Chapter 3. Span Engineering
Pulse Transmission
Pulses generated by the terminal equipment (e.g. channel bank) and repeaters are subject to distortion by attenuation and phase characteristics of the cable. In the line and office repeater units, just preceding the actual regenerator is an ALBO equalizer which restores adequate pulse shape for detection and regeneration. Pulses generated in the terminal equipment must reach the office repeater in a predictable fashion even if it is in the same room. In many office repeaters, the line buildout (LBO) setting tells it through how many feet of cable the signal has traveled since the DSX-1 cross-connect. The office repeater adapts to that attenuated signal. Most LBO settings assume the use of 22 gauge twisted-pair cable (with around 14 to 16 pF per foot of capacitance). Note that 24 gauge cable has approximately 25% more loss than 22 gauge. By default, many office repeaters are shipped set for 0 to 133 feet of cable (assuming 22 gauge). For transmission calculations, the cable attenuation at 772 kHz is used. This is half of the 1.544 Mbps clock rate, but this is valid because the power spectrum of the pulse stream is maximum at approximately 772 kHz.
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 logic circuit determines that there is a pulse, the repeater outputs a new pulse that is free of noise, distortion, or interference. As a result of many possible factors, a few pulses may be incorrectly regenerated. A Zero is sent instead of a One, or vice versa. The ratio of error pulses to the total number of time slots is called the error rate or the bit error rate (BER). One error in 1000 is referred to as BER 10-3. One error in a million is BER 10-6. For strictly voice circuit application to T1, a BER of 10-6 or 10-7 might be considered acceptable performance, although a BER of 10-8 or 10-9 might be required for some data purposes. In the most common systems, once live traffic errors have crept into the transmission stream, they cannot be sorted out or corrected. Error rates tend to accumulate through an end-to-end system, although the rates tend to be low for digital systems compared to analog systems. The exception to this is found in a few sophisticated microwave radio systems where forward error correction is used. However, its expense limits application.
Overall System Length
Each repeater in the series adds a small amount of jitter to every pulse of the bit stream. A conservative limit of 200 tandem repeaters in a system ensures that the accumulated jitter won't exceed the synchronization capability of terminal equipment or higher-order multiplexers. Based on the accumulation of error rates in tandem repeater sections, and particularly in end sections (the repeater section next to the central office), the system should not include more than ten tandem span lines (nine intermediate offices).
Design Criteria
Long-range design of the span line is necessary to plan the expected cross section of the span. The selection of one-cable or two-cable operation, locations for the line repeaters, and repeater section length will depend on the future requirements of the route. Also, the cable plant must be carefully studied in terms of the number, type, age of cables; freedom from bridge taps and branches; splicing integrity; suitability for line repeater locations; and minimum exposure to electrical and mechanical hazards. Major factors that control the design of the span include:
1. ultimate number of systems within the cable
2. cable pair attenuation at 772 kHz
3. crosstalk coupling loss between cable pairs
4. central office noise
5. ambient temperature range
Single-Cable or Dual-Cable Operation
In normal one-cable operation, low-level repeater inputs and high-level repeater outputs appear at the same point of the cable. As a result, near-end crosstalk (NEXT) is the limiting factor in repeatered line design. The number of systems that can be installed in a single cable is mainly controlled by the physical separation of the pairs in the two directions of transmission. Greater separation increases the coupling loss, resulting in decreased interference. A general rule is that if transmit and receive pairs are in the same cable binder group, the maximum section loss should be reduced to 15 dB to prevent crosstalk.
In two-cable operation, NEXT does not limit the number of systems for one cable. The choice of one-cable or two-cable operation is based on cable route, circuit requirements, availability of suitable cables, and economics.
