s into or from the data are bipolar violations (BPVs). A bipolar violation is a violation of stream. It is the direct result of equipment buffer overflow or bipolar coding in which two pulses occur consecutively with the underflow, resulting from improperly timed network equipment. same polarity. As will be discussed later in this chapter, T1 and E1 Digital equipment uses input buffers of finite length to accommo- signals are encoded with a system that inverts the polarity of date the momentary frequency fluctuations that can occur between alternate one bits SO that two pulses of the same polarity will not the receiver and transmitter of a digital network node. These fre- occur in a row. On metallic circuits, a bipolar violation will occur if quency differences become persistent when connecting to (or se- a zero is changed to a one, or if a one is changed to a zero. Since lecting) the wrong clocking source, causing buffers to overflow or bipolar violations occur one for one with bit errors, the BPV rate underflow and then reset themselves. It is the buffer resetting that (the ratio of BPVs to correct bits) corresponds to the bit error rate. results in the addition or deletion of data bits from the bit stream. CRC errors are a second possible T1/E1 error condition. The cyclic redundancy check error measurement is an alternative to Based on the source of the slips and their effects on the network, frame error measurement, and is available to T1 circuits that employ all slips can be placed in one of two categories, controlled slips or the Extended Superframe (ESF) format and to E1 circuits that uncontrolled slips. Controlled slips are bit additions or deletions that employ the CRC-4 multiframing. do not disrupt frame synchronisation. Uncontrolled slips are bit CRC-n (n = 4 with PCM-30, n = 6 with T1 ESF) is an error additions or deletions that cause both framing and data to be dis- checking method that uses an n-bit code to represent an entire placed. This framing and data displacement results in a loss of frame mul