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Chapter 15: DS-1 Signal Details Data Error Rates Pulses sent along the repeatered line are regenerated at each repeater point. The repeater looks at each time slot and decides whether or not a pulse is present. If the repeater logic determines that there is a pulse, the repeater puts out a new pulse free of noise, distortion, or interference incurred in the preceding repeater section. Owing to degradation factors, a small number of pulses may be incorrectly regenerated; that is, a pulse will be transmitted where it was not present or vice versa. The ratio of transmitted time periods (pulse or no pulse) that are received incorrectly at the end point to the total number of time periods is called the "error rate." The total error rate is the arithmetic sum of the error rates of the individual repeater sections. Because of the effect of impulse noise from central office equipment, end sections (the section nearest the central office) are the principal source of errors and, therefore, are shortened to increase the signal-to-noise ratio at the office repeater. Between the terminal ends, a maximum error rate of 1 in 106 results in good voice communications. Pulse errors cause transients in the individual voice channels, but at this rate they are not noticeable to the average listener. On the other hand, a higher error rate of 1 in 105 (BER=10-5) results in acceptable voice transmission although audible clicks are noticeable. Many data systems are less tolerant of bit errors, and facilities are engineered to meet error rates of 1 in 108, 109, or 1010 wherever possible. Any bit error on the span line results in a Bipolar Violation (BPV). There is one specific type of error that could show up in a T1 span line. A BPV occurs as a result of bad wiring connections anywhere. A normal DS-1 signal uses Alternate Mark Inversion as the line code. In other words, if the terminal data stream is 11111111, then the line code is transmitted as marks (pulses) of alternating polarity, so we would see +1, -1, +1, -1, +1, -1, +1, -1. If a one BPV has been created, we would see two consecutive pulses of the same polarity. In this case, we might see +1, -1, +1, +1, -1, +1, -1, +1. B8ZS is a different line code that intentionally sends and receives BPVs in a specific pattern to carry a special meaning related to 64 kbps Clear Channel unrestricted data. This is often used for Primary Rate ISDN. Most basic transmission line elements such as line repeaters are completely transparent to any type of framing (SF, ESF, or otherwise). In contrast, most pieces of DS-1 terminal equipment, such as channel banks, are very sensitive to proper framing format. Between these two types of elements, there might be intermediate elements, such as higher order multiplexers and automatic protection switches. These elements may or may not be sensitive to framing format. Some elements even convert from one framing format to another. A few also have the ability to auto-configure depending on the signal format that is first received. Voltage and Temperature Factors Normal simplex loop current is 60 mA for modern line repeaters. A few older repeaters use more current (100, 120, or 150 mA). Occasionally, problems with AC power induction can be overcome by increasing the loop current within the tolerance of the repeaters. However, in the absence of 60 Hertz induction, most simplex loops are set between 55 mA and 65 mA. Copper cable tends to have more resistance at higher ambient temperatures (it will take more voltage to drive the constant current through the loop), so knowing the expected cable temperature extremes for a locality helps set a strategy for optimizing loop current for best performance. Aerial cable is exposed to much hotter temperatures than buried cable, and this must be calculated into the design.
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