Chapter 10. Spare Lines and Automatic Spare Line Switching

Murphy's Law states "Anything that can go wrong, will go wrong."

Although modern line repeaters are very reliable, they are used in a hostile environment (temperature extremes, vibration, lightning, automobile damage, rodent damage, etc.). As a result, line repeaters can fail any time. Poor cable splicing techniques might lead to intermittent noise after installation. When channel banks or customer services have gone into alarm at the central office, how long the service disruption will remain is a function of the restoration technique. Most user companies have implemented spare span lines within each T1 trunk cable. When a working span begins to fail, the traffic at the ends is rerouted to the spare span line. Some choose to do this manually (via patch cords) at each attended office. Others who place more value on availability of the span line for live traffic have chosen to implement automatic transfer equipment.

The Automatic Protective Switch (APS) is located at each end of a span line facility and takes care of signal failure detection and automatic transfer. APS products generally fall into two categories: one-for-one switching (1:1) or one-for-N switching (1:N). The term 1:1 means that there is one protection line for each normal working line. Another way of stating this is one standby line and one normal line. This is most commonly used in ring networks and in applications involving mixed transmission media (one fiber facility and one copper facility, for example). In contrast, 1:N means that there is one protection line for a number (N) of normal working lines. In practice, the integer N could be 1, and the upper limit on N may be dozens. The number N is commonly 2 to 8. This type is most commonly used on "straight T1 cable carrier" applications, especially for subscriber loop carrier.

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