as it gets. There is a positive voltage for every Oone,O and zero voltage for every Ozero.O The problem with unipolar line coding is that there is no Oreturn to zeroO between positive pulses, without which the hardware can lose track of how many positive pulses have been received. This problem is called loss of synchronization.

Bipolar line coding, or alternate mark inversion (ami), also uses zero voltage to represent a Ozero,O but it uses alternating positive and negative voltages to represent Oones.O The danger is that a large number of sequential zeros will cause a loss of synchronization. So ami has a Oones density ruleO that no more that 15 zeros can be transmitted consecutively. But in a random bit stream it would not be difficult to find 15 zeros in a row.

An early solution was Opulse stuffing,O whereby every eighth bit was taken out of the signal and forced to be a pulse. Pulse stuffing eliminated the possibility of 15 zeros in a row by reducing the bandwidth of the channel, but less bandwidth was not part of the objective.

Pulse stuffing has been replaced by the bipolar 8 zero substitution (b8zs), whereby hardware at either end of the signal listens for eight consecutive zeros and replaces the entire 8-bit word with a fictional word. To differentiate between a real and fictional word, the hardware creates a bipolar violation?two consecutive positive pulses.

If you want to try distance-learning via the Internet, go to dt-fad.mpt.go.jp. There, the Ministry of Posts and Telecommunications of Japan offers a course, comtec, which covers the fundamentals of data transmission. It is free of charge.

Donna Ballast is a communications analyst at the University of Texas at Austin and a bicsi registered communications distribution designer (rcdd). Questions can be sent to her at Cabling Installation & Maintenance or at

PO Drawer 7580,

The University of Texas, Austin, TX 78713;

tel: (512) 471-0112, fax: (512) 471-8883,

e-mail: ballast@utexas.edu.

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